Re: [Qemu-devel] [PATCH] [RFC] qcow2: add compression type feature

[Stefano Garzarella]

Hi Denis,
few simple comments below regarding the CODING_STYLE (point 7. Comment style)

On Tue, Feb 05, 2019 at 12:08:25PM +0300, Denis Plotnikov wrote:
> The patch adds some preparation parts for incompatible compression type
> feature into 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 convertion, thus the only compression
> algorithm is used for the image.
> 
> The plan is to add support for ZSTD and then may be something more effective
> in the future.
> 
> ZSDT compression algorithm consumes 3-5 times less CPU power with a
> comparable comression ratio with zlib. It would be wise to use it for
> data compression f.e. for backups.
> 
> The default compression is ZLIB.
> 
> Signed-off-by: Denis Plotnikov <address@hidden>
> ---
>  block/qcow2.c | 25 +++++++++++++++++++++++++
>  block/qcow2.h | 26 ++++++++++++++++++++++----
>  2 files changed, 47 insertions(+), 4 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 8c91b92865..cb3d6cc1c0 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -73,6 +73,7 @@ typedef struct {
>  #define  QCOW2_EXT_MAGIC_FEATURE_TABLE 0x6803f857
>  #define  QCOW2_EXT_MAGIC_CRYPTO_HEADER 0x0537be77
>  #define  QCOW2_EXT_MAGIC_BITMAPS 0x23852875
> +#define  QCOW2_EXT_MAGIC_COMPRESSION_TYPE 0x434D5052
>  
>  static int coroutine_fn
>  qcow2_co_preadv_compressed(BlockDriverState *bs,
> @@ -397,6 +398,9 @@ static int qcow2_read_extensions(BlockDriverState *bs, 
> uint64_t start_offset,
>  #endif
>              break;
>  
> +        case QCOW2_EXT_MAGIC_COMPRESSION_TYPE:
> +            /* Setting compression type to BDRVQcow2State->compression_type 
> */
> +            /* from the image header is going to be here */

I'd use a multiline comment block here.

>          default:
>              /* unknown magic - save it in case we need to rewrite the header 
> */
>              /* If you add a new feature, make sure to also update the fast
> @@ -2431,6 +2435,11 @@ 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,
> @@ -2461,6 +2470,22 @@ int qcow2_update_header(BlockDriverState *bs)
>          buflen -= ret;
>      }
>  
> +    /* Compression type extension */
> +    if (s->compression_type != 0) {
> +        Qcow2CompressionTypeExt comp_header = {
> +            .compression_type = cpu_to_be32(s->compression_type),
> +        };
> +        ret = header_ext_add(buf, QCOW2_EXT_MAGIC_COMPRESSION_TYPE,
> +                             &comp_header,
> +                             cpu_to_be64(sizeof(comp_header)),
> +                             buflen);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +        buf += ret;
> +        buflen -= ret;
> +    }
> +
>      /* Keep unknown header extensions */
>      QLIST_FOREACH(uext, &s->unknown_header_ext, next) {
>          ret = header_ext_add(buf, uext->magic, uext->data, uext->len, 
> buflen);
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 32cce9eee2..fdde5bbefd 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -112,6 +112,10 @@
>  #define QCOW2_OPT_REFCOUNT_CACHE_SIZE "refcount-cache-size"
>  #define QCOW2_OPT_CACHE_CLEAN_INTERVAL "cache-clean-interval"
>  
> +/* Compression types */
> +#define QCOW2_COMPRESSION_TYPE_ZLIB    0
> +#define QCOW2_COMPRESSION_TYPE_ZSTD    1
> +
>  typedef struct QCowHeader {
>      uint32_t magic;
>      uint32_t version;
> @@ -197,10 +201,13 @@ enum {
>  
>  /* Incompatible feature bits */
>  enum {
> -    QCOW2_INCOMPAT_DIRTY_BITNR   = 0,
> -    QCOW2_INCOMPAT_CORRUPT_BITNR = 1,
> -    QCOW2_INCOMPAT_DIRTY         = 1 << QCOW2_INCOMPAT_DIRTY_BITNR,
> -    QCOW2_INCOMPAT_CORRUPT       = 1 << QCOW2_INCOMPAT_CORRUPT_BITNR,
> +    QCOW2_INCOMPAT_DIRTY_BITNR            = 0,
> +    QCOW2_INCOMPAT_CORRUPT_BITNR          = 1,
> +    QCOW2_INCOMPAT_COMPRESSION_TYPE_BITNR = 2,
> +    QCOW2_INCOMPAT_DIRTY                  = 1 << QCOW2_INCOMPAT_DIRTY_BITNR,
> +    QCOW2_INCOMPAT_CORRUPT                = 1 << 
> QCOW2_INCOMPAT_CORRUPT_BITNR,
> +    QCOW2_INCOMPAT_COMPRESSION_TYPE       =
> +                                    1 << 
> QCOW2_INCOMPAT_COMPRESSION_TYPE_BITNR,
>  
>      QCOW2_INCOMPAT_MASK          = QCOW2_INCOMPAT_DIRTY
>                                   | QCOW2_INCOMPAT_CORRUPT,
> @@ -256,6 +263,10 @@ typedef struct Qcow2BitmapHeaderExt {
>      uint64_t bitmap_directory_offset;
>  } QEMU_PACKED Qcow2BitmapHeaderExt;
>  
> +typedef struct Qcow2CompressionTypeExt {
> +    uint32_t compression_type;
> +} QEMU_PACKED Qcow2CompressionTypeExt;
> +
>  typedef struct BDRVQcow2State {
>      int cluster_bits;
>      int cluster_size;
> @@ -340,6 +351,13 @@ typedef struct BDRVQcow2State {
>  
>      CoQueue compress_wait_queue;
>      int nb_compress_threads;
> +    /**

I'd remove the second asterisk here.

> +     * 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 compresion type set
> +     */
> +    uint32_t compression_type;
>  } BDRVQcow2State;
>  
>  typedef struct Qcow2COWRegion {
> -- 
> 2.17.0
> 
> 

Thanks,
Stefano