Metadata hit ratio
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 thexfs_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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Quoted 11 lines · Yannis Klonatos
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,
Quoted 8 lines
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 thesubmit_biofunction. 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 thexfs_statscounters? 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.
Signature · 7 lines
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