Raid 5 & 6 are erasure coding as an example.

See my other comment but RAID 5/6 are not usually considered erasure coding implementations [0]. Mustly, because AFAIK RAID 5 and 6 aren't standards, so it depends on the controller or software manufacturer implementation. Some use XOR, some use Reed Solomon, some use other stuff...

For example, Linux MD RAID 5 uses XOR, and RAID 6 uses Galois fields. [1][2]

  0: https://www.itprotoday.com/storage/erasure-coding-vs-raid-which-right-and-when
  1: https://raid.wiki.kernel.org/index.php/A_guide_to_mdadm#Raid_5
  2: http://igoro.com/archive/how-raid-6-dual-parity-calculation-works/

Of course RAID 5 and 6 are erasure coding. They use a code (e.g. XOR/parity) to tolerate the behavior of a binary erasure channel (BEC). Systematic codes (where some of the code stripes are just identity chunks of the data), like RAID 5 and 6, are probably the most common case of erasure codes.

Now, we could have an argument about whether RAID 1 is an erasure code, but that wouldn't teach us much.

Well, I stand corrected (and I don't have much knowledge to argue about this).

Re-reading, I see that in the storage world "simple" correction systems don't tend to be classified as erasure coding.

Were I say "simple", I think I'd say any correction system that's not parametrizable, as in "let's do a 17+3 in this ordinary dataset and 4+2 in this one that's very important"

About RAID 1, technically probably it is... although bit very efficient :D

Technically xor is a special case of erasure coding.

https://en.wikipedia.org/wiki/RozoFS <-- never heard of it before I googled. But apparently this.

I wonder if Brtfs is using this for raid. Though last I read it Raid-5 was still broken.

- ZFS with data loss protection

- Ceph with data loss protection

- Any filesystem on linux-md without data loss protection, BTRFS on linux-md on synology

I'm not sure what are you referring to with "data loss protection" in ZFS, but if you refer to the 'copies' parameter, that's not erasure coding, that's just full copies of the data blocks [0]. They mostly only work for some sector errors on disk. If you lose a whole disk, THEY DO NOTHING. [1]

"The block pointer can store up to three copies of the data each pointed by a unique DVA. These blocks are referred to as “ditto” blocks in ZFS terminology." [2]

And although VMWare calls RAID5/6 "Erasure Coding" [3](!!), I'd say it's not [4].

  0: https://docs.oracle.com/en/operating-systems/solaris/oracle-solaris/11.4/manage-zfs/copies-property.html
  1: https://jrs-s.net/2016/05/02/zfs-copies-equals-n/
  2: https://pages.cs.wisc.edu/~kadav/zfs/zfsrel.pdf
  3: https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.virtualsan.doc/GUID-AD408FA8-5898-4541-9F82-FE72E6CD6227.html
  4: https://www.itprotoday.com/storage/erasure-coding-vs-raid-which-right-and-when

--

Edit: some extra info and links

Sorry I meant to write protection against data corruption (checksums)

Note that ZFS (and probably other fs) checksums are not data corruption protection, but data corruption detection ;)

If a checksum error is detected, you depend (mostly) on your RAIDZ protection to recover from it [0]. If you're on a single drive or striped array without mirrors or raidz1/2/3... you're SOL. (ditto blocks can help but they aren't the best. I'd only recommend them if you're stuck on a single drive setup without raid)

But you at least know that your data is corrupt, and that's very important too :)

  0: https://openzfs.github.io/openzfs-docs/Basic%20Concepts/Checksums.html

> For double or triple parity, we use a special case of Reed-Solomon coding

https://github.com/openzfs/zfs/blob/zfs-2.1-release/module/z...

It's not mainline yet but bcachefs[0][1][2] includes erasure coding as one of its defining features.

Maybe, if we're lucky, Kent Overstreet (koverstreet[3]) will magically appear and talk more on his latest creation.

[0] https://bcachefs.org/

[1] https://bcachefs.org/bcachefs-principles-of-operation.pdf

[2] https://www.patreon.com/bcachefs (de facto bcachefs blog)

[3] https://news.ycombinator.com/user?id=koverstreet

Oh, interesting! I still haven't played with bcachefs, so I didn't know you can do erasure coding replicas there. Although they says the implementation is still simple so it suffers from "write hole".

> Although they says the implementation is still simple so it suffers from "write hole".

That's what I was hoping Kent would show up and expand on. His March 29 update[0] says that erasure coding "has seen a lot of progress" which makes me hopeful.

[0] https://www.patreon.com/posts/your-irregular-80720060

I am very stoked for bcachefs to make it into the kernel.

While not a traditional file system, Tahoe-LAFS[0] uses erasure coding in its decentralized cloud store.

[0] https://tahoe-lafs.org/trac/tahoe-lafs