Comments on: Reclaiming unused VMDK space with storage thin provisioning

By: smgj

smgj — Mon, 09 Nov 2009 18:15:28 +0000

And further:
VCB skips zero-blocks, but then I run into what you write about – deleted, but not reclaimed blocks.

So I guess good housekeeping will be necessary anyway.

By: smgj

smgj — Mon, 09 Nov 2009 17:42:24 +0000

Itâ€™s time for a RL example, I think. (This is Esx 3,5 – to be upgraded soon)

I have a guest, Hobb-Term-01 that has:
c: 30,0GB, used: 20,4GB, free: 9,53GB – reported by guest OS

Hobb-Term-01-flat.vmdk: 32 227 695 616bytes (30GB) shown on VMware cluster host

“du -b Hobb-Term-01/” yields 34 381 496 320bytes (32GB), but includes 2GB swap ( excluded from backup) and some log files (included in backup)

Net disk usage on host (including log files and such) is 30,2GB which is fully backed by VCB + Backup Exec.

Backup exec log states: “Processed 21 302 371 215 bytes (= 19.83938GB) in 18 minutes and 14 seconds.”

So it seems that compression of the empty space in the vmdk-file is done at this point in the processing – while the snapshot are moved from the host to the backup server by the VCB scripts. Just as you wrote, except that it's not Backup Exec that makes up for not using thin disk but the VCB prescript. (That is “disk usage” on backup server, not disk usage on hosts of course.)

Backup Exec also states: “Software compression ratio: 1,6:1″ This is BEs internal compression.
Backup exec's final disk usage on BE-server for this job is nearly 12GB – which is not bad at all.

So I guess I've finally answered my own question. Thanks for helping me reach it.

By: Gabrie van Zanten

Gabrie van Zanten — Mon, 09 Nov 2009 16:10:33 +0000

Well if Backup Exec makes a backup of the whole VMDK, yes the backup will be smaller. It could be that a VM with 50GB assigned VMDK, will in reality only have a 10GB VMDK and therefore Backup Exec has less data (even though it is empty) to backup. I just don't know if Backup Exec can look inside the VMDK and skip all the empty blocks or if it can compress a thick disk of 50GB with just 10GB of data to a compressed backup of 10GB.

In other words, can compression of Backup Exec make up for not using a thin disk? Not sure on that.

By: smgj

smgj — Mon, 09 Nov 2009 15:42:32 +0000

Ah. We're using VCB + a stand alone server with Backup Exec (12,5) (+ LUN mounting). And in the end Backup Exec's sw-compression on the disk based backup.

I thought the new snapshots also could be smaller after shrinking the vmdks – that would shrink the backup as well.

By: Gabrie van Zanten

Gabrie van Zanten — Mon, 09 Nov 2009 15:22:24 +0000

Actually this depends on your backup solution. VCB only makes the snapshot and frees the disk for making a backup.

By: smgj

smgj — Mon, 09 Nov 2009 15:19:14 +0000

Thanks.
Just to be sure:
The backup will have the same space requirements as earlier then?

By: Gabrie van Zanten

Gabrie van Zanten — Mon, 09 Nov 2009 14:47:15 +0000

No, it will not influence your VCB Backup.

By: smgj

smgj — Mon, 09 Nov 2009 14:40:59 +0000

Hi. I read the article with great interest and have a question: How does this work with snapshotbased VCB-backup? Does it have any impact at all?

By: smgj

smgj — Mon, 09 Nov 2009 11:15:28 +0000

And further:
VCB skips zero-blocks, but then I run into what you write about – deleted, but not reclaimed blocks.

So I guess good housekeeping will be necessary anyway.

By: smgj

smgj — Mon, 09 Nov 2009 10:42:24 +0000