Forced Rescan
Hi all,
I have a strange issue where our Monday night incremental backups for our Vmware policies run very slowly, this has been going on for some time & support have so far been unable to help, all other nights run as expected.
I have however had some thoughts on what this could be & would like to discuss them with the community. It’s around the Vmware Forced Rescan option.
On our weekly full backups, taken on a Friday, we had this setting mistakenly on. So every Friday Night we were carrying out a forced rescan for the entirety of our estate. Funnily enough, Friday night backups are generally ok and usually complete sometime on Saturday.
I’d like to understand what effect a ‘forced rescan’ has on the next incremental backup (a Monday night for us) and if this potentially could be much slower than a ‘typical’ incremental due to the extra work involved around the data extents of the unchanged blocks (Blurb from the manual below) as there will obviously be a large number of ‘unchanged blocks’ after a forced rescan and thus extra to work to complete.
We’ve now changed the policies to not carry out a ‘forced rescan’ every Friday but we’ll need to wait a week to find out the result so in the mean time I’d like someone in the know to confirm or deny this :)
Finally, how often are people using a forced rescan for VMware policies? I see the docs refer to 6 months.
Matt.
Blurb
From NBU 7.6 Vmware Admin Guide:
The NetBackup Accelerator creates the backup stream and backup image for each virtual machine as follows:
■ If the virtual machine has no previous backup, NetBackup performs a full backup
and uses VMware Changed Block Tracking to track the data in use for each VMDK.
■ At the next backup, NetBackup identifies data that has changed since the
previous backup. Only changed blocks and the header information are included
in the backup, to create a full virtual disk backup.
■ The backup host sends to the media server a tar backup stream that consists
of the following: The virtual machine's changed blocks, and the previous backup
ID and data extents (block offset and size) of the unchanged blocks.
■ The media server reads the virtual machine's changed blocks, the backup ID,
and information about the data extents of the unchanged blocks. From the
backup ID and data extents, the media server locates the rest of the virtual
machine's data in existing backups.
■ The media server directs the storage server to create a new full image that
consists of the following: The newly changed blocks, and the existing unchanged
blocks that reside on the storage server. The storage server may not write the
existing blocks but rather link them to the image.
