4 ms·

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

sp332 12y ago
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.
prakashsurya 12y ago
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).
- mrb 12y ago
  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_protection 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.
  - prakashsurya 12y ago
    yup, that's why I added "by default". a user won't get that functionality without explicitly asking for it.
X-Istence 12y ago
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".
nisa 12y ago
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.

sp332 12y ago

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.

prakashsurya 12y ago

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).

mrb 12y ago
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_protection 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.
- prakashsurya 12y ago
  yup, that's why I added "by default". a user won't get that functionality without explicitly asking for it.

X-Istence 12y ago

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".

nisa 12y ago

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.