On Fri, 18 Jan 2013, Travis Rhoden wrote:
> On Fri, Jan 18, 2013 at 5:43 PM, Gregory Farnum <greg@xxxxxxxxxxx> wrote:
> > 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!
>
> I wondered about that. But wasn't confident enough to ask about it.
> >
> >> 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.
>
> Is "max sync interval" a hard stop, though? I mean, once 5 seconds
> pass, it's going to flush/sync no matter what, right? So there is no
> point in making it much bigger than what can be written to the journal
> in those 5 seconds. I feel like I must be missing something, though,
> otherwise the recommendation wouldn't to make the journal 2x that
> size.
The sync itself can take time, and we *initiate* the sync at that time.
Hence the 2x. When the journal fills up there is a hefty performance
hit, too. When you adjust this down, check back at some ponit and make
sure you don't see JOURNAL FULL messages in your log that point to a
problem (with the code or the tuning logic).
Thanks!
sage
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