Quoting Dave Chinner (2013-05-03 21:15:47)
> Hi folks,
>
> It's that time again - I ran fsmark on btrfs and found performance
> was awful.
>
> tl;dr: memory pressure causes random writeback of metadata ("bad"),
> fragmenting the underlying sparse storage. This causes a downward
> spiral as btrfs cycles through "good" IO patterns that get
> fragmented at the device level due to the "bad" IO patterns
> fragmenting the underlying sparse device.
>
Really interesting Dave, thanks for all this analysis.
We're going to have hard time matching xfs fragmentation just because
the files are zero size and we don't have the inode tables. But, I'll
take a look at the metadata memory pressure based writeback, sounds like
we need to push a bigger burst.
-chris
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