> In my case, the replica-3 and k2m2 are stored on the same spinning disks.
That is exactly what I meant by same pool. The only way for a cache to make sense would be if the data being written or read will be modified or heavily read for X amount of time and then ignored.
If things are rarely read, and randomly so, them prompting then into a cache tier just makes you wait for the object to be promoted to cache before you read it once or twice before it sits in there until it's demoted again. If you have random io and anything can really be read next, then a cache tier on the same disks as the EC pool will only cause things to be promoted and demoted for no apparent reason.
You can always test this for your use case and see if it helps enough to create a pool and tier that you need to manage or not. I'm planning to remove my cephfs cache tier once I upgrade to Luminous as I only have it as a requirement. It causes me to show down my writes heavily as eviction io is useless and wasteful of cluster io for me. I haven't checked on the process for that yet, but I'm assuming it's a set command on the pool that will then allow me to disable and remove the cache tier. I mention that because if it is that easy to enable/disable, then testing it should be simple and easy to compare.
Hi David,
Thanks for the clarification. Reminded me of some details I forgot
to mention.
In my case, the replica-3 and k2m2 are stored on the same spinning
disks. (Mainly using EC for "compression" b/c with the EC k2m2 setting
PG only takes up the same amount of space as a replica-2 while allowing
2 disks to fail like replica-3 without loss.)
I'm using this setup as RBDs and cephfs to store things like local
mirrors of linux packages and drive images to be broadcast over network.
Seems to be about as fast as a normal hard drive. :)
So is this the situation where the "cache tier [is] ont the same root
of osds as the EC pool"?
Thanks for the advice!
Chad.
On 09/30/2017 12:32 PM, David Turner wrote:
> I can only think of 1 type of cache tier usage that is faster if you are
> using the cache tier on the same root of osds as the EC pool. That is
> cold storage where the file is written initially, modified and read door
> the first X hours, and then remains in cold storage for the remainder of
> its life with rate reads.
>
> Other than that there are a few use cases using a faster root of osds
> that might make sense, but generally it's still better to utilize that
> faster storage in the rest of the osd stack either as journals for
> filestore or Wal/DB partitions for bluestore.
>
>
> On Sat, Sep 30, 2017, 12:56 PM Chad William Seys
> <cwseys@xxxxxxxxxxxxxxxx <mailto:cwseys@xxxxxxxxxxxxxxxx>> wrote:
>
> Hi all,
> Now that Luminous supports direct writing to EC pools I was
> wondering
> if one can get more performance out of an erasure-coded pool with
> overwrites or an erasure-coded pool with a cache tier?
> I currently have a 3 replica pool in front of a k2m2 erasure coded
> pool. Luminous documentation on cache tiering
> http://docs.ceph.com/docs/luminous/rados/operations/cache-tiering/#a-word-of-caution
> makes it sound like cache tiering is usually not recommonded.
>
> Thanks!
> Chad.
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