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[Robert van Leeuwen <Robert.vanLeeuwen@xxxxxxxxxxxxx>]

> I'm facing the issue about the performance degradation, and once I glanced that changing the value in /proc/sys
> /vm/vfs_cache_pressure will do a favour.

> Can anyone explain to me whether and why it is useful?

Hi,

When this is set to a lower value the kernel will try to keep the inode/dentry cache longer in memory.
Since the swift replicator is scanning the filesystem continuously it will eat up a lot of iops if those are not in memory.

To see if a lot of cache misses are happening, for xfs, you can look at xs_dir_lookup and xs_ig_missed.
( look at http://xfs.org/index.php/Runtime_Stats )

We greatly benefited from setting this to a low value but we have quite a lot of files on a node ( 30 million)
Note that setting this to zero will result in the OOM killer killing the machine sooner or later. 
(especially if files are moved around due to a cluster change ;)

Cheers,
Robert van Leeuwen