[PATCH v9 0/2] qcow2: add zstd cluster compression

[PATCH v9 0/2] qcow2: add zstd cluster compression

[Vladimir Sementsov-Ogievskiy]

Hi all!

Here is my proposal, about how to correctly update qcow2 specification
to introduce new field, keeping in mind currently existing images and
downstream Qemu instances.

v9: Merge 01 and 02
    Change wordings
    Require header alignment

Vladimir Sementsov-Ogievskiy (2):
  docs: improve qcow2 spec about extending image header
  docs: qcow2: introduce compression type feature

 docs/interop/qcow2.txt | 53 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 50 insertions(+), 3 deletions(-)

-- 
2.18.0

[PATCH v9 1/2] docs: improve qcow2 spec about extending image header

[Vladimir Sementsov-Ogievskiy]

Make it more obvious how to add new fields to the version 3 header and
how to interpret them.

The specification is adjusted so for new defined optional fields:

1. Software may support some of these optional fields and ignore the
   others, which means that features may be backported to downstream
   Qemu independently.
2. If we want to add incompatible field (or a field, for which some its
   values would be incompatible), it must be accompanied by
   incompatible feature bit.

Also the concept of "default is zero" is clarified, as it's strange to
say that the value of the field is assumed to be zero for the software
version which don't know about the field at all and don't know how to
treat it be it zero or not.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 docs/interop/qcow2.txt | 39 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 36 insertions(+), 3 deletions(-)

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index af5711e533..d92c827763 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -79,9 +79,9 @@ The first cluster of a qcow2 image contains the file header:
                     Offset into the image file at which the snapshot table
                     starts. Must be aligned to a cluster boundary.
 
-If the version is 3 or higher, the header has the following additional fields.
-For version 2, the values are assumed to be zero, unless specified otherwise
-in the description of a field.
+For version 2, the header is exactly 72 bytes in length, and finishes here.
+For version 3 or higher, the header length is at least 104 bytes, including
+the next fields through header_length.
 
          72 -  79:  incompatible_features
                     Bitmask of incompatible features. An implementation must
@@ -164,6 +164,39 @@ in the description of a field.
         100 - 103:  header_length
                     Length of the header structure in bytes. For version 2
                     images, the length is always assumed to be 72 bytes.
+                    For version 3 it's at least 104 bytes and must be a multiply
+                    of 8.
+
+Additional fields (version 3 and higher)
+
+In general, these fields are optional and may be safely ignored by the software,
+as well as filled by zeros (which is equal to field absence), if software needs
+to set field B, but don't want to care about field A, which precedes B. More
+formally, additional fields have the following compatibility rules:
+
+1. If the value of the additional field must not be ignored for correct
+handling of the file, it will be accompanied by a corresponding incompatible
+feature bit.
+
+2. If there are no unrecognized incompatible feature bits set, an unknown
+additional field may be safely ignored other than preserving its value when
+rewriting the image header.
+
+3. An explicit value of 0 will have the same behavior as when the field is not
+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 ... >
+
+Header padding
+
+@header_length must be a multiply of 8, which means that if last additional field
+end is not aligned, some padding is needed. This padding must be zeroed, so that,
+if some existing (or future) additional field will fall into the padding, it
+will be interpreted accordingly to point [3.] of the previous paragraph, i.e.
+in same manner as when this field is not present.
 
 Directly after the image header, optional sections called header extensions can
 be stored. Each extension has a structure like the following:
-- 
2.18.0

[PATCH v9 2/2] docs: qcow2: introduce compression type feature

[Vladimir Sementsov-Ogievskiy]

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 <dplotnikov@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@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
+                                clusters. compression_type field must be
+                                present and not zero.
+
                     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.
+                    If incompatible compression type bit is set: the field must
+                    be present and non-zero (which means non-zlib compression type)
+                    If incompatible compression type bit is unset: the field
+                    may not exist or it must be zero (which means zlib).
+                    Available compression type values:
+                        0: zlib <https://www.zlib.net/>
 
 Header padding
 
-- 
2.18.0

Re: [PATCH v9 0/2] qcow2: add zstd cluster compression

[Vladimir Sementsov-Ogievskiy]

ping

16.12.2019 15:17, Vladimir Sementsov-Ogievskiy wrote:
> Hi all!
> 
> Here is my proposal, about how to correctly update qcow2 specification
> to introduce new field, keeping in mind currently existing images and
> downstream Qemu instances.
> 
> v9: Merge 01 and 02
>      Change wordings
>      Require header alignment
> 
> Vladimir Sementsov-Ogievskiy (2):
>    docs: improve qcow2 spec about extending image header
>    docs: qcow2: introduce compression type feature
> 
>   docs/interop/qcow2.txt | 53 +++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 50 insertions(+), 3 deletions(-)
> 


-- 
Best regards,
Vladimir

Re: [PATCH v9 1/2] docs: improve qcow2 spec about extending image header

[Max Reitz]

