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Re: [Qemu-devel] [PATCH] rbd: hook up cache options

From: Josh Durgin
Subject: Re: [Qemu-devel] [PATCH] rbd: hook up cache options
Date: Tue, 22 May 2012 09:24:55 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

On 05/22/2012 02:18 AM, Paolo Bonzini wrote:
Il 17/05/2012 22:42, Josh Durgin ha scritto:
+     * Fallback to more conservative semantics if setting cache
+     * options fails. Ignore errors from setting rbd_cache because the
+     * only possible error is that the option does not exist, and
+     * librbd defaults to no caching. If write through caching cannot
+     * be set up, fall back to no caching.
+     */
+    if (flags&  BDRV_O_NOCACHE) {
+        rados_conf_set(s->cluster, "rbd_cache", "false");
+    } else {
+        rados_conf_set(s->cluster, "rbd_cache", "true");
+        if (!(flags&  BDRV_O_CACHE_WB)) {
+            r = rados_conf_set(s->cluster, "rbd_cache_max_dirty", "0");
+            if (r<  0) {
+                rados_conf_set(s->cluster, "rbd_cache", "false");
+            }
+        }
+    }

Last time I looked at ceph, rbd_flush was not a full flush of the cache;
it only ensured that the pending requests were sent.  So my questions are:

I'm not sure which version you were looking at, but this hasn't been
the case since caching was implemented. I don't think it was ever the
case, actually. rbd_flush has always waited for pending I/Os to complete
(be on disk on all replicas), not just be in flight.

If you're interested in the current implementation, you can see:

src/librbd.cc: librbd::flush()

which goes into:

src/osdc/ObjectCacher.cc: ObjectCacher::commit_set()


src/librados/IoCtxImpl.cc: IoCtxImpl::flush_aio_writes()

1) has this changed?  does rbd_flush now flush dirty items when
rbd_cache_max_dirty>  0?

The rbd_cache_* options did not exist before 0.46.

2) should the usage of a cache be conditional on LIBRBD_VERSION_CODE>=

It doesn't matter if you use an older version because the non-existent
options don't have any effect.


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