From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:47494)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <josh.durgin@inktank.com>) id 1SWrtX-0004uO-5h
	for qemu-devel@nongnu.org; Tue, 22 May 2012 12:25:12 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <josh.durgin@inktank.com>) id 1SWrtO-0003wv-3O
	for qemu-devel@nongnu.org; Tue, 22 May 2012 12:25:10 -0400
Received: from mail-pb0-f45.google.com ([209.85.160.45]:39206)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <josh.durgin@inktank.com>) id 1SWrtN-0003tG-TG
	for qemu-devel@nongnu.org; Tue, 22 May 2012 12:25:02 -0400
Received: by pbbro12 with SMTP id ro12so10369159pbb.4
	for <qemu-devel@nongnu.org>; Tue, 22 May 2012 09:24:58 -0700 (PDT)
Message-ID: <4FBBBDD7.6030006@inktank.com>
Date: Tue, 22 May 2012 09:24:55 -0700
From: Josh Durgin <josh.durgin@inktank.com>
MIME-Version: 1.0
References: <1337287349-7664-1-git-send-email-josh.durgin@inktank.com>
	<4FBB59F4.90600@redhat.com>
In-Reply-To: <4FBB59F4.90600@redhat.com>
Content-Type: text/plain; charset=ISO-8859-15; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH] rbd: hook up cache options
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kwolf@redhat.com, qemu-devel@nongnu.org

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()

or

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>=
> LIBRBD_VERSION(0, 1, 1)?

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

Thanks,
Josh