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Re: [PATCH for-4.2 1/4] Revert "qcow2: skip writing zero buffers to empt

From: Eric Blake
Subject: Re: [PATCH for-4.2 1/4] Revert "qcow2: skip writing zero buffers to empty COW areas"
Date: Fri, 1 Nov 2019 13:40:23 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1

On 11/1/19 11:00 AM, Max Reitz wrote:
This reverts commit c8bb23cbdbe32f5c326365e0a82e1b0e68cdcd8a.

This commit causes fundamental performance problems on XFS (because
fallocate() stalls the AIO pipeline), and as such it is not clear that
we should unconditionally enable this behavior.

We expect subclusters to alleviate the performance penalty of small
writes to newly allocated clusters, so when we get them, the originally
intended performance gain may actually no longer be significant.

If we want to reintroduce something similar to c8bb23cbdbe, it will
require extensive benchmarking on various systems with subclusters

Cc: address@hidden
Signed-off-by: Max Reitz <address@hidden>

+++ b/qapi/block-core.json
@@ -3304,8 +3304,6 @@
  # @cor_write: a write due to copy-on-read (since 2.11)
-# @cluster_alloc_space: an allocation of file space for a cluster (since 4.1)
  # @none: triggers once at creation of the blkdebug node (since 4.1)

Deleting released qapi is not backwards-compatible. However, given that the only known user of this interface is debug testing via iotests, I'm not too concerned that we would be impacting any external users.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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