Re: Metadata hit ratio

[Dave Chinner <david@fromorbit.com>]

On Thu, Jun 24, 2010 at 11:30:44PM +0300, Yannis Klonatos wrote:
> Hello all,
> 
>         Along with my other (yet pending :-( ) question/issue, I
> have another question (i hope this is the last :-) )
> now.
>         I would like to measure the hit ratio of the metadata
> accesses of XFS (inode+internal buffers). It is my
> understanding that XFS uses its own data structures, and does not
> rely on the buffercache mechanisms of
> the Linux kernel. However, even doing so, there may be cases that
> the metadata may not fit in the RAM,
> and I/O operations are required to fetch them from the underlying
> storage. I have found out that there are
> two places that XFS uses the submit_bio function. If i add some
> counters there, would it suffice to measure all
> the metadata misses?
>         Or is this information available in one (or more) of the
> xfs_stats counters? And if so, what do i need to sum
> up in order to get the total metadata hit and miss ratio?

Start by looking here:

http://xfs.org/index.php/Runtime_Stats

But you probably want some of the buf counters which are
undocumented on that page, so a rough description of them is
(from PCP):

$ pminfo -t xfs.buffer
xfs.buffer.get [number of request buffer calls]
xfs.buffer.create [number of buffers created]
xfs.buffer.get_locked [number of requests for a locked buffer which succeeded]
xfs.buffer.get_locked_waited [number of requests for a locked buffer which waited]
xfs.buffer.busy_locked [number of non-blocking requests for a locked buffer which failed]
xfs.buffer.miss_locked [number of requests for a locked buffer which failed due to no buffer]
xfs.buffer.page_retries [number of retry attempts when allocating a page for insertion in a buffer]
xfs.buffer.page_found [number of hits in the page cache when looking for a page]
xfs.buffer.get_read [number of buffer get calls requiring immediate device reads]

So you might be able to get something from those stats.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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