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