Quoting "Austin S. Hemmelgarn" <ahferro...@gmail.com>:
Also, I don't quite understand what the poster means by "the
snapshot duplication of defrag only affects the fragmented
portions". Possibly it means approximately: if a file wasn't
modified in the current (latest) subvolume, it doesn't need to be
unshared. But, that would still unshare all the log files, for
example, even all files that have been appended, etc... that's
quite bad. Even if just one byte was appended to a log file, then
defrag will unshare the entire file (I suppose).
What it means is that defrag will only ever touch a file if that
file has extents that require defragmentation, and will then only
touch extents that are smaller than the target extent size (32M by
default, configurable at run-time with the `-t` option for the
defrag command) and possibly those directly adjacent to such extents
(because it might merge the small extents into larger neighbors,
which will in turn rewrite the larger extent too).
Umm... it seems to me that it's quite a poor defrag you got there.
* There's almost no net benefit to not defragmenting when dealing
with very large files that mostly see internal rewrites (VM disk
images, large databases, etc) because every internal rewrite will
implicitly unshare extents anyway.
Ok, so if you have a database, and then you snapshot its subvolume,
you might be in trouble because of all the in-place writes that
databases do, right?
It would almost be better if you could, manually, order the database
file to be unshared and defragmented. So, that would be the use-case
for defrag-unsharing. Interesting. Ok, I would agree with that. So,
there needs to be the operation called defrag-unshare, but that has
nothing to do with the real defrag.
I mean, this defrag-unsharing is just a glorified copy operation, but
there are a few twists, because it must be consistent, as opposed to
online copy, which would fail the consistency criteria.
But, you and other developers here seem to be confusing this
defrag-unshare with the real defrag. I bet you haven't even considered
what it means to "defrag without usharing" in terms of: what the final
result of such defrag should be, when it is done perfectly.
