Hey folks,
The Ceph docs give the following recommendation on sizing your journal:
osd journal size = {2 * (expected throughput * filestore min sync interval)}
The default value of min sync interval is .01. If you use throughput
of a mediocre 7200RPM drive of 100MB/sec, this comes to 2 MB. That
seems like the lower bound to have the journal do anything at all.
My question is what is the upper bound? There's clearly a limit to
how big make, such that it just becomes wasted space. The reason I
want to know is that since I will be journals on SSDs, with each
journal being a dedicated partition, there is a benefit to not making
the partition bigger than it needs to be. All that unpartitioned
space can be used by the SSD firmware for wear-leveling and other
things (so long as it remains unpartitioned).
Would the following calc be appopriate?
Assume an SSD write speed of 400MB/sec. Default max sync interval is 5.
2 * (400 MB/sec * 5sec) = 4 GB.
So is it appropriate to assume that if I can't write to an SSD faster
than 400 MB/sec, and I keep the default sync interval values, a
journal greater than 4GB is just a waste?
I had been using 10GB journals... seems like overkill.
Or put another way, if I want to use 10GB journals, I should bump the
max sync interval to 12.5.
- Travis
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