Re: [Qemu-devel] [PATCH v4 3/8] mirror: Do zero write on target if sectors not allocated

[Paolo Bonzini <pbonzini@redhat.com>]

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On 22/05/2015 22:20, Eric Blake wrote:
> But I'm worried about a shallow copy.  If I start with "base <-
> active", where "active" has an explicit zero cluster that is
> overwriting an allocated non-zero cluster in "base", and I'm
> creating the shallow clone to "base <- copy", then the default of
> 'unmap=true' says that bdrv_aio_write_zeroes() may attempt to unmap
> the cluster in "copy".  At which point, doesn't that mean that
> reading from "copy" will dredge up the non-zero data from "base",
> which is NOT a faithful mirroring of "active"?

No, bdv_aio_write_zeroes+BDRV_REQ_MAY_UNMAP only unmaps if it results
in zeroes.  In addition, unlike bdrv_aio_discard,
bdrv_aio_write_zeroes will do a real write of zeroes if [sector_num,
sector_num+nb_sectors) is not aligned to the disk's unmap granularity.

Paolo
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