What is the size of a vSphere Changed Block Tracking block (CBT)

  • 23 October 2020
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An interesting question arose some time ago. A customer changed permissions of files on a Windows file-server VM. Changes were replicated using DFS. So far so good, but the amount of data that was processed at next incremental backup of target VM was even more than a full backup. Because incremental backup is based on VMware Changed Block Tracking (CBT) feature, it was suddenly interesting to know whats the size of a CBT block. After some recherche I did not find an answer to this question. But I did find a great script that calculates CBT incremental backup size. Based on this script I created the following script to list the amount and the size of blocks changed in between of two snapshots.

 

Outcome in short

When using the script – described in detail later – to show CBT blocks I was surprised. I thought there will be a number of fix-sized blocks that are marked to be backed up. But – assumed, script works correct – there isn’t a fixed size of CBT blocks. There seems to be a wide variance of block sizes. I observed sizes from 64KB to many MBs. Probably because of that and because of the cluster-size of the file-server, a lot more data is processed during incremental backup compared to actual changed data.

How CBT works in (very) short

When enabled, CBT tracks changes in VMDKs and saves maps of changes to *-ctk.vmdk file for each *.vmdk. When backup-software (like Veeam B&R) queries CBT it can ask for changes between the current snapshot – taken just now – and the snapshot that was taken at last backup. CBT returns a number of blocks that contain all changes during this period.

About the script

You can do you own testing using the following script. Basically the script works for one VM. It creates a snapshot, to set the start-point of changes. In reality this would be the snapshot created during last incremental backup. After removing this snapshot, script pauses and you can do your changes within the VM. To continue the script, press Enter. The script creates a new snapshot to set the end-point of change-interval. Now CBT is queried for changes that backup-software would process during a incremental backup. Furthermore the amount of blocks are shown: number of unique block-size.

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# select your test-VM here

$VM = Get-VM test-VM

 

Write-Host "Starting to create Snapshot of VM " $vm.Name

$Snapshot = New-Snapshot -VM $VM -Quiesce -Name "Snapshot for CBT-script"

$SnapView = Get-View $Snapshot

$AllSnapDisks = $SnapView.Config.Hardware.Device | where {$_ -is [VMware.Vim.VirtualDisk]}

$AllDisksStart = @()

foreach ($SnapDisk in $AllSnapDisks) {

    $entry = [PSCustomObject] @{

        Key = $SnapDisk.Key

        ChangeID = $SnapDisk.Backing.ChangeId

        Info = $SnapDisk.DeviceInfo.Label

        Capacity = $SnapDisk.CapacityInBytes

    }

    $AllDisksStart += $entry

}

Write-Host "Remove Snapshot of VM " $vm.Name

Remove-Snapshot $Snapshot -Confirm:$false

 

Read-Host "`n" "Press enter to continue"

 

$Snapshot = New-Snapshot -VM $VM -Quiesce -Name "Snapshot for CBT-script"

$SnapView = Get-View $Snapshot

$VmView = Get-View $VM

foreach ($DiskStart in $AllDisksStart) {

    try {

        $Offset = 0

        [double]$GBChanged = 0

        $changesDB = $null

        Do {

            $changes = $VmView.QueryChangedDiskAreas($SnapView.MoRef,$DiskStart.key,$Offset,$DiskStart.changeid)

            $GBchanged += ($changes.ChangedArea | foreach {$_.length} | Measure-Object -sum).sum/1024/1024

            $LastChange = $changes.changedarea | Sort Start | select -last 1

            $Offset = $LastChange.start + $LastChange.Length

            $changesDB += $changes.ChangedArea

        }

        While ($DiskStart.CapacityInBytes -gt $Offset -and $Changes.ChangeArea.Count -gt 0)

    }

    catch {$changesDB = "Something went wrong; is CBT enabled?"}

 

    Write-Host "`n" "Disk: " $DiskStart.Info -ForegroundColor Black -BackgroundColor White

    Write-Host "Changed (MB): " $GBchanged

    Write-Host "Blocks (Bytes): "

    $($changesDB | Group-Object Length  | select Count, @{N='BlockSize'; E={$_.Name}} | ft -AutoSize)

}

Write-Host "`n" "Remove Snapshot of VM " $vm.Name

Remove-Snapshot $Snapshot -Confirm:$false

Outcome in long

With the tool in hands I did a few tests. I used a VM (Windows 2016) with two drives. One just for testing. This partition is formatted in NTFS using a 4KB cluster-size.

