Hopefully a Ceph developer will be able to clarify how small writes are
journaled?
The write-through 'bug' seems to explain small-block performance I've
measured in various configurations (I find similar results to you).
I've not still tested the patch cited, but it would be *very*
interesting to know the impact on clusters running with spinning-disk
journals in particular.
On 2013-12-06 16:05, nicolasc wrote:
Hi James,
Thank you for this clarification. I am quite aware of that, which is
why the journals are on SAS disks in RAID0 (SSDs out of scope).
I still have trouble believing that fast-but-not-super-fast journals
is the main reason for the poor performances observed. Maybe I am
mistaken?
Best regards,
Nicolas Canceill
Scalable Storage Systems
SURFsara (Amsterdam, NL)
On 12/03/2013 03:01 PM, James Pearce wrote:
I would really appreciate it if someone could:
- explain why the journal setup is way more important than
striping settings;
I'm not sure if it's what you're asking, but any write must be
physically written to the journal before the operation is
acknowledged. So the overall cluster performance (or rather write
latency) is always governed by the speed of those journals. Data is
then gathered up into (hopefully) larger blocks and committed to OSDs
later.
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