This probably applies to normal ext4 filesystems as well as LVM ones. <\/p>\n
This post is because I deleted 60Gb of data from a qcow2 disk, and when I did a compress only got the disk image to shrink about 5Gb. Obviously something was not working as I expected.<\/p>\n
In the past my qcow images used only ext4 filesystems so my initial thought was “bugger, why did I use LVM”… but then I started thinking. What is available to compress ?.<\/p>\n
Silly me… if you delete 60Gb of data of course you will not get space back, deleting a file only removes indexes (vtoc entries, pointer to the file etc) and the actual data is still clogging the disk until it is overwritten; a qemu-img compress won’t be able to get data back while that junk is still left there.<\/p>\n
I wrote zeros to the free space on the disk… as it was a live running system I tried to guestimate a safe filesize, I guessed wrong and filled the filesystem, thats life.<\/p>\n
\r\n[mark@vosprey2 ~]$ dd if=\/dev\/zero of=junk count=1000000 bs=1024000\r\ndd: error writing \u2018junk\u2019: No space left on device\r\n80188+0 records in\r\n80187+0 records out\r\n82112401408 bytes (82 GB) copied, 28142.7 s, 2.9 MB\/s\r\n[mark@vosprey2 ~]$ df -k .\/\r\nFilesystem 1K-blocks Used Available Use% Mounted on\r\n\/dev\/mapper\/fedora-root 99653104 94338024 229896 100% \/\r\n[mark@vosprey2 ~]$ rm junk\r\n[mark@vosprey2 ~]$ df -k .\/\r\nFilesystem 1K-blocks Used Available Use% Mounted on\r\n\/dev\/mapper\/fedora-root 99653104 14149764 80418156 15% \/\r\n[mark@vosprey2 ~]$ \r\n<\/pre>\nThen shutdown the VM and use qemu-img to compress the qcow2 disk image. Well that certainly resolved my problem.<\/p>\n
\r\n[root@vmhost1 VM_disks]# ls -ltr osp*\r\n-rw-r--r--. 1 root root 68162533376 Sep 9 22:17 osprey_bkp_aftercompress.qcow2 < == compress of a 75Gb disk before I wrote zeros\r\n-rw-r--r--. 1 root root 93995008000 Sep 11 18:12 osprey_precomp.qcow2 <== filesize after writing zeros filled the disk\r\n-rw-r--r--. 1 root root 11430384640 Sep 11 18:54 osprey.qcow2 <== size after compress of zero filled filesystem\r\n<\/pre>\nWell. I got well over the 60Gb I expected back.<\/p>\n
So qemu-img compress works great, if you do a little manual work before hand to make sure it is able to compress :-).<\/p>\n","protected":false},"excerpt":{"rendered":"
This probably applies to normal ext4 filesystems as well as LVM ones. This post is because I deleted 60Gb of data from a qcow2 disk, and when I did a compress only got the disk image to shrink about 5Gb. … Continue reading