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+id = 1139
+title = "block/nbd.c and drive backup to a remote nbd server"
+state = "closed"
+created_at = "2022-08-03T09:26:02.923Z"
+closed_at = "2024-05-13T12:24:52.546Z"
+labels = ["block:nbd"]
+url = "https://gitlab.com/qemu-project/qemu/-/issues/1139"
+host-os = "Debian 10"
+host-arch = "n/a"
+qemu-version = "EMU emulator version 6.2.0 (Debian 1:6.2+dfsg-2~bpo11+1)"
+guest-os = "n/a"
+guest-arch = "n/a"
+description = """Good afternoon!
+
+I trying to copy attached drive content to remote NBD server via drive-backup QMP method. I'he tested two very similar ways but with very different performance. First is a backuping to exported NBD at another server. Second way is a backuping to same server but with connecting to /dev/nbd*. 
+
+Exporting qcow2 via nbd:
+```
+(nbd) ~ # qemu-nbd -p 12345 -x backup --cache=none --aio=native --persistent -f qcow2 backup.qcow2
+
+(qemu) ~ # qemu-img info nbd://10.0.0.1:12345/backup
+image: nbd://10.0.0.1:12345/backup
+file format: raw
+virtual size: 10 GiB (10737418240 bytes)
+disk size: unavailable
+```
+
+Starting drive backuping via QMP:
+
+```
+{
+\t"execute": "drive-backup",
+\t"arguments": {
+\t\t"device": "disk",
+\t\t"sync": "full",
+\t\t"target": "nbd://10.0.0.1:12345/backup",
+\t\t"mode": "existing"
+\t}
+}
+```
+
+With process starting qemu notifying about warning:
+
+> warning: The target block device doesn't provide information about the block size and it doesn't have a backing file. The default block size of 65536 bytes is used. If the actual block size of the target exceeds this default, the backup may be unusable
+
+And backup process is limited by speed around 30MBps, watched by iotop
+
+
+Second way to creating backup
+
+Exporting qcow2 via nbd:
+```
+(nbd) ~ # qemu-nbd -p 12345 -x backup --cache=none --aio=native --persistent -f qcow2 backup.qcow2
+```
+
+```
+(qemu) ~ # qemu-img info nbd://10.0.0.1:12345/backup
+image: nbd://10.0.0.1:12345/backup
+file format: raw
+virtual size: 10 GiB (10737418240 bytes)
+disk size: unavailable
+(qemu) ~ # qemu-nbd -c /dev/nbd0 nbd://10.0.0.1:12345/backup
+(qemu) ~ # qemu-img info /dev/nbd0
+image: /dev/nbd0
+file format: raw
+virtual size: 10 GiB (10737418240 bytes)
+disk size: 0 B
+```
+
+Starting drive backuping via QMP to local nbd device:
+
+```
+{
+\t"execute": "drive-backup",
+\t"arguments": {
+\t\t"device": "disk",
+\t\t"sync": "full",
+\t\t"target": "/dev/nbd0",
+\t\t"mode": "existing"
+\t}
+}
+```
+
+Backup process started without previous warning, and speed limited around 100MBps (network limit)
+
+So I have question: how I can get same performance without connection network device to local block nbd device at the qemu host?
+
+Kind regards"""
+reproduce = "n/a"
+additional = "n/a"