diff --git a/CHANGELOG.unreleased.md b/CHANGELOG.unreleased.md
index 8b0373858..38c4c0ad8 100644
--- a/CHANGELOG.unreleased.md
+++ b/CHANGELOG.unreleased.md
@@ -6,6 +6,7 @@
 - [Cloud Config] Support both NoCloud and Config Drive 2 datasources for maximum compatibility (PR [#4053](https://github.com/vatesfr/xen-orchestra/pull/4053))
 - [Advanced] Configurable cookie validity (PR [#4059](https://github.com/vatesfr/xen-orchestra/pull/4059))
 - [Plugins] Display number of installed plugins [#4008](https://github.com/vatesfr/xen-orchestra/issues/4008) (PR [#4050](https://github.com/vatesfr/xen-orchestra/pull/4050))
+- [Continuous Replication] Opt-in mode to guess VHD size, should help with XenServer 7.1 CU2 and various `VDI_IO_ERROR` errors (PR [#3726](https://github.com/vatesfr/xen-orchestra/pull/3726))
 
 ### Bug fixes
 
@@ -17,6 +18,7 @@
 
 ### Released packages
 
+- vhd-lib v0.6.0
 - @xen-orchestra/fs v0.8.0
 - xo-server v5.38.0
 - xo-web v5.38.0
diff --git a/packages/vhd-lib/merge.integ.spec.js b/packages/vhd-lib/merge.integ.spec.js
index 93a236cb4..f9e342cef 100644
--- a/packages/vhd-lib/merge.integ.spec.js
+++ b/packages/vhd-lib/merge.integ.spec.js
@@ -1,38 +1,40 @@
 /* eslint-env jest */
 
+import asyncIteratorToStream from 'async-iterator-to-stream'
 import execa from 'execa'
 import fs from 'fs-extra'
 import getStream from 'get-stream'
 import rimraf from 'rimraf'
 import tmp from 'tmp'
-import { fromEvent, pFromCallback } from 'promise-toolbox'
 import { getHandler } from '@xen-orchestra/fs'
+import { pFromCallback } from 'promise-toolbox'
+import { pipeline } from 'readable-stream'
 import { randomBytes } from 'crypto'
 
 import Vhd, { chainVhd, createSyntheticStream, mergeVhd as vhdMerge } from './'
 
 import { SECTOR_SIZE } from './src/_constants'
 
-const initialDir = process.cwd()
+let tempDir = null
 
 jest.setTimeout(60000)
 
 beforeEach(async () => {
-  const dir = await pFromCallback(cb => tmp.dir(cb))
-  process.chdir(dir)
+  tempDir = await pFromCallback(cb => tmp.dir(cb))
 })
 
 afterEach(async () => {
-  const tmpDir = process.cwd()
-  process.chdir(initialDir)
-  await pFromCallback(cb => rimraf(tmpDir, cb))
+  await pFromCallback(cb => rimraf(tempDir, cb))
 })
 
-async function createRandomFile(name, sizeMb) {
-  await execa('bash', [
-    '-c',
-    `< /dev/urandom tr -dc "\\t\\n [:alnum:]" | head -c ${sizeMb}M >${name}`,
-  ])
+async function createRandomFile(name, sizeMB) {
+  const createRandomStream = asyncIteratorToStream(function*(size) {
+    while (size-- > 0) {
+      yield Buffer.from([Math.floor(Math.random() * 256)])
+    }
+  })
+  const input = createRandomStream(sizeMB * 1024 * 1024)
+  await pFromCallback(cb => pipeline(input, fs.createWriteStream(name), cb))
 }
 
 async function checkFile(vhdName) {
@@ -53,31 +55,35 @@ async function convertFromRawToVhd(rawName, vhdName) {
 
 test('blocks can be moved', async () => {
   const initalSize = 4
-  await createRandomFile('randomfile', initalSize)
-  await convertFromRawToVhd('randomfile', 'randomfile.vhd')
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  const originalSize = await handler.getSize('randomfile')
-  const newVhd = new Vhd(handler, 'randomfile.vhd')
+  const rawFileName = `${tempDir}/randomfile`
+  await createRandomFile(rawFileName, initalSize)
+  const vhdFileName = `${tempDir}/randomfile.vhd`
+  await convertFromRawToVhd(rawFileName, vhdFileName)
+  const handler = getHandler({ url: 'file://' })
+  const originalSize = await handler.getSize(rawFileName)
+  const newVhd = new Vhd(handler, vhdFileName)
   await newVhd.readHeaderAndFooter()
   await newVhd.readBlockAllocationTable()
   await newVhd._freeFirstBlockSpace(8000000)
-  await recoverRawContent('randomfile.vhd', 'recovered', originalSize)
-  expect(await fs.readFile('recovered')).toEqual(
-    await fs.readFile('randomfile')
+  const recoveredFileName = `${tempDir}/recovered`
+  await recoverRawContent(vhdFileName, recoveredFileName, originalSize)
+  expect(await fs.readFile(recoveredFileName)).toEqual(
+    await fs.readFile(rawFileName)
   )
 })
 
