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Comment by icantthinkofone | Hacker News Reader

I'm a bit confused. I always thought that proper use of ZFS required multiple drives just like RAID. Then I read an article where RAID was used with ZFS. Now I'm really confused so I'm taking the lazy way and asking here: is ZFS appropriate for a single drive system?

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Sure, COW(copy on write), snapshots, error checksum on files, compression, support for SSD's. You just don't have redundancy.

I thought the error checking (scrubbing) only works if you have multiple drives set up in a RAID.

ZFS writes a checksum when it writes data. So you can tell whether your data is corrupted, even if you don't have the redundancy to restore the corrupted data.

Error detection can always be used, but error correction may or may not be available (it depends on the type of block). Metadata blocks are redundant even on a single drive pool; so if you just have a partial failure (e.g. overwrite a metadata block) it might be able to correct the block using another redundant copy on the same drive. Data blocks will require a redundant pool configuration, though, as these are not store redundantly by default (e.g. multiple drives in a raidz or mirror).

Actually you can correct data blocks, on a single-drive ZFS pool, if you set the attribute copies=2 (ditto blocks): https://blogs.oracle.com/relling/entry/zfs_copies_and_data_p... Obviously this redundancy feature makes you use twice the disk space you would normally use.

yup, that's why I added "by default". a user won't get that functionality without explicitly asking for it.

It only works in the sense that if it finds an error it could recover when it is a multi-drive RAID, but a scrub can verify that the data on disk is still whole and matches what it expects.

Not only that, but a scrub can have ZFS tell you exactly what files are now "damaged".

You can also use it on a single drive. You need to set copies=n for a ZFS dataset. If one copy gets corrupted a zpool scrub fixes this problem on a single drive. However HDDs are usually dying fast after signalling bad blocks to the OS so it's not a good strategy to avoid data loss.

Besides that the metadata is still replicated in a single-drive setup. So if you loose data you'll at least know exactly where the error was. This is still a benefit over ext4.

Yes, I run ZFS on Linux on my single drive laptop currently.

ZFS doesn't _need_ multiple drives to work well, but it is capable of using them if they're available. One can also run a HW raid solution underneath ZFS, where the RAID engine just presents ZFS with a small number of LUNs. It's all up to the admin; and what sort of performance, redundancy, and maintenance guarantees are required.

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