thanks, sounds very promising On Thu, Jul 15, 2010 at 12:13 AM, Sean Bartell <wingedtachikoma@xxxxxxxxx> wrote: > On Wed, Jul 14, 2010 at 11:27:39PM +0200, Mathijs Kwik wrote: >> Hi all, >> >> I'm used to snapshots with LVM and I would like to compare them to btrfs. >> >> The case I want to compare is the following: >> At the moment a snapshot is created, no extra space is needed (maybe >> some metadata overhead) and all data is shared between the original >> and the snapshot. >> In LVM, snapshots work at the block-level, so any changes done to the >> original volume trigger a COW to the snapshot. >> If LVM is configured to use 4Mb blocks (default), this means that >> overwriting a 100k file, will lead to 4Mb "snapshot data" to be backed >> up. >> A 800Mb file will take around 800Mb. >> So, for small files (that are not on the same extent/block) this can >> waste quite some space, while for bigger files, or lots of files >> "close" to each other, it doesn't matter much. >> >> How is this for btrfs snapshots? >> Do they work at the file-level? or also at blocks/extents? >> >> I mean, does changing a 100k file lead to 100k being snapshotted? > > Btrfs CoWs file extents, and files can use only the parts of an extent > they need, so a 1-byte change would only require one additional 4K data > block. Of course, metadata also needs to be updated, and will require > a number of additional blocks. > >> What would happen if I have a 20G file (for example a disk image for kvm)? >> Would minor changes in that file lead to the entire 20G to be COWed/"backed up"? > > No, only the relevant portion. > >> Is there a distinction between data and metadata? >> Or does touching (ctime/mtime) or visiting (atime) a file cause it to be COWed? > > Metadata is CoWed separately, so there will still only be one copy of > the data. > >> Thanks for any info on this. >> Mathijs > -- 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
