Re: [Qemu-devel] [PATCH 2/2] specs/qcow2: Clarify that compressed clusters have the COPIED bit reset

[Eric Blake]

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 
> ---
>  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 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3266
Virtualization:  qemu.org | libvirt.org



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[Qemu-devel] [PATCH 2/2] specs/qcow2: Clarify that compressed clusters have the COPIED bit reset

[Alberto Garcia]

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 
---
 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.
 
 Standard Cluster Descriptor:
 
-- 
2.11.0