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: [email protected] > Signed-off-by: Eric Blake <[email protected]> > --- > 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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