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.
> -Greg
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