On 03/04/2015 05:34 AM, John Spray wrote:
On 04/03/2015 08:26, Nick Fisk wrote:
To illustrate the difference a proper write back cache can make, I put
a 1GB (512mb dirty threshold) flashcache in front of my RBD and
tweaked the flush parameters to flush dirty blocks at a large queue
depth. The same fio test (128k iodepth=1) now runs at 120MB/s and is
limited by the performance of SSD used by flashcache, as everything is
stored as 4k blocks on the ssd. In fact since everything is stored as
4k blocks, pretty much all IO sizes are accelerated to max speed of
the SSD. Looking at iostat I can see all the IO’s are getting
coalesced into nice large 512kb IO’s at a high queue depth, which Ceph
easily swallows.
If librbd could support writing its cache out to SSD it would
hopefully achieve the same level of performance and having it
integrated would be really neat.
What are you hoping to gain from building something into ceph instead of
using flashcache/bcache/dm-cache on top of it? It seems like since you
would anyway need to handle your HA configuration, setting up the actual
cache device would be the simple part.
Agreed regarding flashcache/bcache/dm-cache. I suspect improving an
existing project rather than reinventing it ourselves would be the way
to go. It may also be worth looking at Luis's work, though I note that
he specifically says write-through:
http://vault2015.sched.org/event/6cc56a5b8a95ead46961697028b59c39#.VPc0uX-etWQ
https://github.com/pblcache/pblcache
Cheers,
John
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