[v6,1/3] qcow2: introduce compression type feature

The patch adds some preparation parts for incompatible compression type
feature to QCOW2 header that indicates that *all* compressed clusters
must be (de)compressed using a certain compression type.

It is implied that the compression type is set on the image creation and
can be changed only later by image conversion, thus compression type
defines the only compression algorithm used for the image.

The goal of the feature is to add support of other compression algorithms
to qcow2. For example, ZSTD which is more effective on compression than ZLIB.
It works roughly 2x faster than ZLIB providing a comparable compression ratio
and therefore provides a performance advantage in backup scenarios.

The default compression is ZLIB. Images created with ZLIB compression type
are backward compatible with older qemu versions.

Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/qcow2.c             | 91 +++++++++++++++++++++++++++++++++++++++
 block/qcow2.h             | 26 ++++++++---
 docs/interop/qcow2.txt    | 18 +++++++-
 include/block/block_int.h |  1 +
 qapi/block-core.json      | 22 +++++++++-
 5 files changed, 148 insertions(+), 10 deletions(-)

05.09.2019 12:31, Denis Plotnikov wrote:
> The patch adds some preparation parts for incompatible compression type
> feature to QCOW2 header that indicates that *all* compressed clusters
> must be (de)compressed using a certain compression type.
> 
> It is implied that the compression type is set on the image creation and
> can be changed only later by image conversion, thus compression type
> defines the only compression algorithm used for the image.
> 
> The goal of the feature is to add support of other compression algorithms
> to qcow2. For example, ZSTD which is more effective on compression than ZLIB.
> It works roughly 2x faster than ZLIB providing a comparable compression ratio
> and therefore provides a performance advantage in backup scenarios.
> 
> The default compression is ZLIB. Images created with ZLIB compression type
> are backward compatible with older qemu versions.
> 
> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> ---
>   block/qcow2.c             | 91 +++++++++++++++++++++++++++++++++++++++
>   block/qcow2.h             | 26 ++++++++---
>   docs/interop/qcow2.txt    | 18 +++++++-
>   include/block/block_int.h |  1 +
>   qapi/block-core.json      | 22 +++++++++-
>   5 files changed, 148 insertions(+), 10 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 039bdc2f7e..2884b9d9f2 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1197,6 +1197,32 @@ static int qcow2_update_options(BlockDriverState *bs, QDict *options,
>       return ret;
>   }
>   
> +static int check_compression_type(BDRVQcow2State *s, Error **errp)
> +{
> +    switch (s->compression_type) {
> +    case QCOW2_COMPRESSION_TYPE_ZLIB:
> +        break;
> +
> +    default:
> +        error_setg(errp, "qcow2: unknown compression type: %u",
> +                   s->compression_type);
> +        return -ENOTSUP;
> +    }
> +
> +    /*
> +     * if the compression type differs from QCOW2_COMPRESSION_TYPE_ZLIB
> +     * the incompatible feature flag must be set
> +     */
> +
> +    if (s->compression_type != QCOW2_COMPRESSION_TYPE_ZLIB &&
> +        !(s->incompatible_features & QCOW2_INCOMPAT_COMPRESSION_TYPE)) {
> +            error_setg(errp, "qcow2: Invalid compression type setting");
> +            return -EINVAL;
> +    }
> +
> +    return 0;
> +}
> +
>   /* Called with s->lock held.  */
>   static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
>                                         int flags, Error **errp)
> @@ -1312,6 +1338,35 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
>       s->compatible_features      = header.compatible_features;
>       s->autoclear_features       = header.autoclear_features;
>   
> +    /*
> +     * Handle compression type
> +     * Older qcow2 images don't contain the compression type header.
> +     * Distinguish them by the header length and use
> +     * the only valid (default) compression type in that case
> +     */
> +    if (header.header_length > offsetof(QCowHeader, compression_type)) {
> +        /* sanity check that we can read a compression type */
> +        size_t min_len = offsetof(QCowHeader, compression_type) +
> +                         sizeof(header.compression_type);
> +        if (header.header_length < min_len) {