 test('the BAT MSB is not used for sign', async () => {
   const randomBuffer = await pFromCallback(cb => randomBytes(SECTOR_SIZE, cb))
-  await execa('qemu-img', ['create', '-fvpc', 'empty.vhd', '1.8T'])
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  const vhd = new Vhd(handler, 'empty.vhd')
+  const emptyFileName = `${tempDir}/empty.vhd`
+  await execa('qemu-img', ['create', '-fvpc', emptyFileName, '1.8T'])
+  const handler = getHandler({ url: 'file://' })
+  const vhd = new Vhd(handler, emptyFileName)
   await vhd.readHeaderAndFooter()
   await vhd.readBlockAllocationTable()
   // we want the bit 31 to be on, to prove it's not been used for sign
   const hugeWritePositionSectors = Math.pow(2, 31) + 200
   await vhd.writeData(hugeWritePositionSectors, randomBuffer)
-  await checkFile('empty.vhd')
+  await checkFile(emptyFileName)
   // here we are moving the first sector very far in the VHD to prove the BAT doesn't use signed int32
   const hugePositionBytes = hugeWritePositionSectors * SECTOR_SIZE
   await vhd._freeFirstBlockSpace(hugePositionBytes)
@@ -85,9 +91,10 @@ test('the BAT MSB is not used for sign', async () => {
   // we recover the data manually for speed reasons.
   // fs.write() with offset is way faster than qemu-img when there is a 1.5To
   // hole before the block of data
-  const recoveredFile = await fs.open('recovered', 'w')
+  const recoveredFileName = `${tempDir}/recovered`
+  const recoveredFile = await fs.open(recoveredFileName, 'w')
   try {
-    const vhd2 = new Vhd(handler, 'empty.vhd')
+    const vhd2 = new Vhd(handler, emptyFileName)
     await vhd2.readHeaderAndFooter()
     await vhd2.readBlockAllocationTable()
     for (let i = 0; i < vhd.header.maxTableEntries; i++) {
@@ -107,7 +114,7 @@ test('the BAT MSB is not used for sign', async () => {
     fs.close(recoveredFile)
   }
   const recovered = await getStream.buffer(
-    await fs.createReadStream('recovered', {
+    await fs.createReadStream(recoveredFileName, {
       start: hugePositionBytes,
       end: hugePositionBytes + randomBuffer.length - 1,
     })
@@ -117,27 +124,33 @@ test('the BAT MSB is not used for sign', async () => {
 
 test('writeData on empty file', async () => {
   const mbOfRandom = 3
-  await createRandomFile('randomfile', mbOfRandom)
-  await execa('qemu-img', ['create', '-fvpc', 'empty.vhd', mbOfRandom + 'M'])
-  const randomData = await fs.readFile('randomfile')
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  const originalSize = await handler.getSize('randomfile')
-  const newVhd = new Vhd(handler, 'empty.vhd')
+  const rawFileName = `${tempDir}/randomfile`
+  const emptyFileName = `${tempDir}/empty.vhd`
+  await createRandomFile(rawFileName, mbOfRandom)
+  await execa('qemu-img', ['create', '-fvpc', emptyFileName, mbOfRandom + 'M'])
+  const randomData = await fs.readFile(rawFileName)
+  const handler = getHandler({ url: 'file://' })
+  const originalSize = await handler.getSize(rawFileName)
+  const newVhd = new Vhd(handler, emptyFileName)
   await newVhd.readHeaderAndFooter()
   await newVhd.readBlockAllocationTable()
   await newVhd.writeData(0, randomData)
-  await recoverRawContent('empty.vhd', 'recovered', originalSize)
-  expect(await fs.readFile('recovered')).toEqual(randomData)
+  const recoveredFileName = `${tempDir}/recovered`
+  await recoverRawContent(emptyFileName, recoveredFileName, originalSize)
+  expect(await fs.readFile(recoveredFileName)).toEqual(randomData)
 })
 
 test('writeData in 2 non-overlaping operations', async () => {
   const mbOfRandom = 3
-  await createRandomFile('randomfile', mbOfRandom)
-  await execa('qemu-img', ['create', '-fvpc', 'empty.vhd', mbOfRandom + 'M'])
-  const randomData = await fs.readFile('randomfile')
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  const originalSize = await handler.getSize('randomfile')
-  const newVhd = new Vhd(handler, 'empty.vhd')
+  const rawFileName = `${tempDir}/randomfile`
+  const emptyFileName = `${tempDir}/empty.vhd`
+  const recoveredFileName = `${tempDir}/recovered`
+  await createRandomFile(rawFileName, mbOfRandom)
+  await execa('qemu-img', ['create', '-fvpc', emptyFileName, mbOfRandom + 'M'])
+  const randomData = await fs.readFile(rawFileName)
+  const handler = getHandler({ url: 'file://' })
+  const originalSize = await handler.getSize(rawFileName)
+  const newVhd = new Vhd(handler, emptyFileName)
   await newVhd.readHeaderAndFooter()
   await newVhd.readBlockAllocationTable()
   const splitPointSectors = 2
@@ -146,18 +159,21 @@ test('writeData in 2 non-overlaping operations', async () => {
     splitPointSectors,
     randomData.slice(splitPointSectors * 512)
   )
-  await recoverRawContent('empty.vhd', 'recovered', originalSize)
-  expect(await fs.readFile('recovered')).toEqual(randomData)
+  await recoverRawContent(emptyFileName, recoveredFileName, originalSize)
+  expect(await fs.readFile(recoveredFileName)).toEqual(randomData)
 })
 
 test('writeData in 2 overlaping operations', async () => {
   const mbOfRandom = 3
-  await createRandomFile('randomfile', mbOfRandom)
-  await execa('qemu-img', ['create', '-fvpc', 'empty.vhd', mbOfRandom + 'M'])
-  const randomData = await fs.readFile('randomfile')
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  const originalSize = await handler.getSize('randomfile')
-  const newVhd = new Vhd(handler, 'empty.vhd')
+  const rawFileName = `${tempDir}/randomfile`
+  const emptyFileName = `${tempDir}/empty.vhd`
+  const recoveredFileName = `${tempDir}/recovered`
+  await createRandomFile(rawFileName, mbOfRandom)
+  await execa('qemu-img', ['create', '-fvpc', emptyFileName, mbOfRandom + 'M'])
+  const randomData = await fs.readFile(rawFileName)
+  const handler = getHandler({ url: 'file://' })
+  const originalSize = await handler.getSize(rawFileName)
+  const newVhd = new Vhd(handler, emptyFileName)
   await newVhd.readHeaderAndFooter()
   await newVhd.readBlockAllocationTable()
   const endFirstWrite = 3
@@ -167,119 +183,138 @@ test('writeData in 2 overlaping operations', async () => {
     startSecondWrite,
     randomData.slice(startSecondWrite * 512)
   )
-  await recoverRawContent('empty.vhd', 'recovered', originalSize)
-  expect(await fs.readFile('recovered')).toEqual(randomData)
+  await recoverRawContent(emptyFileName, recoveredFileName, originalSize)
+  expect(await fs.readFile(recoveredFileName)).toEqual(randomData)
 })
 
