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 majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