> +            error_setg(errp,
> +                       "Could not read compression type, "
> +                       "qcow2 header is too short");
> +            ret = -EINVAL;
> +            goto fail;
> +        }
> +
> +        header.compression_type = be32_to_cpu(header.compression_type);
> +        s->compression_type = header.compression_type;
> +    } else {
> +        s->compression_type = QCOW2_COMPRESSION_TYPE_ZLIB;
> +    }
> +
> +    ret = check_compression_type(s, errp);
> +    if (ret) {
> +        goto fail;
> +    }
> +
>       if (s->incompatible_features & ~QCOW2_INCOMPAT_MASK) {
>           void *feature_table = NULL;
>           qcow2_read_extensions(bs, header.header_length, ext_end,
> @@ -2516,6 +2571,12 @@ int qcow2_update_header(BlockDriverState *bs)
>       total_size = bs->total_sectors * BDRV_SECTOR_SIZE;
>       refcount_table_clusters = s->refcount_table_size >> (s->cluster_bits - 3);
>   
> +    ret = check_compression_type(s, NULL);
> +
> +    if (ret) {
> +        goto fail;
> +    }
> +
>       *header = (QCowHeader) {
>           /* Version 2 fields */
>           .magic                  = cpu_to_be32(QCOW_MAGIC),
> @@ -2538,6 +2599,7 @@ int qcow2_update_header(BlockDriverState *bs)
>           .autoclear_features     = cpu_to_be64(s->autoclear_features),
>           .refcount_order         = cpu_to_be32(s->refcount_order),
>           .header_length          = cpu_to_be32(header_length),
> +        .compression_type       = cpu_to_be32(s->compression_type),
>       };
>   
>       /* For older versions, write a shorter header */
> @@ -2635,6 +2697,11 @@ int qcow2_update_header(BlockDriverState *bs)
>                   .bit  = QCOW2_COMPAT_LAZY_REFCOUNTS_BITNR,
>                   .name = "lazy refcounts",
>               },
> +            {
> +                .type = QCOW2_FEAT_TYPE_INCOMPATIBLE,
> +                .bit  = QCOW2_INCOMPAT_COMPRESSION_TYPE_BITNR,
> +                .name = "compression type",
> +            },
>           };
>   
>           ret = header_ext_add(buf, QCOW2_EXT_MAGIC_FEATURE_TABLE,
> @@ -3202,6 +3269,7 @@ qcow2_co_create(BlockdevCreateOptions *create_options, Error **errp)
>           .refcount_table_offset      = cpu_to_be64(cluster_size),
>           .refcount_table_clusters    = cpu_to_be32(1),
>           .refcount_order             = cpu_to_be32(refcount_order),
> +        .compression_type           = cpu_to_be32(QCOW2_COMPRESSION_TYPE_ZLIB),
>           .header_length              = cpu_to_be32(sizeof(*header)),
>       };
>   
> @@ -3221,6 +3289,21 @@ qcow2_co_create(BlockdevCreateOptions *create_options, Error **errp)
>               cpu_to_be64(QCOW2_AUTOCLEAR_DATA_FILE_RAW);
>       }
>   
> +    if (qcow2_opts->has_compression_type &&
> +        qcow2_opts->compression_type != QCOW2_COMPRESSION_TYPE_ZLIB) {
> +
> +        switch (qcow2_opts->compression_type) {
> +        default:
> +            error_setg_errno(errp, -EINVAL, "Unknown compression type");
> +            goto out;
> +        }
> +
> +        header->compression_type = cpu_to_be32(qcow2_opts->compression_type);
> +
> +        header->incompatible_features |=
> +            cpu_to_be64(QCOW2_INCOMPAT_COMPRESSION_TYPE);
> +    }
> +
>       ret = blk_pwrite(blk, 0, header, cluster_size, 0);
>       g_free(header);
>       if (ret < 0) {
> @@ -3402,6 +3485,7 @@ static int coroutine_fn qcow2_co_create_opts(const char *filename, QemuOpts *opt
>           { BLOCK_OPT_ENCRYPT,            BLOCK_OPT_ENCRYPT_FORMAT },
>           { BLOCK_OPT_COMPAT_LEVEL,       "version" },
>           { BLOCK_OPT_DATA_FILE_RAW,      "data-file-raw" },
> +        { BLOCK_OPT_COMPRESSION_TYPE,   "compression-type" },
>           { NULL, NULL },
>       };
>   
> @@ -4598,6 +4682,7 @@ static ImageInfoSpecific *qcow2_get_specific_info(BlockDriverState *bs,
>               .data_file          = g_strdup(s->image_data_file),
>               .has_data_file_raw  = has_data_file(bs),
>               .data_file_raw      = data_file_is_raw(bs),
> +            .compression_type   = s->compression_type,
>           };
>       } else {
>           /* if this assertion fails, this probably means a new version was
> @@ -5163,6 +5248,12 @@ static QemuOptsList qcow2_create_opts = {
>               .help = "Width of a reference count entry in bits",
>               .def_value_str = "16"
>           },
> +        {
