Re: [PATCH] kvm tools: Use mmap for working with disk image V2

Re: [PATCH] kvm tools: Use mmap for working with disk image V2

[Christoph Hellwig]

How do you plan to handle I/O errors or ENOSPC conditions?  Note that
shared writeable mappings are by far the feature in the VM/FS code
that is most error prone, including the impossiblity of doing sensible
error handling.

The version that accidentally used MAP_PRIVATE actually makes a lot of
sense for an equivalent of qemu's snapshot mode where the image is
readonly and changes are kept private as long as the amount of modified
blocks is small enough to not kill the host VM, but using shared
writeable mappings just sems dangerous.

Re: [PATCH] kvm tools: Use mmap for working with disk image V2

[Pekka Enberg]

On Mon, Apr 11, 2011 at 9:41 PM, Christoph Hellwig <hch@infradead.org> wrote:
> How do you plan to handle I/O errors or ENOSPC conditions?  Note that
> shared writeable mappings are by far the feature in the VM/FS code
> that is most error prone, including the impossiblity of doing sensible
> error handling.

Good point. I reverted the commit. Thanks!

On Mon, Apr 11, 2011 at 9:41 PM, Christoph Hellwig <hch@infradead.org> wrote:
> The version that accidentally used MAP_PRIVATE actually makes a lot of
> sense for an equivalent of qemu's snapshot mode where the image is
> readonly and changes are kept private as long as the amount of modified
> blocks is small enough to not kill the host VM, but using shared
> writeable mappings just sems dangerous.

Yup, Sasha, mind submitting a MAP_PRIVATE version that's enabled with
'--snapshot' (or equivalent) command line option.

                        Pekka
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