 test('BAT can be extended and blocks moved', async () => {
   const initalSize = 4
-  await createRandomFile('randomfile', initalSize)
-  await convertFromRawToVhd('randomfile', 'randomfile.vhd')
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  const originalSize = await handler.getSize('randomfile')
-  const newVhd = new Vhd(handler, 'randomfile.vhd')
+  const rawFileName = `${tempDir}/randomfile`
+  const recoveredFileName = `${tempDir}/recovered`
+  const vhdFileName = `${tempDir}/randomfile.vhd`
+  await createRandomFile(rawFileName, initalSize)
+  await convertFromRawToVhd(rawFileName, vhdFileName)
+  const handler = getHandler({ url: 'file://' })
+  const originalSize = await handler.getSize(rawFileName)
+  const newVhd = new Vhd(handler, vhdFileName)
   await newVhd.readHeaderAndFooter()
   await newVhd.readBlockAllocationTable()
   await newVhd.ensureBatSize(2000)
-  await recoverRawContent('randomfile.vhd', 'recovered', originalSize)
-  expect(await fs.readFile('recovered')).toEqual(
-    await fs.readFile('randomfile')
+  await recoverRawContent(vhdFileName, recoveredFileName, originalSize)
+  expect(await fs.readFile(recoveredFileName)).toEqual(
+    await fs.readFile(rawFileName)
   )
 })
 
 test('coalesce works with empty parent files', async () => {
   const mbOfRandom = 2
-  await createRandomFile('randomfile', mbOfRandom)
-  await convertFromRawToVhd('randomfile', 'randomfile.vhd')
+  const rawFileName = `${tempDir}/randomfile`
+  const emptyFileName = `${tempDir}/empty.vhd`
+  const vhdFileName = `${tempDir}/randomfile.vhd`
+  const recoveredFileName = `${tempDir}/recovered`
+  await createRandomFile(rawFileName, mbOfRandom)
+  await convertFromRawToVhd(rawFileName, vhdFileName)
   await execa('qemu-img', [
     'create',
     '-fvpc',
-    'empty.vhd',
+    emptyFileName,
     mbOfRandom + 1 + 'M',
   ])
-  await checkFile('randomfile.vhd')
-  await checkFile('empty.vhd')
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  const originalSize = await handler._getSize('randomfile')
-  await chainVhd(handler, 'empty.vhd', handler, 'randomfile.vhd', true)
-  await checkFile('randomfile.vhd')
-  await checkFile('empty.vhd')
-  await vhdMerge(handler, 'empty.vhd', handler, 'randomfile.vhd')
-  await recoverRawContent('empty.vhd', 'recovered', originalSize)
-  expect(await fs.readFile('recovered')).toEqual(
-    await fs.readFile('randomfile')
+  await checkFile(vhdFileName)
+  await checkFile(emptyFileName)
+  const handler = getHandler({ url: 'file://' })
+  const originalSize = await handler._getSize(rawFileName)
+  await chainVhd(handler, emptyFileName, handler, vhdFileName, true)
+  await checkFile(vhdFileName)
+  await checkFile(emptyFileName)
+  await vhdMerge(handler, emptyFileName, handler, vhdFileName)
+  await recoverRawContent(emptyFileName, recoveredFileName, originalSize)
+  expect(await fs.readFile(recoveredFileName)).toEqual(
+    await fs.readFile(rawFileName)
   )
 })
 
 test('coalesce works in normal cases', async () => {
   const mbOfRandom = 5
-  await createRandomFile('randomfile', mbOfRandom)
-  await createRandomFile('small_randomfile', Math.ceil(mbOfRandom / 2))
+  const randomFileName = `${tempDir}/randomfile`
+  const random2FileName = `${tempDir}/randomfile2`
+  const smallRandomFileName = `${tempDir}/small_randomfile`
+  const parentFileName = `${tempDir}/parent.vhd`
+  const child1FileName = `${tempDir}/child1.vhd`
+  const child2FileName = `${tempDir}/child2.vhd`
+  const recoveredFileName = `${tempDir}/recovered`
+  await createRandomFile(randomFileName, mbOfRandom)
+  await createRandomFile(smallRandomFileName, Math.ceil(mbOfRandom / 2))
   await execa('qemu-img', [
     'create',
     '-fvpc',
-    'parent.vhd',
+    parentFileName,
     mbOfRandom + 1 + 'M',
   ])
-  await convertFromRawToVhd('randomfile', 'child1.vhd')
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  await execa('vhd-util', ['snapshot', '-n', 'child2.vhd', '-p', 'child1.vhd'])
-  const vhd = new Vhd(handler, 'child2.vhd')
+  await convertFromRawToVhd(randomFileName, child1FileName)
+  const handler = getHandler({ url: 'file://' })
+  await execa('vhd-util', [
+    'snapshot',
+    '-n',
+    child2FileName,
+    '-p',
+    child1FileName,
+  ])
+  const vhd = new Vhd(handler, child2FileName)
   await vhd.readHeaderAndFooter()
   await vhd.readBlockAllocationTable()
   vhd.footer.creatorApplication = 'xoa'
   await vhd.writeFooter()
 
