On 2017-07-31 06:51, Sebastian Ochmann wrote:
Hello,
I have a quite simple and possibly stupid question. Since I'm
occasionally seeing warnings about failed loading of free space cache, I
wanted to clear and rebuild space cache. So I mounted the filesystem(s)
with -o clear_cache and subsequently with my regular options which
includes space_cache. Indeed, dmesg tells me:
[ 60.285190] BTRFS info (device dm-1): force clearing of disk cache
and then
[ 137.151845] BTRFS info (device dm-1): use ssd allocation scheme
[ 137.151850] BTRFS info (device dm-1): disk space caching is enabled
[ 137.151852] BTRFS info (device dm-1): has skinny extents
To my understanding, btrfs-freespace should then start working to
rebuild the free space cache. However, I can't remember that I have ever
seen btrfs work hard after clearing the space cache. The drives aren't
working much, and the btrfs-freespace processes (which are indeed there)
don't do anything either.
So simple question: Can anyone try to clear their space cache and
confirm that btrfs actually does something after doing so? Is there
anything I could do to confirm that something is happening?
Based on my (limited) understanding of that code, assuming you're using
the original free space cache (which I think is the case, since you said
you're using the regular 'space_cache' option instead of
'space_cache=v2'), there's not _much_ work that needs to be done unless
free space is heavily fragmented and the disk is reasonably full. The
original free space cache is pretty similar to an allocation bitmap, and
computing that is not hard to do (you just figure out which blocks are
actually used).
Based on my own experience, you'll see almost zero activity most of the
time when rebuilding the free space cache regardless of which you are
using (the original, or the new version), although the newer free space
tree code appears to do a bit more work.
The drives in question are a SSD and a HDD, both in the range of 1-2 TB
in size.
I'm on Arch Linux, kernel 4.12.3, btrfs-progs 4.11.1
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