drive-backup: added support for data compression

The idea is simple - backup is "written-once" data. It is written block
by block and it is large enough. It would be nice to save storage
space and compress it.

The patch adds a flag to the qmp/hmp drive-backup command which enables
block compression. Compression should be implemented in the format driver
to enable this feature.

There are some limitations of the format driver to allow compressed writes.
We can write data only once. Though for backup this is perfectly fine.
These limitations are maintained by the driver and the error will be
reported if we are doing something wrong.
Signed-off-by: NPavel Butsykin <pbutsykin@virtuozzo.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NDenis V. Lunev <den@openvz.org>
CC: Jeff Cody <jcody@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: John Snow <jsnow@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/backup.c

@@ -47,6 +47,7 @@ typedef struct BackupBlockJob {
     uint64_t sectors_read;
     unsigned long *done_bitmap;
     int64_t cluster_size;
+    bool compress;
     NotifierWithReturn before_write;
     QLIST_HEAD(, CowRequest) inflight_reqs;
 } BackupBlockJob;
@@ -154,7 +155,8 @@ static int coroutine_fn backup_do_cow(BackupBlockJob *job,
                                        bounce_qiov.size, BDRV_REQ_MAY_UNMAP);
         } else {
             ret = blk_co_pwritev(job->target, start * job->cluster_size,
-                                 bounce_qiov.size, &bounce_qiov, 0);
+                                 bounce_qiov.size, &bounce_qiov,
+                                 job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
         }
         if (ret < 0) {
             trace_backup_do_cow_write_fail(job, start, ret);
@@ -477,6 +479,7 @@ static void coroutine_fn backup_run(void *opaque)
 void backup_start(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, int64_t speed,
                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
+                  bool compress,
                   BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   BlockCompletionFunc *cb, void *opaque,
@@ -507,6 +510,12 @@ void backup_start(const char *job_id, BlockDriverState *bs,
         return;
     }
 
+    if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
+        error_setg(errp, "Compression is not supported for this drive %s",
+                   bdrv_get_device_name(target));
+        return;
+    }
+
     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
         return;
     }
@@ -555,6 +564,7 @@ void backup_start(const char *job_id, BlockDriverState *bs,
     job->sync_mode = sync_mode;
     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
                        sync_bitmap : NULL;
+    job->compress = compress;
 
     /* If there is no backing file on the target, we cannot rely on COW if our
      * backup cluster size is smaller than the target cluster size. Even for

blockdev.c

@@ -3132,6 +3132,9 @@ static void do_drive_backup(DriveBackup *backup, BlockJobTxn *txn, Error **errp)
     if (!backup->has_job_id) {
         backup->job_id = NULL;
     }
+    if (!backup->has_compress) {
+        backup->compress = false;
+    }
 
     bs = qmp_get_root_bs(backup->device, errp);
     if (!bs) {
@@ -3210,8 +3213,8 @@ static void do_drive_backup(DriveBackup *backup, BlockJobTxn *txn, Error **errp)
     }
 
     backup_start(backup->job_id, bs, target_bs, backup->speed, backup->sync,
-                 bmap, backup->on_source_error, backup->on_target_error,
-                 block_job_cb, bs, txn, &local_err);
+                 bmap, backup->compress, backup->on_source_error,
+                 backup->on_target_error, block_job_cb, bs, txn, &local_err);
     bdrv_unref(target_bs);
     if (local_err != NULL) {
         error_propagate(errp, local_err);
@@ -3277,7 +3280,7 @@ void do_blockdev_backup(BlockdevBackup *backup, BlockJobTxn *txn, Error **errp)
         }
     }
     backup_start(backup->job_id, bs, target_bs, backup->speed, backup->sync,
-                 NULL, backup->on_source_error, backup->on_target_error,
+                 NULL, false, backup->on_source_error, backup->on_target_error,
                  block_job_cb, bs, txn, &local_err);
     if (local_err != NULL) {
         error_propagate(errp, local_err);