-  const originalSize = await handler._getSize('randomfile')
-  await chainVhd(handler, 'parent.vhd', handler, 'child1.vhd', true)
-  await execa('vhd-util', ['check', '-t', '-n', 'child1.vhd'])
-  await chainVhd(handler, 'child1.vhd', handler, 'child2.vhd', true)
-  await execa('vhd-util', ['check', '-t', '-n', 'child2.vhd'])
-  const smallRandom = await fs.readFile('small_randomfile')
-  const newVhd = new Vhd(handler, 'child2.vhd')
+  const originalSize = await handler._getSize(randomFileName)
+  await chainVhd(handler, parentFileName, handler, child1FileName, true)
+  await execa('vhd-util', ['check', '-t', '-n', child1FileName])
+  await chainVhd(handler, child1FileName, handler, child2FileName, true)
+  await execa('vhd-util', ['check', '-t', '-n', child2FileName])
+  const smallRandom = await fs.readFile(smallRandomFileName)
+  const newVhd = new Vhd(handler, child2FileName)
   await newVhd.readHeaderAndFooter()
   await newVhd.readBlockAllocationTable()
   await newVhd.writeData(5, smallRandom)
-  await checkFile('child2.vhd')
-  await checkFile('child1.vhd')
-  await checkFile('parent.vhd')
-  await vhdMerge(handler, 'parent.vhd', handler, 'child1.vhd')
-  await checkFile('parent.vhd')
-  await chainVhd(handler, 'parent.vhd', handler, 'child2.vhd', true)
-  await checkFile('child2.vhd')
-  await vhdMerge(handler, 'parent.vhd', handler, 'child2.vhd')
-  await checkFile('parent.vhd')
-  await recoverRawContent(
-    'parent.vhd',
-    'recovered_from_coalescing',
-    originalSize
-  )
-  await execa('cp', ['randomfile', 'randomfile2'])
-  const fd = await fs.open('randomfile2', 'r+')
+  await checkFile(child2FileName)
+  await checkFile(child1FileName)
+  await checkFile(parentFileName)
+  await vhdMerge(handler, parentFileName, handler, child1FileName)
+  await checkFile(parentFileName)
+  await chainVhd(handler, parentFileName, handler, child2FileName, true)
+  await checkFile(child2FileName)
+  await vhdMerge(handler, parentFileName, handler, child2FileName)
+  await checkFile(parentFileName)
+  await recoverRawContent(parentFileName, recoveredFileName, originalSize)
+  await execa('cp', [randomFileName, random2FileName])
+  const fd = await fs.open(random2FileName, 'r+')
   try {
     await fs.write(fd, smallRandom, 0, smallRandom.length, 5 * SECTOR_SIZE)
   } finally {
     await fs.close(fd)
   }
-  expect(await fs.readFile('recovered_from_coalescing')).toEqual(
-    await fs.readFile('randomfile2')
+  expect(await fs.readFile(recoveredFileName)).toEqual(
+    await fs.readFile(random2FileName)
   )
 })
 
