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zfs(8): improve document of compression behaviours 

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Signed-off-by: DHE <git@dehacked.net>
Closes #4660 
Closes #8423
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DeHackEd 2019-02-25 14:10:16 -05:00 committed by Brian Behlendorf
parent 8eef997679
commit ba7b05cb25
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man/man8/zfs.8
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@ -1275,6 +1275,19 @@ compression algorithm compresses runs of zeros.
This property can also be referred to by its shortened column name
.Sy compress .
Changing this property affects only newly-written data.
.Pp
When any setting except
.Sy off
is selected, compression will explicitly check for blocks consisting of only
zeroes (the NUL byte). When a zero-filled block is detected, it is stored as
a hole and not compressed using the indicated compression algorithm.
.Pp
Any block being compressed must be no larger than 7/8 of its original size
after compression, otherwise the compression will not be considered worthwhile
and the block saved uncompressed. Note that when the logical block is less than
8 times the disk sector size this effectively reduces the necessary compression
ratio; for example 8k blocks on disks with 4k disk sectors must compress to 1/2
or less of their original size.
.It Xo
.Sy context Ns = Ns Sy none Ns | Ns
.Em SELinux_User:SElinux_Role:Selinux_Type:Sensitivity_Level