20.01.2020 19:14, Max Reitz wrote: > On 16.12.19 13:17, Vladimir Sementsov-Ogievskiy wrote: >> The patch add new additional field to qcow2 header: compression_type, >> which specifies compression type. If field is absent or zero, default >> compression type is set: ZLIB, which corresponds to current behavior. >> >> New compression type (ZSTD) is to be added in further commit. >> >> Suggested-by: Denis Plotnikov <dplotni...@virtuozzo.com> >> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> >> --- >> docs/interop/qcow2.txt | 16 +++++++++++++++- >> 1 file changed, 15 insertions(+), 1 deletion(-) >> >> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt >> index d92c827763..77146b5169 100644 >> --- a/docs/interop/qcow2.txt >> +++ b/docs/interop/qcow2.txt >> @@ -109,6 +109,11 @@ the next fields through header_length. >> An External Data File Name header >> extension may >> be present if this bit is set. >> >> + Bit 3: Compression type bit. If this bit is set, >> + non-default compression is used for >> compressed > > s/non-default/a non-default/ > >> + clusters. compression_type field must be >> + present and not zero. > > s/compression_type field/The compression_type field/ > >> + >> Bits 3-63: Reserved (set to 0) >> >> 80 - 87: compatible_features >> @@ -188,7 +193,16 @@ present*, if not altered by specific incompatible bit. >> *. Field is not present when header_length is less or equal to field's >> offset. >> Also, all additional fields are not present for version 2. >> >> - < ... No additional fields in the header currently ... > >> + 104: compression_type >> + Defines the compression method used for compressed >> clusters. >> + A single compression type is applied to all compressed >> image >> + clusters. > > Sounds a bit too complicated. Maybe „All compressed clusters in an > image use the same compression type.” instead? (Or s/an/the same/)
Sounds better, yes. > >> + If incompatible compression type bit is set: the field >> must > > Hmm, this sounds like there was an “incompatible compression type” bit, > instead of an incompatible bit called “compression type”. So maybe “If > the incompatible bit "compression type" is set, this field must...”? OK) > >> + be present and non-zero (which means non-zlib >> compression type) > > s/$/./ > >> + If incompatible compression type bit is unset: the field > > I’d just make this “Otherwise, this field...” > >> + may not exist or it must be zero (which means zlib). > > “must not be present or must be zero”? > > (“exist” sounds a bit weird; the spec only defined “not present” so far. > As for the “may not”, that isn’t in RFC 2119. :-)) Agreed, thanks. > > Max > >> + Available compression type values: >> + 0: zlib <https://www.zlib.net/> >> >> Header padding >> >> > > -- Best regards, Vladimir
