On 2016-10-20 05:29, Timofey Titovets wrote:
Hi, i use btrfs for NFS VM replica storage and for NFS shared VM storage.
At now i have a small problem what VM image deletion took to long time
and NFS client show a timeout on deletion
(ESXi Storage migration as example).
Kernel: Linux nfs05 4.7.0-0.bpo.1-amd64 #1 SMP Debian 4.7.5-1~bpo8+2
(2016-10-01) x86_64 GNU/Linux
Mount options: noatime,compress-force=zlib,space_cache,commit=180
Feature enabled:
big_metadata:1
compress_lzo:1
extended_iref:1
mixed_backref:1
no_holes:1
skinny_metadata:1
AFAIK, unlink() return only when all references to all extents from
unlinked inode will be deleted
So with compression enabled files have a many many refs to each
compressed chunk.
So, it's possible to return unlink() early? or this a bad idea(and why)?
I may be completely off about this, but I could have sworn that unlink()
returns when enough info is on the disk that both:
1. The file isn't actually visible in the directory.
2. If the system crashes, the filesystem will know to finish the cleanup.
Out of curiosity, what are the mount options (and export options) for
the NFS share? I have a feeling that that's also contributing. In
particular, if you're on a reliable network, forcing UDP for mounting
can significantly help performance, and if your server is reliable, you
can set NFS to run asynchronously to make unlink() return almost
immediately.
Now, on top of that, you should probably look at adding 'lazytime' to
the mount options for BTRFS. This will cause updates to file
time-stamps (not just atime, but mtime also, it has no net effect on
ctime though, because a ctime update means something else in the inode
got updated) to be deferred up to 24 hours or until the next time the
inode would be written out, which can significantly improve performance
on BTRFS because of the write-amplification. It's not hugely likely to
improve performance for unlink(), but it should improve write
performance some, which may help in general.
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