Messages in this thread Patch in this message | | | Date | Wed, 5 Dec 2012 21:17:07 +0000 | From | Alun <> | Subject | PATCH reduce impact of FIFREEZE on userland processes |
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This patch is against kernel version 3.7-rc7.
The FIFREEZE ioctl blocks userland writes, then calls sync_filesystem. If there is a large amount of dirty data, this sync can take a substantial time to complete, with corresponding loss of responsiveness to any userland processes wishing to write.
This patch simply adds an extra call to sync_filesystem prior to blocking writes, so that (hopefully) the majority of outstanding dirty data has been flushed before we impact on userland.
I'm a complete kernel newbie and have only done some pretty minimal testing on my own machine, but with the patch in place the impact of running "fsfreeze -f" immediately followed by "fsfreeze -u" on a moderately loaded filesystem (as measured by time taken for a write() to complete) was reduced from 2.5 to 0.2 seconds. Hopefully there's no subtlety in how all this works, and that adding the extra call has no scary implications...
Signed-off-by: Alun Jones <alun.linux@ty-penguin.org.uk>
--- linux-3.7-rc7/fs/super.c.orig 2012-11-29 17:35:37.000000000 +0000 +++ linux-3.7-rc7/fs/super.c 2012-12-05 20:56:38.730631855 +0000 @@ -1314,6 +1314,11 @@ int freeze_super(struct super_block *sb) return 0; } + /* Sync before we block writes to reduce the amount of + * work that has to be done afterwards. + */ + sync_filesystem(sb); + /* From now on, no new normal writers can start */ sb->s_writers.frozen = SB_FREEZE_WRITE; smp_wmb();
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