Journal on SSD should effectively double your throughput because data will not be written to the same device twice to ensure transactional integrity. Additionally, by placing the OSD journal on an SSD you should see less latency, the disk head no longer has to seek back and forth between the journal and data partitions. For large writes it's not as critical to have a device that supports high IOPs or throughput because large writes are striped across many 4MB rados objects, relatively evenly distributed across the cluster. Small write operations will benefit the most from an OSD data partition with a writeback cache like btier/flashcache because it can absorbs an order of magnitude more IOPs and allow a slower spinning device catch up when there is less activity.
On Tue, Oct 8, 2013 at 12:09 AM, Robert van Leeuwen <Robert.vanLeeuwen@xxxxxxxxxxxxx> wrote:
> I tried putting Flashcache on my spindle OSDs using an Intel SSL and it works great.Small note that on Red Hat based distro's + Flashcache + XFS:
> This is getting me read and write SSD caching instead of just write performance on the journal.
> It should also allow me to protect the OSD journal on the same drive as the OSD data and still get benefits of SSD caching for writes.
There is a major issue (kernel panics) running xfs + flashcache on a 6.4 kernel. (anything higher then 2.6.32-279)
It should be fixed in kernel 2.6.32-387.el6 which, I assume, will be 6.5 which only just entered Beta.
Fore more info, take a look here:
https://github.com/facebook/flashcache/issues/113
Since I've hit this issue (thankfully in our dev environment) we are slightly less enthusiastic about running flashcache :(
It also adds a layer of complexity so I would rather just run the journals on SSD, at least on Redhat.
I'm not sure about the performance difference of just journals v.s. Flashcache but I'd be happy to read any such comparison :)
Also, if you want to make use of the SSD trim func
P.S. My experience with Flashcache is on Openstack Swift & Nova not Ceph.
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Kyle
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