Hi,
On 03/19/2015 10:41 PM, Nick Fisk wrote:
I'm looking at trialling OSD's with a small flashcache device over them to
hopefully reduce the impact of metadata updates when doing small block io.
Inspiration from here:-
http://comments.gmane.org/gmane.comp.file-systems.ceph.devel/12083
One thing I suspect will happen, is that when the OSD node starts up udev
could possibly mount the base OSD partition instead of flashcached device,
as the base disk will have the ceph partition uuid type. This could result
in quite nasty corruption.
I ran into this problem with an enhanceio based cache for one of our
database servers.
I think you can prevent this problem by using bcache, which is also
integrated into the official kernel tree. It does not act as a drop in
replacement, but creates a new device that is only available if the
cache is initialized correctly. A GPT partion table on the bcache device
should be enough to allow the standard udev rules to kick in.
I haven't used bcache in this scenario yet, and I cannot comment on its
speed and reliability compared to other solutions. But from the
operational point of view it is "safer" than enhanceio/flashcache.
Best regards,
Burkhard
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