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qcow2: Document the Extended L2 Entries feature

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Subcluster allocation in qcow2 is implemented by extending the
existing L2 table entries and adding additional information to
indicate the allocation status of each subcluster.

This patch documents the changes to the qcow2 format and how they
affect the calculation of the L2 cache size.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <5199f2e1c717bcaa58b48142c9062b803145ff7f.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Alberto Garcia 2020-07-10 18:12:49 +02:00 • committed by Max Reitz
parent 12c6aebedf
commit 30afc12085
2 changed files with 83 additions and 4 deletions
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19
docs/qcow2-cache.txt
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@ -1,6 +1,6 @@
qcow2 L2/refcount cache configuration
=====================================
Copyright (C) 2015, 2018 Igalia, S.L.
Copyright (C) 2015, 2018-2020 Igalia, S.L.
Author: Alberto Garcia <berto@igalia.com>
This work is licensed under the terms of the GNU GPL, version 2 or
@ -222,3 +222,20 @@ support this functionality, and is 0 (disabled) on other platforms.
This functionality currently relies on the MADV_DONTNEED argument for
madvise() to actually free the memory. This is a Linux-specific feature,
so cache-clean-interval is not supported on other systems.
Extended L2 Entries
-------------------
All numbers shown in this document are valid for qcow2 images with normal
64-bit L2 entries.
Images with extended L2 entries need twice as much L2 metadata, so the L2
cache size must be twice as large for the same disk space.
disk_size = l2_cache_size * cluster_size / 16
i.e.
l2_cache_size = disk_size * 16 / cluster_size
Refcount blocks are not affected by this.