Created zfs-check (markdown)

zfs-check.md

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+# Checking backups with zfs-check
+
+## What does it do?
+
+zfs-check is a tool to generate checksum streams of zfs datasets. (it also can be used on regular block devices and files)
+
+Its kind of like sha1sum but incremental: It generates a hash per "chunk" of data. This allows you to use it on huge datasets and only do a partial check of the data.
+
+## Why?
+
+A tool like this wouldn't seem necessary for ZFS: ZFS has full check summing, so you can be 100% sure the data doesn't get corrupted, right?
+
+While this is true to a certain extend, it IS possible that data gets corrupted during transfer with zfs send/recv. This can happen because of certain bugs in ZFS. For example: https://github.com/openzfs/zfs/issues/12762
+
+In such cases the checksums are still OK, but they are of the wrong (corrupt) data.
+
+So a tool to actually compare the data of 2 datasets was needed, hence zfs-check.
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