On Fri, May 10, 2013 Liu Bo wrote:
On Thu, May 09, 2013 at 03:41:49PM -0500, Kyle Gates wrote:
I'll preface that I'm running Ubuntu 13.04 with the standard 3.8
series kernel so please disregard if this has been fixed in higher
versions. This is on a btrfs RAID1 with 3 then 4 disks.
My use case is to set the nocow 'C' flag on a directory and copy in
some files, then make lots of writes (same file sizes) and note that
the number of extents stays the same, good.
Then run a balance (I added a disk) and start making writes again,
now the number of extents starts climbing, boo.
Is this standard behavior? I realize a balance will cow the files.
Are they also being checksummed thereby breaking the nocow flag?
I have made no snapshots and made no writes to said files while the
balance was running.
Hi Kyle,
It's hard to say if it's standard, it is a side effect casued by balance.
During balance, our reloc root works like a snapshot, so we set
last_snapshot on the fs root, and this makes new nocow writes think that
we have to do cow as the extent is created before taking snapshot.
But the nocow 'C' flag on the file is still there, if you make new
writes on the new extent after balance, you still get the same number of
extents.
thanks,
liubo
Thank you for the explanation.
On my machine this didn't happen however. IIRC one 10GiB file had 24 extents
before balance, 26 extents after balance, and 1000+ and growing when I
checked the following day.
I'll add that I am running a relatively recent version of btrfs-tools from a
ppa.
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