Re: poor OSD performance using kernel 3.4

[Stefan Priebe <s.priebe@profihost.ag>]

Am 27.05.2012 13:33, schrieb Alexandre DERUMIER:
>> how much time to flush from journal to disks ?
>>> I don't know how to measure this.
> Do an iostat, you must see timelapse of write inactivity on disk (datas are written to journal) , then after a timelapse
 > of write activity on disk.(data flushed from journal to disk)
No it always starts in parallel. Journal is set to 1GB. I've now moved 
the journal to disk - so i can use iostat.

>>> As ceph starts to write to journal and
>>> disk in parallel
>
> this is strange, from doc:
> http://ceph.com/wiki/OSD_journal
>
> the journal mode should be write-ahead with xfs.
> So write to journal first then flush to disk each 30sec.
I'm not quite sure as:
http://ceph.com/wiki/Ceph.conf#filestore_journal_writeahead

says there are two options:
filestore journal writeahead
and
filestore journal parallel
but even
filestore journal writeahead = 1
filestore journal parallel = 0

results in a parallel start.

> maybe your tmpfs is too small, and flushs occurs at 50% of free space on journal.
> If by exemple, your flush occurs each 1 or 2seconds, this can cause very slow write.
1GB? My 1Gbit/s LAN test connection can't handle more than about 
120MB/s. So there's at least room for 8-10s.

;-(

Stefan