Missing good documentation for TRIM on L2ARC cache devices

[mabod]

I just added an SSD as cache device to my main pool (4 HDD RAID10). But I did not find any good documentation about how the trim operation is performaned on SSD cache devices.

I read about persistent and non-persistent L2ARC. I read about parameters like l2arc_trim_ahead and l2arc_write_max and I also read PR #9789

But I still have no clue if and how trim is performed on my new cache SSD. I still do not understand what I need to configure. etc. etc. Should I go for a persistent l2arc? Why? How?

From my point view the easiest setup would be if a trim is performant on every import of a pool.

[amotin]

Persistent L2ARC is enabled by default, unless you disable it via module parameter. But AFAIK it should have nothing to do with TRIM. Generally TRIM for L2ARC is a questionable concept, since L2ARC is written sequentially, and so in some opinions should not cause excessive flash fragmentation to solve with TRIM. But if you wish, according to zpoolprops(7): "TRIM on L2ARC devices is enabled by setting l2arc_trim_ahead > 0" and "autotrim=on".

[mabod]

Thank you, but where can I read about this? Especially about autotrim=on in conjunction with cache devices.

[amotin]

Is section for autotrim in man 7 zpoolprops not enough? Please propose the extension.

[mabod]

To me it is not clear that the autotrim property includes log and cache devices. If that is the case a "zpool trim" command would also do the trick. But I have 4 HDD in the pool plus the SSD for cache. And when I initiate a zpool trim for that pool, I get the error that trim is not supported.

╰─# zpool status zstore               
  pool: zstore
 state: ONLINE
  scan: scrub repaired 0B in 05:42:11 with 0 errors on Tue Oct  1 05:46:43 2024
config:

	NAME                               STATE     READ WRITE CKSUM
	zstore                             ONLINE       0     0     0
	  mirror-0                         ONLINE       0     0     0
	    WD40EFRX-68N32N0-WCC7K4JK1111  ONLINE       0     0     0
	    WD40EFRX-68N32N0-WCC7K6YX2222  ONLINE       0     0     0
	  mirror-1                         ONLINE       0     0     0
	    WD40EFRX-68N32N0-WCC7KA693333  ONLINE       0     0     0
	    WD40EFRX-68N32N0-WC6ALFKX4444  ONLINE       0     0     0
	cache
	  sde                              ONLINE       0     0     0

╰─# zpool trim zstore  
cannot trim: no devices in pool support trim operations

Why would autotrim for that pool than work?

PS
I am 100 % sure that the SSD (dev/sde) is trimming fine. I mounted it as xfs filesystem and successfully did an fstrim before I added it as cache device to the pool.

[amotin]

Both zpool trim (manual TRIM) and autotrim work for special and log vdevs, since they are pretty much a normal vdevs from space allocation point of view. L2ARC vdevs are different, they require explicit TRIM support. As I commented in other ticket, autotrim is supported there if you enable it via the module parameter. zpool trim I guess may be not implemented, but it may not have much sense in many cases when persistent L2ARC is full of data.

[mabod]

Thank you for your explanations. Very helpful for me. I will close this issue now.