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[Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification
From: |
Stefan Hajnoczi |
Subject: |
[Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification |
Date: |
Mon, 11 Oct 2010 14:42:41 +0100 |
User-agent: |
Mutt/1.5.20 (2009-06-14) |
On Mon, Oct 11, 2010 at 03:04:03PM +0200, Avi Kivity wrote:
> On 10/11/2010 12:09 PM, Stefan Hajnoczi wrote:
> >On Sun, Oct 10, 2010 at 11:20:09AM +0200, Avi Kivity wrote:
> >> On 10/08/2010 05:48 PM, Stefan Hajnoczi wrote:
> >> >Signed-off-by: Stefan Hajnoczi<address@hidden>
> >> >---
> >> > docs/specs/qed_spec.txt | 94
> >> +++++++++++++++++++++++++++++++++++++++++++++++
> >> > 1 files changed, 94 insertions(+), 0 deletions(-)
> >> >
> >> >+Feature bits:
> >> >+* QED_F_BACKING_FILE = 0x01. The image uses a backing file.
> >> >+* QED_F_NEED_CHECK = 0x02. The image needs a consistency check before
> >> use.
> >>
> >> Great that QED_F_NEED_CHECK is now non-optional. However, suppose
> >> we add a new structure (e.g. persistent freelist); it's now
> >> impossible to tell whether the structure is updated or not:
> >>
> >> 1 new qemu opens image
> >> 2 writes persistent freelist
> >> 3 clears need_check
> >> 4 shuts down
> >> 5 old qemu opens image
> >> 6 doesn't update persistent freelist
> >> 7 clears need_check
> >> 8 shuts down
> >>
> >> The image is now inconsistent, but has need_check cleared.
> >>
> >> We can address this by having a third feature bitmask that is
> >> autocleared by guests that don't recognize various bits; so the
> >> sequence becomes:
> >>
> >> 1 new qemu opens image
> >> 2 writes persistent freelist
> >> 3 clears need_check
> >> 4 sets persistent_freelist
> >> 5 shuts down
> >> 6 old qemu opens image
> >> 7 clears persistent_freelist (and any other bits it doesn't recognize)
> >> 8 doesn't update persistent freelist
> >> 9 clears need_check
> >> 10 shuts down
> >>
> >> The image is now consistent, since the persistent freelist has
> >> disappeared.
> >
> >It is more complicated than just the feature bit. The freelist requires
> >space in the image file. Clearing the persistent_freelist bit leaks the
> >freelist.
> >
> >We can solve this by using a compat feature bit and an autoclear feature
> >bit. The autoclear bit specifies whether or not the freelist is valid
> >and the compat feature bit specifices whether or not the freelist
> >exists.
> >
> >When the new qemu opens the image again it notices that the autoclear
> >bit is unset but the compat bit is set. This means the freelist is
> >out-of-date and its space can be reclaimed.
> >
> >I don't like the idea of doing these feature bit acrobatics because they
> >add complexity. On the other hand your proposal ensures backward
> >compatibility in the case of an optional data structure that needs to
> >stay in sync with the image. I'm just not 100% convinced it's worth it.
>
> My scenario ends up with data corruption if we move to an old qemu
> and then back again, without any aborts.
>
> A leak is acceptable (it won't grow; it's just an unused, incorrect
> freelist), but data corruption is not.
The alternative is for the freelist to be a non-compat feature bit.
That means older QEMU binaries cannot use a QED image that has enabled
the freelist.
Stefan
- [Qemu-devel] [PATCH v2 0/7] qed: Add QEMU Enhanced Disk format, Stefan Hajnoczi, 2010/10/08
- [Qemu-devel] [PATCH v2 3/7] docs: Add QED image format specification, Stefan Hajnoczi, 2010/10/08
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Avi Kivity, 2010/10/10
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Stefan Hajnoczi, 2010/10/11
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Avi Kivity, 2010/10/11
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification,
Stefan Hajnoczi <=
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Avi Kivity, 2010/10/11
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Stefan Hajnoczi, 2010/10/11
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Avi Kivity, 2010/10/11
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Anthony Liguori, 2010/10/11
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Avi Kivity, 2010/10/11
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Anthony Liguori, 2010/10/11
- [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Avi Kivity, 2010/10/11
- Re: [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Anthony Liguori, 2010/10/11
- Re: [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Avi Kivity, 2010/10/11
- Re: [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification, Anthony Liguori, 2010/10/11