We are using Hammer - latest released version. How do I check if it's getting promoted into the cache?
We're using the latest ceph kernel client. Where do I poke at readahead settings there?
On Tue, Sep 8, 2015 at 8:29 AM, Gregory Farnum <gfarnum@xxxxxxxxxx> wrote:
On Thu, Sep 3, 2015 at 11:58 PM, Kyle Hutson <kylehutson@xxxxxxx> wrote:
> I was wondering if anybody could give me some insight as to how CephFS does
> its caching - read-caching in particular.
>
> We are using CephFS with an EC pool on the backend with a replicated cache
> pool in front of it. We're seeing some very slow read times. Trying to
> compute an md5sum on a 15GB file twice in a row (so it should be in cache)
> takes the time from 23 minutes down to 17 minutes, but this is over a 10Gbps
> network and with a crap-ton of OSDs (over 300), so I would expect it to be
> down in the 2-3 minute range.
A single sequential read won't necessarily promote an object into the
cache pool (although if you're using Hammer I think it will), so you
want to check if it's actually getting promoted into the cache before
assuming that's happened.
>
> I'm just trying to figure out what we can do to increase the performance. I
> have over 300 TB of live data that I have to be careful with, though, so I
> have to have some level of caution.
>
> Is there some other caching we can do (client-side or server-side) that
> might give us a decent performance boost?
Which client are you using for this testing? Have you looked at the
readahead settings? That's usually the big one; if you're only asking
for 4KB at once then stuff is going to be slow no matter what (a
single IO takes at minimum about 2 milliseconds right now, although
the RADOS team is working to improve that).
-Greg
>
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