-test('createSyntheticStream passes vhd-util check', async () => {
+test.only('createSyntheticStream passes vhd-util check', async () => {
   const initalSize = 4
-  const expectedVhdSize = 4197888
-  await createRandomFile('randomfile', initalSize)
-  await convertFromRawToVhd('randomfile', 'randomfile.vhd')
-  const handler = getHandler({ url: 'file://' + process.cwd() })
-  const stream = await createSyntheticStream(handler, 'randomfile.vhd')
-  expect(stream.length).toEqual(expectedVhdSize)
-  await fromEvent(
-    stream.pipe(await fs.createWriteStream('recovered.vhd')),
-    'finish'
+  const rawFileName = `${tempDir}/randomfile`
+  const vhdFileName = `${tempDir}/randomfile.vhd`
+  const recoveredVhdFileName = `${tempDir}/recovered.vhd`
+  await createRandomFile(rawFileName, initalSize)
+  await convertFromRawToVhd(rawFileName, vhdFileName)
+  await checkFile(vhdFileName)
+  const handler = getHandler({ url: 'file://' })
+  const stream = await createSyntheticStream(handler, vhdFileName)
+  const expectedVhdSize = (await fs.stat(vhdFileName)).size
+  expect(stream.length).toEqual((await fs.stat(vhdFileName)).size)
+  await pFromCallback(cb =>
+    pipeline(stream, fs.createWriteStream(recoveredVhdFileName), cb)
   )
-  await checkFile('recovered.vhd')
-  const stats = await fs.stat('recovered.vhd')
+  await checkFile(recoveredVhdFileName)
+  const stats = await fs.stat(recoveredVhdFileName)
   expect(stats.size).toEqual(expectedVhdSize)
-  await execa('qemu-img', ['compare', 'recovered.vhd', 'randomfile'])
+  await execa('qemu-img', ['compare', recoveredVhdFileName, rawFileName])
 })
diff --git a/packages/vhd-lib/package.json b/packages/vhd-lib/package.json
index c49cd4ea4..43897b0c8 100644
--- a/packages/vhd-lib/package.json
+++ b/packages/vhd-lib/package.json
@@ -42,6 +42,7 @@
     "fs-promise": "^2.0.0",
     "get-stream": "^4.0.0",
     "index-modules": "^0.3.0",
+    "readable-stream": "^3.0.6",
     "rimraf": "^2.6.2",
     "tmp": "^0.0.33"
   },
diff --git a/packages/vhd-lib/src/_checkFooter.js b/packages/vhd-lib/src/_checkFooter.js
new file mode 100644
index 000000000..1d2803055
--- /dev/null
+++ b/packages/vhd-lib/src/_checkFooter.js
@@ -0,0 +1,20 @@
+import assert from 'assert'
+
+import {
+  DISK_TYPE_DIFFERENCING,
+  DISK_TYPE_DYNAMIC,
+  FILE_FORMAT_VERSION,
+  FOOTER_COOKIE,
+  FOOTER_SIZE,
+} from './_constants'
+
+export default footer => {
+  assert.strictEqual(footer.cookie, FOOTER_COOKIE)
+  assert.strictEqual(footer.dataOffset, FOOTER_SIZE)
+  assert.strictEqual(footer.fileFormatVersion, FILE_FORMAT_VERSION)
+  assert(footer.originalSize <= footer.currentSize)
+  assert(
+    footer.diskType === DISK_TYPE_DIFFERENCING ||
+      footer.diskType === DISK_TYPE_DYNAMIC
+  )
+}
diff --git a/packages/vhd-lib/src/_checkHeader.js b/packages/vhd-lib/src/_checkHeader.js
new file mode 100644
index 000000000..332dcd759
--- /dev/null
+++ b/packages/vhd-lib/src/_checkHeader.js
@@ -0,0 +1,14 @@
+import assert from 'assert'
+
+import { HEADER_COOKIE, HEADER_VERSION, SECTOR_SIZE } from './_constants'
+
+export default (header, footer) => {
+  assert.strictEqual(header.cookie, HEADER_COOKIE)
+  assert.strictEqual(header.dataOffset, undefined)
+  assert.strictEqual(header.headerVersion, HEADER_VERSION)
+  assert(Number.isInteger(Math.log2(header.blockSize / SECTOR_SIZE)))
+
+  if (footer !== undefined) {
+    assert(header.maxTableEntries >= footer.currentSize / header.blockSize)
+  }
+}
diff --git a/packages/vhd-lib/src/_getFirstAndLastBlocks.js b/packages/vhd-lib/src/_getFirstAndLastBlocks.js
new file mode 100644
index 000000000..dadd29ee0
--- /dev/null
+++ b/packages/vhd-lib/src/_getFirstAndLastBlocks.js
@@ -0,0 +1,47 @@
+import assert from 'assert'
+
+import { BLOCK_UNUSED } from './_constants'
+
+// get the identifiers and first sectors of the first and last block
+// in the file
+export default bat => {
+  const n = bat.length
+  assert.notStrictEqual(n, 0)
+  assert.strictEqual(n % 4, 0)
+
+  let i = 0
+  let j = 0
+  let first, firstSector, last, lastSector
+
+  // get first allocated block for initialization
+  while ((firstSector = bat.readUInt32BE(j)) === BLOCK_UNUSED) {
+    i += 1
+    j += 4
+
+    if (j === n) {
+      const error = new Error('no allocated block found')
+      error.noBlock = true
+      throw error
+    }
+  }
+  lastSector = firstSector
+  first = last = i
+
+  while (j < n) {
+    const sector = bat.readUInt32BE(j)
+    if (sector !== BLOCK_UNUSED) {
+      if (sector < firstSector) {
+        first = i
+        firstSector = sector
+      } else if (sector > lastSector) {
+        last = i
+        lastSector = sector
+      }
+    }
+
+    i += 1
+    j += 4
+  }
+
+  return { first, firstSector, last, lastSector }
+}
diff --git a/packages/vhd-lib/src/_readChunk.js b/packages/vhd-lib/src/_readChunk.js
new file mode 100644
index 000000000..0d049b797
--- /dev/null
+++ b/packages/vhd-lib/src/_readChunk.js
@@ -0,0 +1,50 @@
+export default async function readChunk(stream, n) {
+  if (n === 0) {
+    return Buffer.alloc(0)
+  }
+  return new Promise((resolve, reject) => {
+    const chunks = []
+    let i = 0
+
+    function clean() {
+      stream.removeListener('readable', onReadable)
+      stream.removeListener('end', onEnd)
+      stream.removeListener('error', onError)
+    }
+
+    function resolve2() {
+      clean()
+      resolve(Buffer.concat(chunks, i))
+    }
+
+    function onEnd() {
+      resolve2()
+      clean()
+    }
+
+    function onError(error) {
+      reject(error)
+      clean()
+    }
+
+    function onReadable() {
+      const chunk = stream.read(n - i)
+      if (chunk === null) {
+        return // wait for more data
+      }
+      i += chunk.length
+      chunks.push(chunk)
+      if (i >= n) {
+        resolve2()
+      }
+    }
+
+    stream.on('end', onEnd)
+    stream.on('error', onError)
+    stream.on('readable', onReadable)
+
+    if (stream.readable) {
+      onReadable()
+    }
+  })
+}
diff --git a/packages/vhd-lib/src/createVhdStreamWithLength.integ.spec.js b/packages/vhd-lib/src/createVhdStreamWithLength.integ.spec.js
new file mode 100644
index 000000000..1737fa599
--- /dev/null
+++ b/packages/vhd-lib/src/createVhdStreamWithLength.integ.spec.js
@@ -0,0 +1,93 @@
+/* eslint-env jest */
+
+import asyncIteratorToStream from 'async-iterator-to-stream'
+import execa from 'execa'
+import fs from 'fs-extra'
+import rimraf from 'rimraf'
+import getStream from 'get-stream'
+import tmp from 'tmp'
+import { createReadStream, createWriteStream } from 'fs'
+import { pFromCallback } from 'promise-toolbox'
