From: Christoph Hellwig <hch@infradead.org>
To: Mike Snitzer <snitzer@redhat.com>
Cc: elder@inktank.com, Sage Weil <sage@inktank.com>,
Christoph Hellwig <hch@infradead.org>, Ugis <ugis22@gmail.com>,
linux-lvm@redhat.com,
"ceph-devel@vger.kernel.org" <ceph-devel@vger.kernel.org>,
"ceph-users@ceph.com" <ceph-users@ceph.com>
Subject: Re: [linux-lvm] poor read performance on rbd+LVM, LVM overload
Date: Mon, 21 Oct 2013 11:06:16 -0700 [thread overview]
Message-ID: <20131021180616.GA7196@infradead.org> (raw)
In-Reply-To: <20131021150129.GA28099@redhat.com>
On Mon, Oct 21, 2013 at 11:01:29AM -0400, Mike Snitzer wrote:
> It isn't DM that splits the IO into 4K chunks; it is the VM subsystem
> no?
Well, it's the block layer based on what DM tells it. Take a look at
dm_merge_bvec
From dm_merge_bvec:
/*
* If the target doesn't support merge method and some of the devices
* provided their merge_bvec method (we know this by looking at
* queue_max_hw_sectors), then we can't allow bios with multiple vector
* entries. So always set max_size to 0, and the code below allows
* just one page.
*/
Although it's not the general case, just if the driver has a
merge_bvec method. But this happens if you using DM ontop of MD where I
saw it aswell as on rbd, which is why it's correct in this context, too.
Sorry for over generalizing a bit.
next prev parent reply other threads:[~2013-10-21 18:06 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-16 14:46 [linux-lvm] poor read performance on rbd+LVM, LVM overload Ugis
2013-10-16 16:16 ` Sage Weil
2013-10-17 9:06 ` David McBride
2013-10-17 15:18 ` Mike Snitzer
2013-10-18 7:56 ` Ugis
2013-10-19 0:01 ` Sage Weil
2013-10-20 15:18 ` Ugis
2013-10-20 18:21 ` [linux-lvm] [ceph-users] " Josh Durgin
2013-10-21 3:58 ` [linux-lvm] " Sage Weil
2013-10-21 14:11 ` Christoph Hellwig
2013-10-21 15:01 ` Mike Snitzer
2013-10-21 15:06 ` Mike Snitzer
2013-10-21 16:02 ` Sage Weil
2013-10-21 17:48 ` Mike Snitzer
2013-10-21 18:05 ` Sage Weil
2013-10-21 18:06 ` Christoph Hellwig [this message]
2013-10-21 18:27 ` Mike Snitzer
2013-10-30 14:53 ` Ugis
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