On Wed, 2009-04-22 at 21:20 +0200, Tomasz Chmielewski wrote: > Right now, the majority of Linux users probably have LVM on their SAN > devices (i.e those being iSCSI targets). > > Using LVM on a SAN device is easy: just create a new logical volume or > its snapshot, make it a target to iSCSI initiators, done. > This is definitely one of the target use cases for btrfs. > I was wondering how btrfs would fit here and if it could replace LVM. > > > I see the following benefits of using btrfs instead of LVM: > > - you can create sparse files which would grow as iSCSI initators use > more space (you can do it with ext3 now as well) > > - you can use btrfs compression, to further reduce used space and > perhaps increase speed (SANs are mostly IO bound, not CPU bound) > > - LVM has a big performance hit when using snapshots; btrfs doesn't > > > > However, with btrfs, I'm not sure about: > > - what happens if SAN machine crashes while the iSCSI file images were > being written to; with LVM and its block devices, I'm somehow more > confident it wouldn't make more data loss than necessary If iscsi is writing with O_DIRECT|O_SYNC it should work. But, tuning for this config is something we have to concentrate more on. > > - taking snapshots of individual files (file images on SAN) is not > possible with btrfs? Probably they would have to be placed in separate > directories first to make snapshots - some minor manageability issue Btrfs can't snapshot a single dir, but it can snapshot a single file. See the bcp command included with btrfs-progs. I'd also suggest using preallocated files (via fallocate) instead of sparse files. I will perform better in general. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
