On Wed, May 4, 2011 at 8:23 PM, Edward Ned Harvey <opensolarisisdeadlongliveopensola...@nedharvey.com> wrote: > Generally speaking, dedup doesn't work on VM images. (Same is true for ZFS > or netapp or anything else.) Because the VM images are all going to have > their own filesystems internally with whatever blocksize is relevant to the > guest OS. If the virtual blocks in the VM don't align with the ZFS (or > whatever FS) host blocks... Then even when you write duplicated data inside > the guest, the host won't see it as a duplicated block.
A zvol with 4k blocks should give you decent results with Windows guests. Recent versions use 4k alignment by default and 4k blocks, so there should be lots of duplicates for a base OS image. > There are some situations where dedup may help on VM images... For example > if you're not using sparse files and you have a zero-filed disk... But in compression=zle works even better for these cases, since it doesn't require DDT resources. > Or if you're intimately familiar with both the guest & host filesystems, and > you choose blocksizes carefully to make them align. But that seems > complicated and likely to fail. Using a 4k block size is a safe bet, since most OSs use a block size that is a multiple of 4k. It's the same reason that the new "Advanced Format" drives use 4k sectors. Windows uses 4k alignment and 4k (or larger) clusters. ext3/ext4 uses 1k, 2k, or 4k blocks. Drives over 512MB should use 4k by default. The block alignment is determined by the partitioning, so some care needs to be taken there. zfs uses 'ashift' size blocks. I'm not sure what ashift works out to be when using a zvol though, so it could be as small as 512b but may be set to the same as the blocksize property. ufs is 4k or 8k on x86 and 8k on sun4u. As with ext4, block alignment is determined by partitioning and slices. -B -- Brandon High : bh...@freaks.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
