From: Robert White <rwhite@pobox.com>
To: Marc MERLIN <marc@merlins.org>,
Larkin Lowrey <llowrey@nuclearwinter.com>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: Heavy nocow'd VM image fragmentation
Date: Fri, 24 Oct 2014 19:41:32 -0700 [thread overview]
Message-ID: <544B0DDC.1020102@pobox.com> (raw)
In-Reply-To: <20141024114916.GI12554@merlins.org>
On 10/24/2014 04:49 AM, Marc MERLIN wrote:
> On Thu, Oct 23, 2014 at 06:04:43PM -0500, Larkin Lowrey wrote:
>> I have a 240GB VirtualBox vdi image that is showing heavy fragmentation
>> (filefrag). The file was created in a dir that was chattr +C'd, the file
>> was created via fallocate and the contents of the orignal image were
>> copied into the file via dd. I verified that the image was +C.
> To be honest, I have the same problem, and it's vexing:
If I understand correctly, when you take a snapshot the file goes into
what I call "1COW" mode. The snapshot is pinning the old image into
place and the new data has to go somewhere. One explainer said it was
"on its head" but I suspect the fragmentation still ends up in the
written segment.
Options may include transferring the VMs to a subvolume that you don't
snapshot very often.
Doing backups by taking the snapshot, transfering it to backup, and then
dropping it before you restart the virtual machine.
Honestly while autodefrag would be your enemy, I don't see a lot of
harm/degradation in leaving the VM images on my drive all normal COW and
doing the occasional defrag.
I could, of course, be wrong.
next prev parent reply other threads:[~2014-10-25 2:41 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-23 23:04 Heavy nocow'd VM image fragmentation Larkin Lowrey
2014-10-24 11:49 ` Marc MERLIN
2014-10-25 2:41 ` Robert White [this message]
2014-10-25 3:28 ` Duncan
2014-10-26 17:20 ` Larkin Lowrey
2014-10-27 6:44 ` Duncan
2014-10-27 12:04 ` Austin S Hemmelgarn
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