[-- Attachment #1.1: Type: text/plain, Size: 5070 bytes --]

On 16.12.19 13:17, Vladimir Sementsov-Ogievskiy wrote:
> Make it more obvious how to add new fields to the version 3 header and
> how to interpret them.
> 
> The specification is adjusted so for new defined optional fields:
> 
> 1. Software may support some of these optional fields and ignore the
>    others, which means that features may be backported to downstream
>    Qemu independently.
> 2. If we want to add incompatible field (or a field, for which some its
>    values would be incompatible), it must be accompanied by
>    incompatible feature bit.
> 
> Also the concept of "default is zero" is clarified, as it's strange to
> say that the value of the field is assumed to be zero for the software
> version which don't know about the field at all and don't know how to
> treat it be it zero or not.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  docs/interop/qcow2.txt | 39 ++++++++++++++++++++++++++++++++++++---
>  1 file changed, 36 insertions(+), 3 deletions(-)

I put review of this off for so long because I always waited for Eric to
give his R-b, but maybe not.

I generally think that he’s stricter on what to write in documentation,
and accordingly I only have nit picks on spelling and structure:

> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index af5711e533..d92c827763 100644
> --- a/docs/interop/qcow2.txt
> +++ b/docs/interop/qcow2.txt
> @@ -79,9 +79,9 @@ The first cluster of a qcow2 image contains the file header:
>                      Offset into the image file at which the snapshot table
>                      starts. Must be aligned to a cluster boundary.
>  
> -If the version is 3 or higher, the header has the following additional fields.
> -For version 2, the values are assumed to be zero, unless specified otherwise
> -in the description of a field.
> +For version 2, the header is exactly 72 bytes in length, and finishes here.
> +For version 3 or higher, the header length is at least 104 bytes, including
> +the next fields through header_length.
>  
>           72 -  79:  incompatible_features
>                      Bitmask of incompatible features. An implementation must
> @@ -164,6 +164,39 @@ in the description of a field.
>          100 - 103:  header_length
>                      Length of the header structure in bytes. For version 2
>                      images, the length is always assumed to be 72 bytes.
> +                    For version 3 it's at least 104 bytes and must be a multiply

s/multiply/multiple/

> +                    of 8.
> +
> +Additional fields (version 3 and higher)

If this is supposed to be a heading, maybe it should enclosed by “===”
on both sides.

> +
> +In general, these fields are optional and may be safely ignored by the software,
> +as well as filled by zeros (which is equal to field absence), if software needs
> +to set field B, but don't want to care about field A, which precedes B. More

s/don't/does not/ (or maybe s/don't want/does not/)

> +formally, additional fields have the following compatibility rules:
> +
> +1. If the value of the additional field must not be ignored for correct
> +handling of the file, it will be accompanied by a corresponding incompatible
> +feature bit.
> +
> +2. If there are no unrecognized incompatible feature bits set, an unknown
> +additional field may be safely ignored other than preserving its value when
> +rewriting the image header.
> +
> +3. An explicit value of 0 will have the same behavior as when the field is not
> +present*, if not altered by specific incompatible bit.

s/by specific/by a specific/

> +
> +*. Field is not present when header_length is less or equal to field's offset.

s/Field/A field/, s/field's/the field's/

(maybe also +considered, as in "A field is considered not present...")

> +Also, all additional fields are not present for version 2.
> +
> +        < ... No additional fields in the header currently ... >

This looks a bit weird to me, but the next patch will remove it again,
so who cares.

> +Header padding

Same heading note here (I’d make this “=== Header padding ===”).

> +
> +@header_length must be a multiply of 8, which means that if last additional field

s/multiply/multiple/

> +end is not aligned, some padding is needed. This padding must be zeroed, so that,

I think s/last additional field end/the last additional field’s end/, or
maybe s/last additional field end/the end of the last additional field/.

> +if some existing (or future) additional field will fall into the padding, it
> +will be interpreted accordingly to point [3.] of the previous paragraph, i.e.
> +in same manner as when this field is not present.

s/in same/in the same/

>  

I think there should be a new heading here
(“=== Header extensions ===”).

Max

>  Directly after the image header, optional sections called header extensions can
>  be stored. Each extension has a structure like the following:
> 


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

Re: [PATCH v9 2/2] docs: qcow2: introduce compression type feature

[Max Reitz]

[-- Attachment #1.1: Type: text/plain, Size: 3114 bytes --]

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 <dplotnikov@virtuozzo.com>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@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/)

> +                    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...”?

