On Tue, Jan 10, 2017 at 08:07:39AM +0100, Christian Theune wrote: > Hi, > > As XFS is gaining CoW support I’d like to understand the > implementation on a specific aspect: we’re using CoW for making disk > image backups as image files in btrfs. This has proven prohibitive > once the chain of CoW reflinks grows too long and everything becomes > too fragmented. btrfs has improved in some places but the issue still > persists. As in making snapshots of a disk image via something like "cp --reflink=always a.img a.img.20170110" ? > We’re currently considering to move away from CoW filesystems for our > use case and implement a higher level strategy. I now wonder whether > XFS will have the same issue or whether the architecture is different > in a significant way that will avoid prohibitive performance > regressions on long CoW chains (think: hundreds to a few thousand). The primary strategies XFS uses to combat fragmentation are a combination of reusing the delayed allocation mechanism to defer CoW block allocation as long as possible in the hopes of being able to make larger requests; and implementing the "CoW extent size hint" (default 32 blocks or 128K) which rounds the start and end of an allocation request to the nearest $cowextsize boundary. So for example if you write to 32 adjacent shared blocks in random order, they'll end up on disk with a single 128K extent, if possible. Note also that XFS only performs CoW if the block is shared, so if you write the same shared block in a file 20 times, the first write goes to a new block and the next 19 overwrite that new block. There will not be another CoW unless you reflink the file again. > I would appreciate a pointer where to look at - I’m a coder but > following kernel code to understand architecture hasn’t been > successful/efficient for me in the past … You might try reading the huge comment blocks in fs/xfs/xfs_reflink.c. --D > > Kind regards, > Christian > > -- > Christian Theune · ct@xxxxxxxxxxxxxxx · +49 345 219401 0 > Flying Circus Internet Operations GmbH · http://flyingcircus.io > Forsterstraße 29 · 06112 Halle (Saale) · Deutschland > HR Stendal HRB 21169 · Geschäftsführer: Christian. Theune, Christian. Zagrodnick > -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
