18.10.2019 17:04, Eric Blake wrote: > On 10/18/19 4:47 AM, 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 | 19 ++++++++++++++++++- >> 1 file changed, 18 insertions(+), 1 deletion(-) >> >> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt >> index b971e59b1a..4eabd81363 100644 >> --- a/docs/interop/qcow2.txt >> +++ b/docs/interop/qcow2.txt >> @@ -109,6 +109,12 @@ least next five fields, up to the @header_length field. >> 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 >> + clusters. In this case, @header_length must >> + be at least 105 and @compression_type field >> + must be non-zero. > > I want to insist on 8-byte alignment, so this should be at least 112. > > Also, as pointed out in v7, use of the decoration '@header_length' and > '@compression_type' is not consistent with the rest of the document. Drop the > @.
Missed this, sorry( > >> + >> Bits 3-63: Reserved (set to 0) >> 80 - 87: compatible_features >> @@ -183,7 +189,18 @@ It's allowed for the header end to cut some field in >> the middle (in this case >> the field is considered as absent), but in this case the part of the field >> which is covered by @header_length must be zeroed. >> - < ... 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. >> + If incompatible compression type bit is set: the field >> must >> + exist (i.e. @header_length >= 105) and must be non-zero >> ( >> + which means non-zlib compression type) >> + If incompatible compression type bit is unset: the field >> + may not exist (if @header_length < 105) or it must be >> zero >> + (which means zlib). >> + Available compression type values: >> + 0: zlib <https://www.zlib.net/> > > One byte for the field is fine, but I'd still explicitly document 7 bytes of > unused padding, since I want to insist on an 8-byte multiple. > OK -- Best regards, Vladimir