> +            .name = BLOCK_OPT_COMPRESSION_TYPE,
> +            .type = QEMU_OPT_STRING,
> +            .help = "Compression method used for image clusters compression",
> +            .def_value_str = "zlib"
> +        },
>           { /* end of list */ }
>       }
>   };
> diff --git a/block/qcow2.h b/block/qcow2.h
> index fc1b0d3c1e..659e4f852a 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -140,6 +140,7 @@ typedef struct QCowHeader {
>   
>       uint32_t refcount_order;
>       uint32_t header_length;
> +    uint32_t compression_type;
>   } QEMU_PACKED QCowHeader;
>   
>   typedef struct QEMU_PACKED QCowSnapshotHeader {
> @@ -203,16 +204,20 @@ enum {
>   
>   /* Incompatible feature bits */
>   enum {
> -    QCOW2_INCOMPAT_DIRTY_BITNR      = 0,
> -    QCOW2_INCOMPAT_CORRUPT_BITNR    = 1,
> -    QCOW2_INCOMPAT_DATA_FILE_BITNR  = 2,
> -    QCOW2_INCOMPAT_DIRTY            = 1 << QCOW2_INCOMPAT_DIRTY_BITNR,
> -    QCOW2_INCOMPAT_CORRUPT          = 1 << QCOW2_INCOMPAT_CORRUPT_BITNR,
> -    QCOW2_INCOMPAT_DATA_FILE        = 1 << QCOW2_INCOMPAT_DATA_FILE_BITNR,
> +    QCOW2_INCOMPAT_DIRTY_BITNR            = 0,
> +    QCOW2_INCOMPAT_CORRUPT_BITNR          = 1,
> +    QCOW2_INCOMPAT_DATA_FILE_BITNR        = 2,
> +    QCOW2_INCOMPAT_COMPRESSION_TYPE_BITNR = 3,
> +    QCOW2_INCOMPAT_DIRTY                  = 1 << QCOW2_INCOMPAT_DIRTY_BITNR,
> +    QCOW2_INCOMPAT_CORRUPT                = 1 << QCOW2_INCOMPAT_CORRUPT_BITNR,
> +    QCOW2_INCOMPAT_DATA_FILE              = 1 << QCOW2_INCOMPAT_DATA_FILE_BITNR,
> +    QCOW2_INCOMPAT_COMPRESSION_TYPE       =
> +        1 << QCOW2_INCOMPAT_COMPRESSION_TYPE_BITNR,
>   
>       QCOW2_INCOMPAT_MASK             = QCOW2_INCOMPAT_DIRTY
>                                       | QCOW2_INCOMPAT_CORRUPT
> -                                    | QCOW2_INCOMPAT_DATA_FILE,
> +                                    | QCOW2_INCOMPAT_DATA_FILE
> +                                    | QCOW2_INCOMPAT_COMPRESSION_TYPE,
>   };
>   
>   /* Compatible feature bits */
> @@ -359,6 +364,13 @@ typedef struct BDRVQcow2State {
>   
>       bool metadata_preallocation_checked;
>       bool metadata_preallocation;
> +    /*
> +     * Compression type used for the image. Default: 0 - ZLIB
> +     * The image compression type is set on image creation.
> +     * The only way to change the compression type is to convert the image
> +     * with the desired compression type set
> +     */
> +    Qcow2CompressionType compression_type;
>   } BDRVQcow2State;
>   
>   typedef struct Qcow2COWRegion {
> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index af5711e533..59e09caacf 100644
> --- a/docs/interop/qcow2.txt
> +++ b/docs/interop/qcow2.txt
> @@ -109,7 +109,12 @@ in the description of a field.
>                                   An External Data File Name header extension may
>                                   be present if this bit is set.
>   
> -                    Bits 3-63:  Reserved (set to 0)
> +                    Bit 3:      Compression type bit. The bit must be set if
> +                                the compression type differs from default of zlib.
> +                                If the compression type is default the bit should
> +                                be unset.
> +
> +                    Bits 4-63:  Reserved (set to 0)
>   
>            80 -  87:  compatible_features
>                       Bitmask of compatible features. An implementation can
> @@ -165,6 +170,17 @@ in the description of a field.
>                       Length of the header structure in bytes. For version 2
>                       images, the length is always assumed to be 72 bytes.
>   
> +        104 - 107:  compression_type
> +                    Defines the compression method used for compressed clusters.
> +                    A single compression type is applied to all compressed image
> +                    clusters.
> +                    The default compression type is zlib (value: 0).
> +                    When the compression type differs from the default
> +                    the compression type bit (incompatible feature bit 3)
> +                    must be set.
> +                    Available compression type values:
> +                        0: zlib <https://www.zlib.net/> (default)
> +
>   Directly after the image header, optional sections called header extensions can
>   be stored. Each extension has a structure like the following:
>   

