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March 28th, 2011 12:00
need help understanding symstat results
Hi, I need help understanding the results of the symstat command. I am a bit confused about the relation between the cache and the actual load on the backend of the DMX.
Here is an exemple:
303 824 1744 98 98 27 34
Was the physical array actually writing at 303 IOPS? If so, what's the link with the 98% cache hits?
Thanks in advance
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Quincy561
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March 28th, 2011 13:00
The read hits are those reads that were in cache when the front end asked for them. It doesn't mean there wasn't a backend IO needed to satisfy them however, as in the case of data that was prefetched before the host asked for it.
Write hits are writes to the Symmetrix while cache is not at any WP limit, or there is no other delay in writing to cache such as slots that are locked or any replication that must take place before the write is accepted.
The first column is the IO rate for reads and writes for each device listed, so if it says 303 for the writes, it means it is doing 303 writes per second to that device, and if the write hit % is 98% then 98% of the time the host doesn't have to wait for anything to write into Symm cache.