[PVE-User] ZFS Disk Usage unexpected high

Yanni M. gianni.milo22 at gmail.com
Mon Apr 5 11:54:40 CEST 2021

This is a common issue on raidz based pools. Assuming 4k sectors
(ashift=12) and zvol with 8K volblocksize, each 8K (2-sector) block uses a
single sector (4k) of parity. So 15TB of 8KB blocks (default
volblocksize=8k) takes up at least 22.5TB space on disk (including parity).
You will use less parity by increasing the volblock size (e.g.
volblocksize=32k, or the default recordsize=128k for filesystems) in
exchange of a possible lower performance. Another solution would be using a
pool of striped mirrors (RAID10). This problem does not exist in such pools
(as there are no parity blocks used).

On Mon, 5 Apr 2021 at 09:23, Konold, Martin <martin.konold at konsec.com>

> Hi,
> I set up a single VM which currently used 15TB of data.
> /dev/sdb is technically a ZFS volume on the Proxmox Host.
> [root at vm ~]# df -h /data
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/sdb         40T   15T   25T  37% /data
> [root at vm ~]# du -s /data/
> 14874345100     /data/
> [root at host /]# zfs list zfs01/PVE-BE/vm-1-disk-1
> NAME                         USED  AVAIL     REFER  MOUNTPOINT
> zfs01/PVE-BE/vm-1-disk-1  31.5T  5.82T     31.5T  -
> Why is the usage on the host about twice as large as within the vm?
> (Yes, I have given fstrim and discard a try).
> --
> Regards
> ppa. Martin Konold
> --
> Martin Konold - Prokurist, CTO
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