On Sunday 2012-12-09 11:41, Roman Mamedov wrote:

quoted

Absolutely. COW snapshots cause severe fragmentation (it's
in their nature).

CoW filesystem incurs fragmentation by its nature, not specifically snapshots.
Even without snapshots, rewriting portions of existing files will write the
new blocks not over the original ones, but elsewhere, thus increasing
fragmentation.

Right, of course.

In the "normal" case, the originals are deleted at some point, so the
length of the CoW "chain" should be bound. However, with snapshots,
there is, I wager to say, no limit to the chain length, so
defragmenting a file also becomes more work.

One can of course use rsync without --inplace, which does a full file
copy-up if the file has changed, which essentially disables CoW,
and thus reduces fragmentation. Unfortunately, not all copy
operations on one's machine do use rsync, so perhaps it would be nice
to have this as a subvolume mount option as well.