Another CBT bug found in VMware ESXi 6.0


VMware just published KB 2136854 regarding a new bug found in ESXi 6.0 that causes virtual machine backups, which use Changed Block Tracking (CBT), to be inconsistent. VMware says the cause of the issue is this:

This issue occurs due to an issue with CBT in the disklib area, this causes the change tracking information of I/Os that occur during snapshot consolidation to be lost. The main backup payload data is never lost and it is always written to the backend device. However, the corresponding change tracking information entries which occur during the consolidation task are missed. Subsequent QueryDiskChangedAreas() calls do not include these missed blocks, hence a backup based on this CBT data is inconsistent.

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How to install CrashPlan on Synology Diskstation

CrashPlan Backup

A little while ago I posted several ways to improve the security on your Synology NAS and was recently asked about how to install CrashPlan on a Synology NAS. Synology doesn’t have it’s own pre-built package available, but luckily PC Load Letter from the Synology community has stepped up and made a Synology package that works beautifully – the only downside is his instructions are a bit dated and I found harder than they needed to be.

If you don’t already have a CrashPlan account, go ahead and sign up now.

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Restore a VM using the {vm}-flat.vmdk file

  • VMware

missing vmdk file error

A couple days ago I was asked the question “An admin deleted the vmdk file associated with a VM, there are no snapshots or backups of the VM . Can we restore this VM somehow?“. I believe in backups, I like to have my VM’s backed up so that I can depend on them for events just like this very issue. At first I was stumped, until the {vm-name}.flat.vmdk was mentioned. That’s when I remembered a method to recover a VM using the vm-flat.vmdk file that I had actually done 2-3 years ago.

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Increase incremental backup speed by enabling CBT

  • VMware

In a recent VMware environment I was working in, we noticed that our incremental backups was taking a long time (like up to 7-8 hours) to complete. After digging around it was found that VMware actually has a feature built-in since version 4 called Change Block Tracking (CBT). CBT will track any blocks that were changed since the last backup and tag them and stores the information in a -CTK file. The obvious benefit is that now the third party software only backs up the changed block and not the entire VM each time, reducing the amount of data being backed thus speeding up backups and even lowers the CPU utilization on the VM host.

CBT is disabled by default though there are some backup tools that will enable it automatically or you can enable it manually which can easily be done following these steps:

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