> +                    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. :-))

Max

> +                    Available compression type values:
> +                        0: zlib <https://www.zlib.net/>
>  
>  Header padding
>  
> 



[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

Re: [PATCH v9 2/2] docs: qcow2: introduce compression type feature

[Vladimir Sementsov-Ogievskiy]

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 <dplotnikov@virtuozzo.com>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@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

Re: [PATCH v9 1/2] docs: improve qcow2 spec about extending image header

[Vladimir Sementsov-Ogievskiy]

20.01.2020 19:03, Max Reitz wrote:
> On 16.12.19 13:17, Vladimir Sementsov-Ogievskiy wrote:
>> Make it more obvious how to add new fields to the version 3 header and
>> how to interpret them.
>>
>> The specification is adjusted so for new defined optional fields:
>>
>> 1. Software may support some of these optional fields and ignore the
>>     others, which means that features may be backported to downstream
>>     Qemu independently.
>> 2. If we want to add incompatible field (or a field, for which some its
>>     values would be incompatible), it must be accompanied by
>>     incompatible feature bit.
>>
>> Also the concept of "default is zero" is clarified, as it's strange to
>> say that the value of the field is assumed to be zero for the software
>> version which don't know about the field at all and don't know how to
>> treat it be it zero or not.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> ---
>>   docs/interop/qcow2.txt | 39 ++++++++++++++++++++++++++++++++++++---
>>   1 file changed, 36 insertions(+), 3 deletions(-)
> 
> I put review of this off for so long because I always waited for Eric to
> give his R-b, but maybe not.
> 
> I generally think that he’s stricter on what to write in documentation,
> and accordingly I only have nit picks on spelling and structure:

This is very helpful too, thanks.

I'll resend now with your suggestions, to make it easier to read for others.

> 
>> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
>> index af5711e533..d92c827763 100644
>> --- a/docs/interop/qcow2.txt
>> +++ b/docs/interop/qcow2.txt
>> @@ -79,9 +79,9 @@ The first cluster of a qcow2 image contains the file header:
>>                       Offset into the image file at which the snapshot table
>>                       starts. Must be aligned to a cluster boundary.
>>   
>> -If the version is 3 or higher, the header has the following additional fields.
>> -For version 2, the values are assumed to be zero, unless specified otherwise
>> -in the description of a field.
>> +For version 2, the header is exactly 72 bytes in length, and finishes here.
>> +For version 3 or higher, the header length is at least 104 bytes, including
>> +the next fields through header_length.
>>   
>>            72 -  79:  incompatible_features
>>                       Bitmask of incompatible features. An implementation must
>> @@ -164,6 +164,39 @@ in the description of a field.
>>           100 - 103:  header_length
>>                       Length of the header structure in bytes. For version 2
>>                       images, the length is always assumed to be 72 bytes.
>> +                    For version 3 it's at least 104 bytes and must be a multiply
> 
> s/multiply/multiple/
> 
>> +                    of 8.
>> +
>> +Additional fields (version 3 and higher)
> 
> If this is supposed to be a heading, maybe it should enclosed by “===”
> on both sides.
> 
>> +
>> +In general, these fields are optional and may be safely ignored by the software,
>> +as well as filled by zeros (which is equal to field absence), if software needs
>> +to set field B, but don't want to care about field A, which precedes B. More
> 
> s/don't/does not/ (or maybe s/don't want/does not/)
> 
>> +formally, additional fields have the following compatibility rules:
>> +
>> +1. If the value of the additional field must not be ignored for correct
>> +handling of the file, it will be accompanied by a corresponding incompatible
>> +feature bit.
>> +
>> +2. If there are no unrecognized incompatible feature bits set, an unknown
>> +additional field may be safely ignored other than preserving its value when
>> +rewriting the image header.
>> +
>> +3. An explicit value of 0 will have the same behavior as when the field is not
>> +present*, if not altered by specific incompatible bit.
> 
> s/by specific/by a specific/
> 
>> +
>> +*. Field is not present when header_length is less or equal to field's offset.
> 
> s/Field/A field/, s/field's/the field's/
> 
> (maybe also +considered, as in "A field is considered not present...")
> 
>> +Also, all additional fields are not present for version 2.
>> +
>> +        < ... No additional fields in the header currently ... >
> 
> This looks a bit weird to me, but the next patch will remove it again,
> so who cares.
> 
>> +Header padding
> 
> Same heading note here (I’d make this “=== Header padding ===”).
> 
>> +
>> +@header_length must be a multiply of 8, which means that if last additional field
> 
> s/multiply/multiple/
> 
>> +end is not aligned, some padding is needed. This padding must be zeroed, so that,
> 
> I think s/last additional field end/the last additional field’s end/, or
> maybe s/last additional field end/the end of the last additional field/.
> 
>> +if some existing (or future) additional field will fall into the padding, it
>> +will be interpreted accordingly to point [3.] of the previous paragraph, i.e.
>> +in same manner as when this field is not present.
> 
> s/in same/in the same/
> 
>>   
> 
> I think there should be a new heading here
> (“=== Header extensions ===”).
> 
> Max
> 
>>   Directly after the image header, optional sections called header extensions can
>>   be stored. Each extension has a structure like the following:
>>
> 


-- 
Best regards,
Vladimir