Lessen Scrub Impact on ZFS Storage

[vix9]

On one of my 4 nodes (all with identical hardware specs), the early stages of the monthly ZFS scrub push the IO_Delay on the host to very, very high levels causing some issues with the VMs on that host. In the past on my FreeNAS boxes, I used the tunable vfs.zfs.scrub_delay to lessen the scrub impact while it was running. I just would like to make sure I'm going about making this change in the right way to see if it has a positive effect. I could need to edit/create /etc/modprobe.d/zfs.conf and add the following line::

options zfs zfs_scrub_delay=8 (or whatever number works best, this worked best for me on FreeNAS)

Then run modprobe zfs.

What am I going to break by doing this?

 

[UdoB]

Good morning,
I am not sure what you might break, but the possible configuration settings are these: ~# modinfo zfs. "zfs_scrub_delay" is not listed there...

Best regards

 

[Dunuin]

Its in the ZFS documentation so it should also work on linux: https://openzfs.github.io/openzfs-docs/Performance and Tuning/Module Parameters.html#zfs-scrub-delay

zfs_scrub_delay​

zfs_scrub_delay sets a time-based delay for scrub I/Os. This delay is in addition to the ZIO scheduler’s treatment of scrub workloads. See also zfs_scan_idle

zfs_scrub_delay	Notes
Tags	scrub, ZIO_scheduler
When to change	increasing can reduce impact of scrub workload on dynamic workloads
Data Type	int
Units	clock ticks
Range	0 to MAX_INT
Default	4
Change	Dynamic
Versions Affected	v0.6.0

You set ZFS options by editing (you might needto crete it first) the file "/etc/modprobe.d/zfs.conf". There you could aad a line like options zfs zfs_scrub_delay=8. You definitely lso need to reboot after doing changes. Not sure if you dditionally need to do a update-initramfs -u.

 

[UdoB]

Its in the ZFS dodumentation so it should also work on linux

Interesting. "modinfo" lists 290 parameters... but no "scrub_delay". Perhaps it was not integrated during compile time?

~# modinfo zfs | head -2; modinfo zfs | grep scrub_delay; modinfo zfs | grep ^parm | wc -l
filename:       /lib/modules/5.15.35-2-pve/zfs/zfs.ko
version:        2.1.4-pve1
290

 

[fabian]

this parameter doesn't exist anymore since ZFS on Linux 0.8 .. https://github.com/openzfs/zfs/commit/d4a72f23863382bdf6d0ae33196f5b5decbc48fd

 

[mathx]

so then how to reduce impact?

 

[Nemesiz]

https://openzfs.github.io/openzfs-docs/Performance and Tuning/Module Parameters.html#scrub