No changes

First test is to run script without changing anything between snapshots. Anyway, as you can see there are still changed blocks returned by CBT. There are 64KB, 128KB and 254KB Blocks. Hard disk 1 is the c-partition, Hard disk 2 is an empty test-partition.

CBT1

A few files

For next test I copied a ~64MB file to the partition in between snapshots.

CBT2

Result of querying CBT is shown in next screenshot. This fits very well: 65 MB to backup.

CBT3

Deleting fine between snapshots:

CBT4

Not much to do for backup.

A lot of small files

Lets continue by testing small files. For this I created 1024 files of 1KB size using the following commands.

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$text = ""

1..1024 | % {$text += "a"}

1..1024 | % { New-Item -Path e:\files -Name "$_.txt" -Value $text -ItemType file}

Because of the cluster-size of 4KB, one file takes 4KB on disk. From Windows NTFS perspective one file (left) and the whole directory (right) looks like this.

CBT5

For CBT, new files causes 8MB of backup. Also the variety of block-sizes is notable. This is no matter of fragmentation, because I re-tried and formatted volume before. Nearly the same result.

CBT6

What happens, when changing file properties without changing the content of the file? Therefore I “touched” files using these commands.

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foreach ($file in (Get-ChildItem -Path C:\Temp\files)) {$file.lastwritetime=$(Get-Date)}

CBT talks about 3MB of changes – just changing meta-data.

CBT7

Deleting all files is nearly the same.

CBT16

A lot of bigger files

Now testing files in 5KB size – bigger than cluster-size.

CBT8

Quit big CBT blocks.

CBT9

Changing meta-data of all files by touching them. Still about 3MB.

.CBT10.png

A lot of random-sized files

Now about 450 files were created with a average size of 50KB.

CBT11

For CBT about 26MB are marked as changed.

CBT12

“Touching” all files results in 1.8MB changed blocks. So this seems to be related to file-count, not size.

CBT13

Now changing each file and append a few letters to each content. The result is in increment even larger than after creating all files.

CBT14.png

Deleting all files results in about 2MB meta-data changes.

CBT15

IMHO

CBT is here for a while. Despite of a few ugly bugs (silent corruption of backups) it works fine. Because it is widely used to enable incremental backups it is important or at least interesting to know how it works and about its weaknesses. So in certain situations much more data is marked for backup than really changed. But meta-data updates seems not to be responsible for this, as you can see in this post. Creating files and file-content changes lets backup grow rapidly.

In my opinion the reasons for this are

  • A lot of small files, because for small files the overhead of the filesystem cluster-size is huge.
  • The smallest observed size of a CBT block was 64KB.

From my perspective CBT is a useful feature with some limitations. Because of the mixed nature of virtual workload/data, over all CBT does a good job!


6 comments

Userlevel 7
Badge +3

This is quite interesting. Did you test anything similar out on Hyper-V to see if similar data could be extracted?

A question, “the amount of data that was processed at next incremental backup of target VM was even more than a full backup.”. Just wondering how much data was processed vs provisioned size of the disk?

Userlevel 7
Badge +6

This is quite interesting. Did you test anything similar out on Hyper-V to see if similar data could be extracted?

A question, “the amount of data that was processed at next incremental backup of target VM was even more than a full backup.”. Just wondering how much data was processed vs provisioned size of the disk?

Sorry, no info about Hyper-V - I am more a VMware than a Microsoft guy :wink:

This script is interesting for sure, but I’ve been testing it (before I even saw this post) and I find that it comes up a bit short compared to Veeam. I’ve seen the original script that I believe this is based on, an I’m not convinced the math in the DoWhile is quite right.

Maybe you've done some comparisons with lab tests of an incremental run right after? Seems it’s always off a pretty decent factor. Maybe it’s related to snapshot consolidation after the script runs, but it’s kind of hard to rely on given how far out it really is from a Veeam Incremental run, so I am more inclined it’s an maths issue.

