qcow2 image usage

# qcow2 image usage ###### tags: `linux` `qcow2` #### create qcow2 image ``` /usr/bin/qemu-img create -f qcow2 -o preallocation=metadata /export/vmimgs/glacier.qcow2 8G ``` > NOTE: At this point in time, preallocation=metadata option is the best we can do to extract max. possible (near RAW) I/O performance out of QCOW2 format. #### show actual usage of image ``` [root@moon tmp]# ls -lash /export/vmimgs/glacier.qcow2 970M -rw-r--r--. 1 qemu qemu 8.1G Sep 24 23:45 /export/vmimgs/glacier.qcow2 ``` #### resize image ``` qemu-img resize ubuntu-server.qcow2 +5GB ``` #### mount qcow2 image load NBD kernel module ``` root@NUC10:~# modprobe nbd max_part=8 nbds_max=4 root@NUC10:~# ls -l /dev/nbd* brw-rw---- 1 root disk 43, 0 May 28 20:49 /dev/nbd0 brw-rw---- 1 root disk 43, 4 May 28 20:49 /dev/nbd1 brw-rw---- 1 root disk 43, 8 May 28 20:49 /dev/nbd2 brw-rw---- 1 root disk 43, 12 May 28 20:49 /dev/nbd3 ``` > max_part - Number of partitions per device (default: 0). > nbds_max - Number of block devices that should be initialized (default: 16). connect image as a NBD ``` root@NUC10:~# qemu-nbd --connect=/dev/nbd0 /mnt/data/storage-pool/lab/lab-test.qcow2 root@NUC10:~# fdisk -l /dev/nbd0 -l Disk /dev/nbd0: 16 GiB, 17179869184 bytes, 33554432 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: gpt Disk identifier: 029F65E0-4F19-4249-A6C1-3D3D3B82F0CF Device Start End Sectors Size Type /dev/nbd0p1 2048 4095 2048 1M BIOS boot /dev/nbd0p2 4096 33552383 33548288 16G Linux filesystem root@NUC10:~# ls -l /dev/nbd* brw-rw---- 1 root disk 43, 0 May 28 20:51 /dev/nbd0 brw-rw---- 1 root disk 43, 1 May 28 20:51 /dev/nbd0p1 brw-rw---- 1 root disk 43, 2 May 28 20:51 /dev/nbd0p2 brw-rw---- 1 root disk 43, 4 May 28 20:49 /dev/nbd1 brw-rw---- 1 root disk 43, 8 May 28 20:49 /dev/nbd2 brw-rw---- 1 root disk 43, 12 May 28 20:49 /dev/nbd3 ``` > The linux filesystem partion is on /dev/nbd0p2 mount the filesystem partition on the image ``` root@NUC10:~# mkdir /tmp/qcow2-image root@NUC10:~# mount /dev/nbd0p2 /tmp/qcow2-image/ root@NUC10:~# ls -l /tmp/qcow2-image/ total 2097240 lrwxrwxrwx 1 root root 7 Feb 2 2021 bin -> usr/bin drwxr-xr-x 3 root root 4096 Mar 23 00:25 boot drwxr-xr-x 2 root root 4096 Sep 12 2021 cdrom drwxr-xr-x 5 root root 4096 Feb 2 2021 dev drwxr-xr-x 94 root root 4096 Mar 23 00:24 etc drwxr-xr-x 3 root root 4096 Sep 12 2021 home lrwxrwxrwx 1 root root 7 Feb 2 2021 lib -> usr/lib lrwxrwxrwx 1 root root 9 Feb 2 2021 lib32 -> usr/lib32 lrwxrwxrwx 1 root root 9 Feb 2 2021 lib64 -> usr/lib64 lrwxrwxrwx 1 root root 10 Feb 2 2021 libx32 -> usr/libx32 drwx------ 2 root root 16384 Sep 12 2021 lost+found drwxr-xr-x 2 root root 4096 Feb 2 2021 media drwxr-xr-x 2 root root 4096 Feb 2 2021 mnt drwxr-xr-x 2 root root 4096 Feb 2 2021 opt drwxr-xr-x 2 root root 4096 Apr 15 2020 proc drwx------ 4 root root 4096 Mar 27 17:48 root drwxr-xr-x 11 root root 4096 Feb 2 2021 run lrwxrwxrwx 1 root root 8 Feb 2 2021 sbin -> usr/sbin drwxr-xr-x 7 root root 4096 Sep 18 2021 snap drwxr-xr-x 2 root root 4096 Feb 2 2021 srv -rw------- 1 root root 2147483648 Sep 12 2021 swap.img drwxr-xr-x 2 root root 4096 Apr 15 2020 sys drwxrwxrwt 8 root root 4096 Mar 27 17:49 tmp drwxr-xr-x 14 root root 4096 Feb 2 2021 usr drwxr-xr-x 13 root root 4096 Feb 2 2021 var ``` To unmount ``` root@NUC10:~# umount /tmp/qcow2-image root@NUC10:~# qemu-nbd --disconnect /dev/nbd0 /dev/nbd0 disconnected root@NUC10:~# rmmod nbd ``` #### Reference https://www.kernel.org/doc/Documentation/blockdev/nbd.txt