80% vs 90% Disk Space Usage Warning

[Mark Holtz]

So, I'm trying to manage the disk space usage on my FreeNAS TrueNAS system. My system was constructed five years ago with eight 5TB drives in a RAIDZ2 configuration, giving me an effective 27TB of storage depending on the math you use. I got a warning a few months ago after hitting 80% disk space used, but with some tricks I pulled off (reducing the number of backup copies used by my computers backup programs, reducing the number of snapshots, clearing out some old snapshots that got missed in the automatic cleanup), I managed to reduce the disk space used to 69%. I also used this as an opportunity to save up for some new NAS hard drives, but alas, that is still some time off, and if the semiconductor shortage hits the hard drive sector, ouch! I just noticed a price jump at Amazon today for NAS drives.

Anyways.... I have a question about the disk space warning in TrueNAS. They are:

• I thought the worry point for disk space usage is 90%, at which point, TrueNAS starts operating in a degraded state due to the way ZFS works. Is this now a lower threshold?
• Is the 80% more of a FYI warning? If so, can I change this to a 85% warning?

 

[sretalla]

• I thought the worry point for disk space usage is 90%, at which point, TrueNAS starts operating in a degraded state due to the way ZFS works. Is this now a lower threshold?
• Is the 80% more of a FYI warning? If so, can I change this to a 85% warning?

Depends what you're doing... 50% can be a worry if you want maximum performance and are doing block storage (seems not your case though).

There have been recent threads discussing it and indeed the concern is mostly around performance at 80% and for pool safety above 90/95% (remembering a 100% full pool can't perform any operations, not even delete).

I don't think the warning level is configurable.

You should act according to your needs and concerns.

 

[jgreco]

• I thought the worry point for disk space usage is 90%, at which point, TrueNAS starts operating in a degraded state due to the way ZFS works. Is this now a lower threshold?
• Is the 80% more of a FYI warning? If so, can I change this to a 85% warning?

For lightweight usage, you can probably get to 95% without the world ending. It might be slow as hell writing big stuff. It's not recommended to pass that point. It may be possible to get to a point where pool operations become nearly impossible, as @sretalla notes. Do panic a bit if you manage to get it up to 98%. ;-)

People get all irate about this and sometimes start spewing about how this doesn't happen on a "normal" filesystem like FFS. Except that these people forget that Berkeley UFS/FFS actually DOES reserve 8% of the disk; there are comments in tunefs(8) that are strangely parallel to the things we say about ZFS. The difference is that FFS artificially lowers the 100% mark, meaning that it is actually possible to write to more than 100% capacity on FFS.

That does not happen with ZFS, which just tells you the numbers.

 

[Mark Holtz]

Depends what you're doing... 50% can be a worry if you want maximum performance and are doing block storage (seems not your case though).

Primary use is file storage for my backups, secondary is for file storage, plus it's a Plex Media Server.