Updated zfs check (markdown)

zfs-check.md

@@ -4,7 +4,7 @@
 
 zfs-check is a tool to generate checksum streams from a 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.
+Its kind of like sha1sum but incremental: It generates a sha1 hash per "chunk" of data. This allows you to use it on huge datasets and only do a partial check of the data.
 
 ## Why?
 
@@ -89,8 +89,30 @@ You can use it via a simple SSH pipe to compare to datasets:
 
 ```
 
-
+## Only checking a small sample of the total data
-
+
-
+You can use the `--skip` option to skip a certain amount of chunks. To only check 10% of your data use --skip 9. (It will check 1 block and then skip 9 blocks)
+
+```shell
+[root@pve1 ~]# zfs-check rpool/data/vm-101-disk-0@offsite-20220308020453 --skip 9 --verbose
+  zfs-check v3.2-alpha2 - (c)2022 E.H.Eefting (edwin@datux.nl)
+  
+  Target               : rpool/data/vm-101-disk-0@kantoor_offsite-20220308020453
+  Block size           : 4096 bytes
+  Block count          : 25600
+  Effective chunk size : 104857600 bytes
+  Skip chunk count     : 9 (checks 10.00% of data)
+  
+0	87a193d73a27aceb38334eca51d180493c9a2348
+10	2c2ceccb5ec5574f791d45b63c940cff20550f9a
+20	2c2ceccb5ec5574f791d45b63c940cff20550f9a
+30	6fdee2a737a0b4db519a70dc2ecd765a90a10bce
+```
+
+Normally you use this on the "generating" side.
+
+You can also use it on the "checking" side: It will then just skip this number of hashes. This is usefull if you already generated a 100% list of checksums, but you only want to compare a smaller sample of it.
+
+Note: Keep in mind that if you use --skip on both sides, it will skip 2 times. If you use --skip=9 on both sides, the sender will only send 10% and the checker will only check 10% of that amount. (resulting in a coverage of 1%)