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Re: [Qemu-block] [PATCH v5 2/7] qcow2: Discard/zero clusters by byte cou

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH v5 2/7] qcow2: Discard/zero clusters by byte count
Date: Wed, 15 Feb 2017 16:46:42 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 14.02.2017 um 20:25 hat Eric Blake geschrieben:
> Passing a byte offset, but sector count, when we ultimately
> want to operate on cluster granularity, is madness.  Clean up
> the external interfaces to take both offset and count as bytes,
> while still keeping the assertion added previously that the
> caller must align the values to a cluster.  Then rename things
> to make sure backports don't get confused by changed units:
> instead of qcow2_discard_clusters() and qcow2_zero_clusters(),
> we now have qcow2_cluster_discard() and qcow2_cluster_zeroize().
> The internal functions still operate on clusters at a time, and
> return an int for number of cleared clusters; but on an image
> with 2M clusters, a single L2 table holds 256k entries that each
> represent a 2M cluster, totalling well over INT_MAX bytes if we
> ever had a request for that many bytes at once.  All our callers
> currently limit themselves to 32-bit bytes (and therefore fewer
> clusters), but by making this function 64-bit clean, we have one
> less place to clean up if we later improve the block layer to
> support 64-bit bytes through all operations (with the block layer
> auto-fragmenting on behalf of more-limited drivers), rather than
> the current state where some interfaces are artificially limited
> to INT_MAX at a time.
> Signed-off-by: Eric Blake <address@hidden>

Reviewed-by: Kevin Wolf <address@hidden>

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