Re: [Qemu-devel] [RFC] Re-evaluating subcluster allocation for qcow2 images

From: Kevin Wolf <kwolf@redhat.com>
To: Alberto Garcia <berto@igalia.com>
Cc: Anton Nefedov <anton.nefedov@virtuozzo.com>,
	Denis Lunev <den@virtuozzo.com>,
	"qemu-block@nongnu.org" <qemu-block@nongnu.org>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	Max Reitz <mreitz@redhat.com>
Subject: Re: [Qemu-devel] [RFC] Re-evaluating subcluster allocation for qcow2 images
Date: Fri, 28 Jun 2019 16:57:08 +0200	[thread overview]
Message-ID: <20190628145708.GN5179@dhcp-200-226.str.redhat.com> (raw)
In-Reply-To: <w51r27dixcm.fsf@maestria.local.igalia.com>

Am 28.06.2019 um 16:43 hat Alberto Garcia geschrieben:
> On Thu 27 Jun 2019 06:05:55 PM CEST, Denis Lunev wrote:
> > Please note, I am not talking now about your case with COW. Here the
> > allocation is performed on the sub-cluster basis, i.e. the abscence of
> > the sub-cluster in the image means hole on that offset. This is
> > important difference.
> 
> I mentioned the possibility that if you have a case like 2MB / 64KB and
> you write to an empty cluster then you could allocate the necessary
> subclusters, and additionally fallocate() the space of the whole cluster
> (2MB) in order to try to keep it contiguous.
> 
> With this we would lose the space saving advantage of having
> subclusters. But perhaps that would work for smaller cluster sizes (it
> would mitigate the fragmentation problem).

There seem to be use cases for both ways. So does this need to be an
option?

Kevin