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Re: [PATCH v2] qcow2: Skip copy-on-write when allocating a zero cluster

From: Alberto Garcia
Subject: Re: [PATCH v2] qcow2: Skip copy-on-write when allocating a zero cluster
Date: Wed, 09 Sep 2020 14:52:04 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)


On Thu 27 Aug 2020 04:53:50 PM CEST, Alberto Garcia wrote:
> Since commit c8bb23cbdbe32f5c326365e0a82e1b0e68cdcd8a when a write
> request results in a new allocation QEMU first tries to see if the
> rest of the cluster outside the written area contains only zeroes.
> In that case, instead of doing a normal copy-on-write operation and
> writing explicit zero buffers to disk, the code zeroes the whole
> cluster efficiently using pwrite_zeroes() with BDRV_REQ_NO_FALLBACK.
> This improves performance very significantly but it only happens when
> we are writing to an area that was completely unallocated before. Zero
> clusters (QCOW2_CLUSTER_ZERO_*) are treated like normal clusters and
> are therefore slower to allocate.
> This happens because the code uses bdrv_is_allocated_above() rather
> bdrv_block_status_above(). The former is not as accurate for this
> purpose but it is faster. However in the case of qcow2 the underlying
> call does already report zero clusters just fine so there is no reason
> why we cannot use that information.
> After testing 4KB writes on an image that only contains zero clusters
> this patch results in almost five times more IOPS.


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