Hmmm

We have this  above  "incompatible_features" field:

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.

So, now we actually documented that all v3 images has compression_type field.. But it's not so.
At least it should be reworded, like:

If the version is 3 or higher, the header has the following additional fields, up to the header_length field (inclusive).
For version 2, the values are assumed to be zero, unless specified otherwise
in the description of a field.


And then, after header_length field, we should add something like:

The following fields may present, if header_length is large enough. However, header_length must be aligned to some field
(or should? I think "should" is better, as it was not documented before)

Also, we need to write somewhere, that software which don't know about new fields may use header_length and it is safe. It definitely
means that all new fields we are going to add are compatible by default (old software may ignore it and may rewrite header to be smaller,
and it is safe).

Actually, in this patch, we add not only incompatible extension, we also add compatible header expansion (with incompatible bit unset, but header is larger)...

Then, let's look at possible combinations of header_length and compression type feature

incompat_flag  length          type
0              old             <absent>    # old image. It is should be still valid, so we need to fix spec like I proposed above
0              old < l < new   <?>         # Should we consider it invalid, or old one?
0              new <= l        0           # new (or from future) image with default zlib, compatible. Old software knows where to find qcow2 extensions, as we have valid header len
0              new <= l        t != 0      # broken, invalid image. Software which don't know about compression type will misread it, software which knows will not open
1              old             <absent>    # Hmmm, is it correct? I will be opened after this patch as valid, but it does _not_ correspond to specification..
1              old < l < new   <?>         # Considered as invalid by this patch, but this is not documented
1              new <= l        0           # new (or from future) default zlib, incomaptible for no reason, but still valid
1              new <= l        t != 0      # new (or from future) not zlib, incompatible, valid


> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 3aa1e832a8..4b254802e5 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -58,6 +58,7 @@
>   #define BLOCK_OPT_REFCOUNT_BITS     "refcount_bits"
>   #define BLOCK_OPT_DATA_FILE         "data_file"
>   #define BLOCK_OPT_DATA_FILE_RAW     "data_file_raw"
> +#define BLOCK_OPT_COMPRESSION_TYPE  "compression_type"
>   
>   #define BLOCK_PROBE_BUF_SIZE        512
>   
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 0d43d4f37c..2c002ca6a9 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -78,6 +78,8 @@
>   #
>   # @bitmaps: A list of qcow2 bitmap details (since 4.0)
>   #
> +# @compression-type: the image cluster compression method (since 4.2)
> +#
>   # Since: 1.7
>   ##
>   { 'struct': 'ImageInfoSpecificQCow2',
> @@ -89,7 +91,8 @@
>         '*corrupt': 'bool',
>         'refcount-bits': 'int',
>         '*encrypt': 'ImageInfoSpecificQCow2Encryption',
> -      '*bitmaps': ['Qcow2BitmapInfo']
> +      '*bitmaps': ['Qcow2BitmapInfo'],
> +      'compression-type': 'Qcow2CompressionType'
>     } }
>   
>   ##
> @@ -4274,6 +4277,18 @@
>     'data': [ 'v2', 'v3' ] }
>   
>   
> +##
> +# @Qcow2CompressionType:
> +#
> +# Compression type used in qcow2 image file
> +#
> +# @zlib:  zlib compression, see <http://zlib.net/>
> +#
> +# Since: 4.2
> +##
> +{ 'enum': 'Qcow2CompressionType',
> +  'data': [ 'zlib' ] }
> +
>   ##
>   # @BlockdevCreateOptionsQcow2:
>   #
> @@ -4297,6 +4312,8 @@
>   #                   allowed values: off, falloc, full, metadata)
>   # @lazy-refcounts   True if refcounts may be updated lazily (default: off)
>   # @refcount-bits    Width of reference counts in bits (default: 16)
> +# @compression-type The image cluster compression method
> +#                   (default: zlib, since 4.2)
>   #
>   # Since: 2.12
>   ##
> @@ -4312,7 +4329,8 @@
>               '*cluster-size':    'size',
>               '*preallocation':   'PreallocMode',
>               '*lazy-refcounts':  'bool',
> -            '*refcount-bits':   'int' } }
> +            '*refcount-bits':   'int',
> +            '*compression-type': 'Qcow2CompressionType' } }
>   
>   ##
>   # @BlockdevCreateOptionsQed:
>