Le 03/05/2013 16:34, Travis Rhoden a écrit :
I have a question about "tell bench" command.
When I run this, is it behaving more or less like a dd on the drive?
It appears to be, but I wanted to confirm whether or not it is
bypassing all the normal Ceph stack that would be writing metadata,
calculating checksums, etc.
One bit of behavior I noticed a while back that I was not expecting is
that this command does write to the journal. It made sense when I
thought about it, but when I have an SSD journal in front of an OSD, I
can't get the "tell bench" command to really show me accurate numbers
of the raw speed of the OSD -- instead I get write speeds of the SSD.
Just a small caveat there.
The upside to that is when do you something like "tell \* bench", you
are able to see if that SSD becomes a bottleneck by hosting multiple
journals, so I'm not really complaining. But it does make a bit tough
to see if perhaps one OSD is performing much differently than others.
But really, I'm mainly curious if it skips any normal
metadata/checksum overhead that may be there otherwise.
Travis,
I'm no expert but, to me, the bench doesn't bypass the ceph stack.
On a test setup, I set up the journal on the same drive as the data
drive, when I "tell bench" I can see ~160MB/s throughoutput on the SSD
block device and the benchmark result is ~80MB/s which leads me to think
the data is written twice : once to the journal and once to the
"permanent" storage.
I see almost no read on the block device but the written data probably
is in the page cache.
Cheers,
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