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Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?
From: |
Richard W.M. Jones |
Subject: |
Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images? |
Date: |
Mon, 11 Jun 2018 09:44:23 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, Jun 06, 2018 at 01:32:47PM +0200, Max Reitz wrote:
> ext2?
I wrote an nbdkit plugin for ext2/ext3/ext4 last week.
https://github.com/libguestfs/nbdkit/tree/master/plugins/ext2
It uses libext2fs from e2fsprogs and I think there are some lessons
for anyone who wants to use ext2 to store disk images.
(1) You cannot have more than one host process accessing a single
filesystem image, even read-only. This is because opening an ext2+
filesystem even read-only causes writes, replaying the journal (for
ext3+) or writing to the superblock.
I'm sure there are some common use-cases such as overlays sharing a
common backing store which are excluded by this restriction.
(2) Within a single process you cannot have more than one libext2fs
handle open on the filesystem image. This could make qemu block
drivers a bit awkward (although not impossible) because if two
instances of an ext2 qemu block driver both opened different disks in
the same filesystem they'd need to share a handle.
(3) You can resize files in the filesystem, although because we're
waiting for the NBD resize extension to be finalized my plugin does
not do that.
(4) Trim/discard appears to be possible (and it should be possible to
punch holes in the filesystem image) but I couldn't actually
understand how to make it work. Also fast zeroing.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines. Supports shell scripting,
bindings from many languages. http://libguestfs.org
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, (continued)
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Michal Suchánek, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Max Reitz, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Michal Suchánek, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Daniel P . Berrangé, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Eduardo Habkost, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Max Reitz, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Daniel P . Berrangé, 2018/06/06
- [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Max Reitz, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Eduardo Habkost, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Max Reitz, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?,
Richard W.M. Jones <=
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Richard W.M. Jones, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Michael S. Tsirkin, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Michael S. Tsirkin, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Eric Blake, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Eric Blake, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Gerd Hoffmann, 2018/06/06
- Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?, Max Reitz, 2018/06/06