ZFS - how to use a file as a ZIL

Terence Eden — Sun, 16 Apr 2023 11:34:51 +0000

ZFS sometimes benefits from having a logging drive. Usually, you need to assign a whole device or partition to it. Unfortunately, I had already partitioned my SSD and didn't feel like repartitioning it.

So, here's how to create a file, mount it as a loopback, and then assign it to your ZFS pool as a logging drive.

Create a 1GB file somewhere on your SSD:
- fallocate -l 1G zil.img
Mount the file as a loopback device:
- sudo losetup -fP zil.img
Find the name of the new loopback device:
- losetup -a
Add the new loopback device to your pool:
- sudo zpool add your_pool log /dev/loop123
Wait a few moments and then check it has been added using:
- zpool status
Note - the loop won't be recreated after a reboot, so your ZFS will be a bit grumpy. Remove it with:
- sudo zpool remove data loop123

Is there any point in doing this? No, not really. In my brief experiments it didn't make any appreciable different to the speed of my writes. Your use-case may be different though.

ZFS – Terence Eden’s Blog

ZFS - how to use a file as a ZIL