News for hackers. Stuff that matters.

A comment on How does rsync work?. Parent comment

Re:How does rsync work? by formerly_proven on Saturday July 02, 2022 @10:17AM (#31959170)
Unless you are also doing FS-level deduplication using the same checksums, it generally makes no sense for these to be cryptographic hashes, so they're not necessarily suitable for this purpose.

IIRC neither ZFS nor btrfs use cryptographic hashes for checksumming by default.

[ Parent | on HN ]
Re:How does rsync work? by throw0101a on Saturday July 02, 2022 @10:33AM (#31959311)
> on is a short hand for fletcher4 for non-deduped datasets and sha256 for deduped datasets

* https://openzfs.github.io/openzfs-docs/Basic%20Concepts/Chec...

* https://people.freebsd.org/~asomers/fletcher.pdf

* https://en.wikipedia.org/wiki/Fletcher%27s_checksum

Strangely enough SHA-512 is actually (50%) faster than -256:

> ZFS actually uses a special version of SHA512 called SHA512t256, it uses a different initial value, and truncates the results to 256 bits (that is all the room there is in the block pointer). The advantage is only that it is faster on 64 bit CPUs.

* https://twitter.com/klarainc/status/1367199461546614788

[ Parent | on HN ]
Re:How does rsync work? by slavik81 on Saturday July 02, 2022 @04:02PM (#31961862)
It seems that SHA512t256 is another name for SHA-512/256. It's a shame that the initialization is different from SHA-512, as it would have been very useful to be able to convert a SHA-512 hash into a SHA-512/256 hash.
[ Parent | on HN ]