 

Did you test anything similar out on Hyper-V to see if similar data could be extracted?

 

HV you need to go differently to attack it.

 

 $wmiclass = Get-WmiObject -Class "Msvm_ImageManagementService" -Namespace "root\virtualization\v2"

#GetVirtualDiskChanges() is a method for ImageManagementService, so set this first for ease of access


GetVirtualDiskChanges accepts a similar set of parameters to QueryDiskChanges(). Set $Vm and $VMname before hand, VM should just be Get-VM and $vmname should be a string of the VM name. You'd fetch $rct from either a backup or just from a checkpoint similar to how you'd get it with VMware.

uint32 GetVirtualDiskChanges(
[in] string Path, #$vhd = Get-VHD -VMid $vm.id
[in] string LimitId, #$rct.RctID.RCTIdentifier
[in] string TargetSnapshotId, #$chkpoint = Get-VMSnapshot -Vmname $vmname
[in] uint64 ByteOffset, #Set to 0
[in] uint64 ByteLength, # Disk length should be fine
[out] uint64 ProcessedByteLength,
[out] uint64 ChangedByteOffsets[],
[out] uint64 ChangedByteLengths[],
[out] CIM_ConcreteJob REF Job
);

Once everything is set, you can use:

$namespace.GetVirtualDiskChanges($vhd.path,$rct.RctId.RctIdentifier,$vhd[0],"0",$vhd.FileSize)

The end result will return a similar output to QueryDiskChanges.

VBR OIBs for the disks will carry the ChangeID/RCTId respectively for each backup and you can pull them from the backup with (get-VBRBackup -name ‘somebackup’).GetLastOibs()

Userlevel 7
Badge +6

Hi @ddomask ! Thanks for your input! The original script, this is based on, you can find here (link is also in first paragraph of this post):

https://www.experts-exchange.com/articles/27059/A-PowerShell-script-to-measure-VM-data-change-rates-using-Changed-Block-Tracking-CBT.html

As I adapted it, I checked the VMware documentation how CBT queries should be implemented. And the script seems to be correct! Does not mean there could be a mistake or rounding-differences. I tested my script in a lab, not in real world.

Hi @ddomask ! Thanks for your input! The original script, this is based on, you can find here (link is also in first paragraph of this post):

https://www.experts-exchange.com/articles/27059/A-PowerShell-script-to-measure-VM-data-change-rates-using-Changed-Block-Tracking-CBT.html

As I adapted it, I checked the VMware documentation how CBT queries should be implemented. And the script seems to be correct! Does not mean there could be a mistake or rounding-differences. I tested my script in a lab, not in real world.


Aha, I appreciate that! Just as I do comparisons from the same ChangeID between what a Veeam job pulls and the script pulls, the script usually does come up short. It’s “closeenough.png” that it’s convincing (I wrote several modules for that can be quickly imported and used as a test in the field), but the discrepancy of sometimes 4+ GB is too much for my taste and I’ve not yet gotten to the bottom of where the difference is.

 

It’s a “when it’s quiet” project so I’ll update if I get more information, and I don’t disagree that the approach is correct, I just think the math is off somewhere.

Userlevel 7
Badge +6

Hi @ddomask ! Thanks for your input! The original script, this is based on, you can find here (link is also in first paragraph of this post):

https://www.experts-exchange.com/articles/27059/A-PowerShell-script-to-measure-VM-data-change-rates-using-Changed-Block-Tracking-CBT.html

As I adapted it, I checked the VMware documentation how CBT queries should be implemented. And the script seems to be correct! Does not mean there could be a mistake or rounding-differences. I tested my script in a lab, not in real world.


Aha, I appreciate that! Just as I do comparisons from the same ChangeID between what a Veeam job pulls and the script pulls, the script usually does come up short. It’s “closeenough.png” that it’s convincing (I wrote several modules for that can be quickly imported and used as a test in the field), but the discrepancy of sometimes 4+ GB is too much for my taste and I’ve not yet gotten to the bottom of where the difference is.

 

It’s a “when it’s quiet” project so I’ll update if I get more information, and I don’t disagree that the approach is correct, I just think the math is off somewhere.

Thanks, would be very interesting to know where the huge differences comes from!

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