+import { pipeline } from 'readable-stream'
+
+import { createVhdStreamWithLength } from '.'
+import { FOOTER_SIZE } from './_constants'
+
+let tempDir = null
+
+beforeEach(async () => {
+  tempDir = await pFromCallback(cb => tmp.dir(cb))
+})
+
+afterEach(async () => {
+  await pFromCallback(cb => rimraf(tempDir, cb))
+})
+
+async function convertFromRawToVhd(rawName, vhdName) {
+  await execa('qemu-img', ['convert', '-f', 'raw', '-Ovpc', rawName, vhdName])
+}
+
+async function createRandomFile(name, size) {
+  const createRandomStream = asyncIteratorToStream(function*(size) {
+    while (size-- > 0) {
+      yield Buffer.from([Math.floor(Math.random() * 256)])
+    }
+  })
+  const input = await createRandomStream(size)
+  await pFromCallback(cb => pipeline(input, fs.createWriteStream(name), cb))
+}
+
+test('createVhdStreamWithLength can extract length', async () => {
+  const initialSize = 4 * 1024
+  const rawFileName = `${tempDir}/randomfile`
+  const vhdName = `${tempDir}/randomfile.vhd`
+  const outputVhdName = `${tempDir}/output.vhd`
+  await createRandomFile(rawFileName, initialSize)
+  await convertFromRawToVhd(rawFileName, vhdName)
+  const vhdSize = fs.statSync(vhdName).size
+  const result = await createVhdStreamWithLength(
+    await createReadStream(vhdName)
+  )
+  expect(result.length).toEqual(vhdSize)
+  const outputFileStream = await createWriteStream(outputVhdName)
+  await pFromCallback(cb => pipeline(result, outputFileStream, cb))
+  const outputSize = fs.statSync(outputVhdName).size
+  expect(outputSize).toEqual(vhdSize)
+})
+
+test('createVhdStreamWithLength can skip blank after last block and before footer', async () => {
+  const initialSize = 4 * 1024
+  const rawFileName = `${tempDir}/randomfile`
+  const vhdName = `${tempDir}/randomfile.vhd`
+  const outputVhdName = `${tempDir}/output.vhd`
+  await createRandomFile(rawFileName, initialSize)
+  await convertFromRawToVhd(rawFileName, vhdName)
+  const vhdSize = fs.statSync(vhdName).size
+  // read file footer
+  const footer = await getStream.buffer(
+    createReadStream(vhdName, { start: vhdSize - FOOTER_SIZE })
+  )
+
+  // we'll override the footer
+  const endOfFile = await createWriteStream(vhdName, {
+    flags: 'r+',
+    start: vhdSize - FOOTER_SIZE,
+  })
+  // write a blank over the previous footer
+  await pFromCallback(cb => endOfFile.write(Buffer.alloc(FOOTER_SIZE), cb))
+  // write the footer after the new blank
+  await pFromCallback(cb => endOfFile.end(footer, cb))
+  const longerSize = fs.statSync(vhdName).size
+  // check input file has been lengthened
+  expect(longerSize).toEqual(vhdSize + FOOTER_SIZE)
+  const result = await createVhdStreamWithLength(
+    await createReadStream(vhdName)
+  )
+  expect(result.length).toEqual(vhdSize)
+  const outputFileStream = await createWriteStream(outputVhdName)
+  await pFromCallback(cb => pipeline(result, outputFileStream, cb))
+  const outputSize = fs.statSync(outputVhdName).size
+  // check out file has been shortened again
+  expect(outputSize).toEqual(vhdSize)
+  await execa('qemu-img', ['compare', outputVhdName, vhdName])
+})
diff --git a/packages/vhd-lib/src/createVhdStreamWithLength.js b/packages/vhd-lib/src/createVhdStreamWithLength.js
new file mode 100644
index 000000000..ac19de8e0
--- /dev/null
+++ b/packages/vhd-lib/src/createVhdStreamWithLength.js
@@ -0,0 +1,80 @@
+import assert from 'assert'
+import { pipeline, Transform } from 'readable-stream'
+
+import checkFooter from './_checkFooter'
+import checkHeader from './_checkHeader'
+import noop from './_noop'
+import getFirstAndLastBlocks from './_getFirstAndLastBlocks'
+import readChunk from './_readChunk'
+import { FOOTER_SIZE, HEADER_SIZE, SECTOR_SIZE } from './_constants'
+import { fuFooter, fuHeader } from './_structs'
+
+class EndCutterStream extends Transform {
+  constructor(footerOffset, footerBuffer) {
+    super()
+    this._footerOffset = footerOffset
+    this._footerBuffer = footerBuffer
+    this._position = 0
+    this._done = false
+  }
+
+  _transform(data, encoding, callback) {
+    if (!this._done) {
+      if (this._position + data.length >= this._footerOffset) {
+        this._done = true
+        const difference = this._footerOffset - this._position
+        data = data.slice(0, difference)
+        this.push(data)
+        this.push(this._footerBuffer)
+      } else {
+        this.push(data)
+      }
+      this._position += data.length
+    }
+    callback()
+  }
+}
+
+export default async function createVhdStreamWithLength(stream) {
+  const readBuffers = []
+  let streamPosition = 0
+
+  async function readStream(length) {
+    const chunk = await readChunk(stream, length)
+    assert.strictEqual(chunk.length, length)
+    streamPosition += chunk.length
+    readBuffers.push(chunk)
+    return chunk
+  }
+
+  const footerBuffer = await readStream(FOOTER_SIZE)
+  const footer = fuFooter.unpack(footerBuffer)
+  checkFooter(footer)
+
+  const header = fuHeader.unpack(await readStream(HEADER_SIZE))
+  checkHeader(header, footer)
+
+  await readStream(header.tableOffset - streamPosition)
+
+  const table = await readStream(header.maxTableEntries * 4)
+
+  readBuffers.reverse()
+  for (const buf of readBuffers) {
+    stream.unshift(buf)
+  }
+
+  const footerOffset =
+    getFirstAndLastBlocks(table).lastSector * SECTOR_SIZE +
+    Math.ceil(header.blockSize / SECTOR_SIZE / 8 / SECTOR_SIZE) * SECTOR_SIZE +
+    header.blockSize
+
+  // ignore any data after footerOffset and push footerBuffer
+  //
+  // this is necessary to ignore any blank space between the last block and the
+  // final footer which would invalidate the size we computed
+  const newStream = new EndCutterStream(footerOffset, footerBuffer)
+  pipeline(stream, newStream, noop)
+
+  newStream.length = footerOffset + FOOTER_SIZE
+  return newStream
+}
diff --git a/packages/vhd-lib/src/index.js b/packages/vhd-lib/src/index.js
index dcbe20080..3d033b241 100644
--- a/packages/vhd-lib/src/index.js
+++ b/packages/vhd-lib/src/index.js
@@ -11,3 +11,6 @@ export {
 } from './createReadableSparseStream'
 export { default as createSyntheticStream } from './createSyntheticStream'
 export { default as mergeVhd } from './merge'
+export {
+  default as createVhdStreamWithLength,
+} from './createVhdStreamWithLength'
diff --git a/packages/vhd-lib/src/vhd.js b/packages/vhd-lib/src/vhd.js
index e0401e61c..6f67b6cf0 100644
--- a/packages/vhd-lib/src/vhd.js
+++ b/packages/vhd-lib/src/vhd.js
@@ -1,19 +1,16 @@
 import assert from 'assert'
 import { fromEvent } from 'promise-toolbox'
 
