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Re: [Qemu-devel] [Qemu-block] [PATCH] qcow2: Clarify that compressed clu
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH] qcow2: Clarify that compressed cluster offset requires shift |
Date: |
Mon, 29 Feb 2016 16:42:41 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 |
On 29.02.2016 16:11, Eric Blake wrote:
> The specs for the host cluster offset of a compressed cluster
> were not clear that the offset is in terms of sectors, and requires
> a shift by 9 to be a byte offset. Add some more text to make the
> interpretation obvious.
>
> CC: address@hidden
> Signed-off-by: Eric Blake <address@hidden>
> ---
> docs/specs/qcow2.txt | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 80cdfd0..7049415 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -323,11 +323,16 @@ Standard Cluster Descriptor:
>
> Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
>
> - Bit 0 - x: Host cluster offset. This is usually _not_ aligned to a
> - cluster boundary!
> + Bit 0 - x: Bits 9-(x+9) of host cluster offset. This is
I'd rather use one of "9..(x+9)", "[9, x+9]", "9 - (x+9)", "9—(x+9)" in
order to clarify that this is not a minus but a range.
Max
> + usually _not_ aligned to a cluster boundary!
>
> x+1 - 61: Compressed size of the images in sectors of 512 bytes
>
> +The bits of the host cluster offset not specified in the cluster descriptor
> +are 0 (bits 0-8 are obvious because a 512-byte sector is the smallest
> +addressable unit, while bits 56-63 implies that a qcow2 file cannot exceed
> +2^56 bytes in size).
> +
> If a cluster is unallocated, read requests shall read the data from the
> backing
> file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
> no backing file or the backing file is smaller than the image, they shall
> read
>
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