hmp-commands.hx

@@ -1182,8 +1182,8 @@ ETEXI
 
     {
         .name       = "drive_backup",
-        .args_type  = "reuse:-n,full:-f,device:B,target:s,format:s?",
-        .params     = "[-n] [-f] device target [format]",
+        .args_type  = "reuse:-n,full:-f,compress:-c,device:B,target:s,format:s?",
+        .params     = "[-n] [-f] [-c] device target [format]",
         .help       = "initiates a point-in-time\n\t\t\t"
                       "copy for a device. The device's contents are\n\t\t\t"
                       "copied to the new image file, excluding data that\n\t\t\t"
@@ -1191,7 +1191,9 @@ ETEXI
                       "The -n flag requests QEMU to reuse the image found\n\t\t\t"
                       "in new-image-file, instead of recreating it from scratch.\n\t\t\t"
                       "The -f flag requests QEMU to copy the whole disk,\n\t\t\t"
-                      "so that the result does not need a backing file.\n\t\t\t",
+                      "so that the result does not need a backing file.\n\t\t\t"
+                      "The -c flag requests QEMU to compress backup data\n\t\t\t"
+                      "(if the target format supports it).\n\t\t\t",
         .mhandler.cmd = hmp_drive_backup,
     },
 STEXI

hmp.c

@@ -1109,6 +1109,7 @@ void hmp_drive_backup(Monitor *mon, const QDict *qdict)
     const char *format = qdict_get_try_str(qdict, "format");
     bool reuse = qdict_get_try_bool(qdict, "reuse", false);
     bool full = qdict_get_try_bool(qdict, "full", false);
+    bool compress = qdict_get_try_bool(qdict, "compress", false);
     Error *err = NULL;
     DriveBackup backup = {
         .device = (char *)device,
@@ -1118,6 +1119,8 @@ void hmp_drive_backup(Monitor *mon, const QDict *qdict)
         .sync = full ? MIRROR_SYNC_MODE_FULL : MIRROR_SYNC_MODE_TOP,
         .has_mode = true,
         .mode = reuse ? NEW_IMAGE_MODE_EXISTING : NEW_IMAGE_MODE_ABSOLUTE_PATHS,
+        .has_compress = !!compress,
+        .compress = compress,
     };
 
     if (!filename) {

include/block/block_int.h

@@ -765,6 +765,7 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
 void backup_start(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, int64_t speed,
                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
+                  bool compress,
                   BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   BlockCompletionFunc *cb, void *opaque,

qapi/block-core.json

@@ -898,6 +898,9 @@
 #          Must be present if sync is "incremental", must NOT be present
 #          otherwise. (Since 2.4)
 #
+# @compress: #optional true to compress data, if the target format supports it.
+#            (default: false) (since 2.7)
+#
 # @on-source-error: #optional the action to take on an error on the source,
 #                   default 'report'.  'stop' and 'enospc' can only be used
 #                   if the block device supports io-status (see BlockInfo).
@@ -915,7 +918,7 @@
 { 'struct': 'DriveBackup',
   'data': { '*job-id': 'str', 'device': 'str', 'target': 'str',
             '*format': 'str', 'sync': 'MirrorSyncMode', '*mode': 'NewImageMode',
-            '*speed': 'int', '*bitmap': 'str',
+            '*speed': 'int', '*bitmap': 'str', '*compress': 'bool',
             '*on-source-error': 'BlockdevOnError',
             '*on-target-error': 'BlockdevOnError' } }
 

qmp-commands.hx

@@ -1217,7 +1217,8 @@ EQMP
     {
         .name       = "drive-backup",
         .args_type  = "job-id:s?,sync:s,device:B,target:s,speed:i?,mode:s?,"
-                      "format:s?,bitmap:s?,on-source-error:s?,on-target-error:s?",
+                      "format:s?,bitmap:s?,compress:b?,"
+                      "on-source-error:s?,on-target-error:s?",
         .mhandler.cmd_new = qmp_marshal_drive_backup,
     },
 
@@ -1253,6 +1254,8 @@ Arguments:
 - "mode": whether and how QEMU should create a new image
           (NewImageMode, optional, default 'absolute-paths')
 - "speed": the maximum speed, in bytes per second (json-int, optional)
+- "compress": true to compress data, if the target format supports it.
+              (json-bool, optional, default false)
 - "on-source-error": the action to take on an error on the source, default
                      'report'.  'stop' and 'enospc' can only be used
                      if the block device supports io-status.