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

[Denis Plotnikov]

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 */
         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;
+    /**
+     * 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