Re: [Qemu-devel] [PATCH v5 0/8] Add metadata overlap checks

[Eric Blake <eblake@redhat.com>]

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On 09/02/2013 01:25 AM, Max Reitz wrote:
> If a qcow2 image file becomes corrupted, any write may inadvertently
> overwrite important metadata structures such as the L1 table. This
> series adds functionality for detecting, preventing and (to some extent)
> repairing such collisions.
> 
> v5:
>  - fixed patch 6 (forgot to update the event_names array for the new
>    event BLKDBG_REFTABLE_UPDATE); no other changes
> 
> v4:
>  - fixed handling of preallocated zero clusters in patch 4
>  - moved OFLAG_COPIED checks into a separate function (this affects
>    patches 4 and 5); functionality remains unchanged
>  - patches 1, 2, 3, 6, 7 and 8 remain unmodified (except for line
>    numbers in block/qcow2-refcount.c)

Just now looking at this series, and I have several questions.

It looks like Kevin applied v4 rather than v5; have we fixed that up?

Next, what sort of overhead do these new checks add to the write case?
Is it something that would be a noticeable slowdown?  I'd love to see
some benchmark numbers (hopefully, the default set of checks are in the
noise compared to the overhead of actual I/O).

Also, is there a way to tune the set of checks used at runtime, or are
we stuck with the compiled-in default?  That is, can a user opt in to
more expensive tests for robustness, or opt out of default tests for
speed, via a runtime command, or is it something where they have to
recompile to choose a different QCOW2_OL_DEFAULT value?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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