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Re: [Qemu-devel] [PATCH 2/2] specs/qcow2: Clarify that compressed cluste
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] specs/qcow2: Clarify that compressed clusters have the COPIED bit reset |
Date: |
Tue, 10 Apr 2018 11:31:33 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 |
On 04/10/2018 11:05 AM, Alberto Garcia wrote:
> Compressed clusters are not supposed to have the COPIED bit set, but
> this is not made explicit in the specs, so let's document it.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
> docs/interop/qcow2.txt | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index feb711fb6a..8e1547ded2 100644
> --- a/docs/interop/qcow2.txt
> +++ b/docs/interop/qcow2.txt
> @@ -400,10 +400,10 @@ L2 table entry:
> 62: 0 for standard clusters
> 1 for compressed clusters
>
> - 63: 0 for a cluster that is unused or requires COW, 1 if its
> - refcount is exactly one. This information is only
> accurate
> - in L2 tables that are reachable from the active L1
> - table.
> + 63: 0 for clusters that are unused, compressed or require
> COW.
> + 1 for standard clusters whose refcount is exactly one.
> + This information is only accurate in L2 tables
> + that are reachable from the active L1 table.
This matches what qemu outputs, so the question becomes whether it is
technically necessary to make this requirement mandatory for 3rd-party
implementations. But I'm in favor of the tighter wording, as it gets
rather hairy to check whether exactly one compressed cluster is
occupying a host cluster, plus I don't want to think about what happens
if a compressed cluster with the bit set crosses a host cluster boundary
(does it mean that compressed cluster is the only [remaining] source of
data for BOTH host clusters at once, where both the head of the first
host cluster and tail of the second host cluster is unused?)
Reviewed-by: Eric Blake <address@hidden>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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