thanx Nick, Gregory and Wido,
So at least, we can say the cache tiering in Jewel is stable enough
I think.
I like cache tiering more than the others, but yes, there is a problem
about cache tiering in
flushing data between different nodes, which are not a problem in local
caching solution.
guys:
Is there any plan to enhance cache tiering to solve such problem?
Or as Nick asked, is
that cache tiering fading away?
Yang
On 15/02/2017, 06:42, Nick Fisk wrote:
-----Original Message-----
From: Gregory Farnum [mailto:gfarnum@xxxxxxxxxx]
Sent: 14 February 2017 21:05
To: Wido den Hollander <wido@xxxxxxxx>
Cc: Dongsheng Yang <dongsheng.yang@xxxxxxxxxxxx>; Nick Fisk
<nick@xxxxxxxxxx>; Ceph Users <ceph-users@xxxxxxxxxxxxxx>
Subject: Re: bcache vs flashcache vs cache tiering
On Tue, Feb 14, 2017 at 8:25 AM, Wido den Hollander <wido@xxxxxxxx>
wrote:
Op 14 februari 2017 om 11:14 schreef Nick Fisk <nick@xxxxxxxxxx>:
-----Original Message-----
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On
Behalf Of Dongsheng Yang
Sent: 14 February 2017 09:01
To: Sage Weil <notifications@xxxxxxxxxx>
Cc: ceph-devel@xxxxxxxxxxxxxxx; ceph-users@xxxxxxxxxxxxxx
Subject: bcache vs flashcache vs cache tiering
Hi Sage and all,
We are going to use SSDs for cache in ceph. But I am not sure
which one is the best solution, bcache? flashcache? or cache
tier?
I would vote for cache tier. Being able to manage it from within
Ceph, instead of having to manage X number of bcache/flashcache
instances, appeals to me more. Also last time I looked Flashcache
seems unmaintained and bcache might be going that way with talk of
this new bcachefs. Another point to consider is that Ceph has had a lot of
work done on it to ensure data consistency; I don't ever want to be in a
position where I'm trying to diagnose problems that might be being caused
by another layer sitting in-between Ceph and the Disk.
However, I know several people on here are using bcache and
potentially getting better performance than with cache tiering, so
hopefully someone will give their views.
I am using Bcache on various systems and it performs really well. The
caching layer in Ceph is slow. Promoting Objects is slow and it also involves
additional RADOS lookups.
Yeah. Cache tiers have gotten a lot more usable in Ceph, but the use cases
where they're effective are still pretty limited and I think in-node caching has
a brighter future. We just don't like to maintain the global state that makes
separate caching locations viable and unless you're doing something
analogous to the supercomputing "burst buffers" (which some people are!),
it's going to be hard to beat something that doesn't have to pay the cost of
extra network hops/bandwidth.
Cache tiers are also not a feature that all the vendors support in their
downstream products, so it will probably see less ongoing investment than
you'd expect from such a system.
Should that be taken as an unofficial sign that the tiering support is likely to fade away?
I think both approaches have different strengths and probably the difference between a tiering system and a caching one is what causes some of the problems.
If something like bcache is going to be the preferred approach, then I think more work needs to be done around certifying it for use with Ceph and allowing its behavior to be more controlled by Ceph as well. I assume there are issues around backfilling and scrubbing polluting the cache? Maybe you would want to be able to pass hints down from Ceph, which could also allow per pool cache behavior??
-Greg
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