On Fri, Jan 18, 2013 at 2:20 PM, Travis Rhoden <trhoden@xxxxxxxxx> wrote:
> 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.
Ah. This should refer to the max sync interval, not the min!
> 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.
It can of course grow as large as you let it, and I would leave some
extra room as a margin. The main consideration is that the journal
doesn't like getting too far ahead of the filestore, and that's what
the above calculation uses to set size.
-Greg
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