Likely it won't since the OSD is already coalescing journal writes.
FWIW, I ran through a bunch of tests using seekwatcher and blktrace at
4k, 128k, and 4m IO sizes on a 4 OSD cluster (3x replication) to get a
feel for what the IO patterns are like for the dm-cache developers. I
included both the raw blktrace data and seekwatcher graphs here:
http://nhm.ceph.com/firefly_blktrace/
there are some interesting patterns but they aren't too easy to spot (I
don't know why the Chris decided to use blue and green by default!)
Mark
On 09/22/2014 04:32 PM, Robert LeBlanc wrote:
We are still in the middle of testing things, but so far we have had
more improvement with SSD journals than the OSD cached with bcache (five
OSDs fronted by one SSD). We still have yet to test if adding a bcache
layer in addition to the SSD journals provides any additional improvements.
Robert LeBlanc
On Sun, Sep 14, 2014 at 6:13 PM, Mark Nelson <[email protected]
<mailto:[email protected]>> wrote:
On 09/14/2014 05:11 PM, Andrei Mikhailovsky wrote:
Hello guys,
Was wondering if anyone uses or done some testing with using
bcache or
enhanceio caching in front of ceph osds?
I've got a small cluster of 2 osd servers, 16 osds in total and
4 ssds
for journals. I've recently purchased four additional ssds to be
used
for ceph cache pool, but i've found performance of guest vms to be
slower with the cache pool for many benchmarks. The write
performance
has slightly improved, but the read performance has suffered a
lot (as
much as 60% in some tests).
Therefore, I am planning to scrap the cache pool (at least until it
matures) and use either bcache or enhanceio instead.
We're actually looking at dm-cache a bit right now. (and talking
some of the developers about the challenges they are facing to help
improve our own cache tiering) No meaningful benchmarks of dm-cache
yet though. Bcache, enhanceio, and flashcache all look interesting
too. Regarding the cache pool: we've got a couple of ideas that
should help improve performance, especially for reads. There are
definitely advantages to keeping cache local to the node though. I
think some form of local node caching could be pretty useful going
forward.
Thanks
Andrei
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