+import checkFooter from './_checkFooter'
+import checkHeader from './_checkHeader'
 import constantStream from './_constant-stream'
+import getFirstAndLastBlocks from './_getFirstAndLastBlocks'
 import { fuFooter, fuHeader, checksumStruct, unpackField } from './_structs'
 import { set as mapSetBit, test as mapTestBit } from './_bitmap'
 import {
   BLOCK_UNUSED,
-  DISK_TYPE_DIFFERENCING,
-  DISK_TYPE_DYNAMIC,
-  FILE_FORMAT_VERSION,
-  FOOTER_COOKIE,
   FOOTER_SIZE,
-  HEADER_COOKIE,
   HEADER_SIZE,
-  HEADER_VERSION,
   PARENT_LOCATOR_ENTRIES,
   PLATFORM_NONE,
   PLATFORM_W2KU,
@@ -170,21 +167,10 @@ export default class Vhd {
     }
 
     const footer = (this.footer = fuFooter.unpack(bufFooter))
-    assert.strictEqual(footer.cookie, FOOTER_COOKIE, 'footer cookie')
-    assert.strictEqual(footer.dataOffset, FOOTER_SIZE)
-    assert.strictEqual(footer.fileFormatVersion, FILE_FORMAT_VERSION)
-    assert(footer.originalSize <= footer.currentSize)
-    assert(
-      footer.diskType === DISK_TYPE_DIFFERENCING ||
-        footer.diskType === DISK_TYPE_DYNAMIC
-    )
+    checkFooter(footer)
 
     const header = (this.header = fuHeader.unpack(bufHeader))
-    assert.strictEqual(header.cookie, HEADER_COOKIE)
-    assert.strictEqual(header.dataOffset, undefined)
-    assert.strictEqual(header.headerVersion, HEADER_VERSION)
-    assert(header.maxTableEntries >= footer.currentSize / header.blockSize)
-    assert(Number.isInteger(Math.log2(header.blockSize / SECTOR_SIZE)))
+    checkHeader(header, footer)
 
     // Compute the number of sectors in one block.
     // Default: One block contains 4096 sectors of 512 bytes.
@@ -242,49 +228,6 @@ export default class Vhd {
     )
   }
 
-  // get the identifiers and first sectors of the first and last block
-  // in the file
-  //
-  _getFirstAndLastBlocks() {
-    const n = this.header.maxTableEntries
-    const bat = this.blockTable
-    let i = 0
-    let j = 0
-    let first, firstSector, last, lastSector
-
-    // get first allocated block for initialization
-    while ((firstSector = bat.readUInt32BE(j)) === BLOCK_UNUSED) {
-      i += 1
-      j += 4
-
-      if (i === n) {
-        const error = new Error('no allocated block found')
-        error.noBlock = true
-        throw error
-      }
-    }
-    lastSector = firstSector
-    first = last = i
-
-    while (i < n) {
-      const sector = bat.readUInt32BE(j)
-      if (sector !== BLOCK_UNUSED) {
-        if (sector < firstSector) {
-          first = i
-          firstSector = sector
-        } else if (sector > lastSector) {
-          last = i
-          lastSector = sector
-        }
-      }
-
-      i += 1
-      j += 4
-    }
-
-    return { first, firstSector, last, lastSector }
-  }
-
   // =================================================================
   // Write functions.
   // =================================================================
@@ -311,7 +254,9 @@ export default class Vhd {
 
   async _freeFirstBlockSpace(spaceNeededBytes) {
     try {
-      const { first, firstSector, lastSector } = this._getFirstAndLastBlocks()
+      const { first, firstSector, lastSector } = getFirstAndLastBlocks(
+        this.blockTable
+      )
       const tableOffset = this.header.tableOffset
       const { batSize } = this
       const newMinSector = Math.ceil(
diff --git a/packages/vhd-lib/vhd.integ.spec.js b/packages/vhd-lib/vhd.integ.spec.js
index c4837441b..ca976aebb 100644
--- a/packages/vhd-lib/vhd.integ.spec.js
+++ b/packages/vhd-lib/vhd.integ.spec.js
@@ -4,22 +4,20 @@ import rimraf from 'rimraf'
 import tmp from 'tmp'
 import { createWriteStream, readFile } from 'fs-promise'
 import { fromEvent, pFromCallback } from 'promise-toolbox'
+import { pipeline } from 'readable-stream'
 
 import { createReadableRawStream, createReadableSparseStream } from './'
 
 import { createFooter } from './src/_createFooterHeader'
 
-const initialDir = process.cwd()
+let tempDir = null
 
 beforeEach(async () => {
-  const dir = await pFromCallback(cb => tmp.dir(cb))
-  process.chdir(dir)
+  tempDir = await pFromCallback(cb => tmp.dir(cb))
 })
 
 afterEach(async () => {
-  const tmpDir = process.cwd()
-  process.chdir(initialDir)
-  await pFromCallback(cb => rimraf(tmpDir, cb))
+  await pFromCallback(cb => rimraf(tempDir, cb))
 })
 
 test('createFooter() does not crash', () => {
@@ -55,9 +53,10 @@ test('ReadableRawVHDStream does not crash', async () => {
   }
   const fileSize = 1000
   const stream = createReadableRawStream(fileSize, mockParser)
-  const pipe = stream.pipe(createWriteStream('output.vhd'))
-  await fromEvent(pipe, 'finish')
-  await execa('vhd-util', ['check', '-t', '-i', '-n', 'output.vhd'])
+  await pFromCallback(cb =>
+    pipeline(stream, createWriteStream(`${tempDir}/output.vhd`), cb)
+  )
+  await execa('vhd-util', ['check', '-t', '-i', '-n', `${tempDir}/output.vhd`])
 })
 
