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Metadata hit ratio

To: xfs@xxxxxxxxxxx
Subject: Metadata hit ratio
From: Yannis Klonatos <klonatos@xxxxxxxxxxxx>
Date: Thu, 24 Jun 2010 23:30:44 +0300
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Hello all,

Along with my other (yet pending :-( ) question/issue, I have another question (i hope this is the last :-) )
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?

Thanks (again!) in advance,
Yannis Klonatos

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