 test('ReadableRawVHDStream detects when blocks are out of order', async () => {
@@ -87,9 +86,9 @@ test('ReadableRawVHDStream detects when blocks are out of order', async () => {
     new Promise((resolve, reject) => {
       const stream = createReadableRawStream(100000, mockParser)
       stream.on('error', reject)
-      const pipe = stream.pipe(createWriteStream('outputStream'))
-      pipe.on('finish', resolve)
-      pipe.on('error', reject)
+      pipeline(stream, createWriteStream(`${tempDir}/outputStream`), err =>
+        err ? reject(err) : resolve()
+      )
     })
   ).rejects.toThrow('Received out of order blocks')
 })
@@ -114,19 +113,19 @@ test('ReadableSparseVHDStream can handle a sparse file', async () => {
     blocks
   )
   expect(stream.length).toEqual(4197888)
-  const pipe = stream.pipe(createWriteStream('output.vhd'))
+  const pipe = stream.pipe(createWriteStream(`${tempDir}/output.vhd`))
   await fromEvent(pipe, 'finish')
-  await execa('vhd-util', ['check', '-t', '-i', '-n', 'output.vhd'])
+  await execa('vhd-util', ['check', '-t', '-i', '-n', `${tempDir}/output.vhd`])
   await execa('qemu-img', [
     'convert',
     '-f',
     'vpc',
     '-O',
     'raw',
-    'output.vhd',
-    'out1.raw',
+    `${tempDir}/output.vhd`,
+    `${tempDir}/out1.raw`,
   ])
-  const out1 = await readFile('out1.raw')
+  const out1 = await readFile(`${tempDir}/out1.raw`)
   const expected = Buffer.alloc(fileSize)
   blocks.forEach(b => {
     b.data.copy(expected, b.offsetBytes)
diff --git a/packages/xo-server/config.toml b/packages/xo-server/config.toml
index baa810cbf..90278752a 100644
--- a/packages/xo-server/config.toml
+++ b/packages/xo-server/config.toml
@@ -9,6 +9,18 @@ datadir = '/var/lib/xo-server/data'
 # Necessary for external authentication providers.
 createUserOnFirstSignin = true
 
+# XAPI does not support chunked encoding in HTTP requests which is necessary
+# when the content length is not know which is the case for many backup related
+# operations in XO.
+#
+# It's possible to work-around this for VHDs  because it's possible to guess
+# their size just by looking at the beginning of the stream.
+#
+# But it is a guess, not a certainty, it depends on how the VHDs are formatted
+# by XenServer, therefore it's disabled for the moment but can be enabled
+# specifically for a user if necessary.
+guessVhdSizeOnImport = false
+
 # Whether API logs should contains the full request/response on
 # errors.
 #
diff --git a/packages/xo-server/src/xapi/index.js b/packages/xo-server/src/xapi/index.js
index 78f2d7e86..5c181784e 100644
--- a/packages/xo-server/src/xapi/index.js
+++ b/packages/xo-server/src/xapi/index.js
@@ -68,6 +68,7 @@ import {
   parseDateTime,
   prepareXapiParam,
 } from './utils'
+import { createVhdStreamWithLength } from 'vhd-lib'
 
 const log = createLogger('xo:xapi')
 
@@ -93,8 +94,10 @@ export const IPV6_CONFIG_MODES = ['None', 'DHCP', 'Static', 'Autoconf']
 
 @mixin(mapToArray(mixins))
 export default class Xapi extends XapiBase {
-  constructor(...args) {
-    super(...args)
+  constructor({ guessVhdSizeOnImport, ...opts }) {
+    super(opts)
+
+    this._guessVhdSizeOnImport = guessVhdSizeOnImport
 
     // Patch getObject to resolve _xapiId property.
     this.getObject = (getObject => (...args) => {
@@ -2095,11 +2098,16 @@ export default class Xapi extends XapiBase {
   // -----------------------------------------------------------------
 
   async _importVdiContent(vdi, body, format = VDI_FORMAT_VHD) {
-    if (__DEV__ && body.length == null) {
-      throw new Error(
-        'Trying to import a VDI without a length field. Please report this error to Xen Orchestra.'
-      )
+    if (typeof body.pipe === 'function' && body.length === undefined) {
+      if (this._guessVhdSizeOnImport && format === VDI_FORMAT_VHD) {
+        body = await createVhdStreamWithLength(body)
+      } else if (__DEV__) {
+        throw new Error(
+          'Trying to import a VDI without a length field. Please report this error to Xen Orchestra.'
+        )
+      }
     }
+
     await Promise.all([
       body.task,
       body.checksumVerified,
diff --git a/packages/xo-server/src/xo-mixins/xen-servers.js b/packages/xo-server/src/xo-mixins/xen-servers.js
index 2b9e96d00..082383a5b 100644
--- a/packages/xo-server/src/xo-mixins/xen-servers.js
+++ b/packages/xo-server/src/xo-mixins/xen-servers.js
@@ -40,7 +40,7 @@ const log = createLogger('xo:xo-mixins:xen-servers')
 // - _xapis[server.id] id defined
 // - _serverIdsByPool[xapi.pool.$id] is server.id
 export default class {
-  constructor(xo, { xapiOptions }) {
+  constructor(xo, { guessVhdSizeOnImport, xapiOptions }) {
     this._objectConflicts = { __proto__: null } // TODO: clean when a server is disconnected.
     const serversDb = (this._servers = new Servers({
       connection: xo._redis,
@@ -49,7 +49,10 @@ export default class {
     }))
     this._serverIdsByPool = { __proto__: null }
     this._stats = new XapiStats()
-    this._xapiOptions = xapiOptions
+    this._xapiOptions = {
+      guessVhdSizeOnImport,
+      ...xapiOptions,
+    }
     this._xapis = { __proto__: null }
     this._xo = xo