Improve xattr property documentation
Extend the xattr property section of zfs(8) such that it covers both styles of supported xattr. A short discussion of the benefits and drawbacks of each type is presented to allow users to make an informed choice. Signed-off-by: Massimo Maggi <me@massimo-maggi.eu> Signed-off-by: Richard Yao <ryao@gentoo.org> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Issue #170
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@ -1176,11 +1176,33 @@ Controls whether regular files should be scanned for viruses when a file is open
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.ne 2
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.mk
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.na
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\fB\fBxattr\fR=\fBon\fR | \fBoff\fR\fR
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\fB\fBxattr\fR=\fBon\fR | \fBoff\fR | \fBsa\fR\fR
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.ad
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.sp .6
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.RS 4n
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Controls whether extended attributes are enabled for this file system. The default value is \fBon\fR.
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Controls whether extended attributes are enabled for this file system. Two
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styles of extended attributes are supported either directory based or system
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attribute based.
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.sp
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The default value of \fBon\fR enables directory based extended attributes.
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This style of xattr imposes no practical limit on either the size or number of
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xattrs which may be set on a file. Although under Linux the \fBgetxattr\fR(2)
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and \fBsetxattr\fR(2) system calls limit the maximum xattr size to 64K. This
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is the most compatible style of xattr and it is supported by the majority of
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ZFS implementations.
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.sp
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System attribute based xattrs may be enabled by setting the value to \fBsa\fR.
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The key advantage of this type of xattr is improved performance. Storing
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xattrs as system attributes significantly decreases the amount of disk IO
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required. Up to 64K of xattr data may be stored per file in the space reserved
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for system attributes. If there is not enough space available for an xattr then
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it will be automatically written as a directory based xattr. System attribute
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based xattrs are not accessable on platforms which do not support the
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\fBxattr=sa\fR feature.
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.sp
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The use of system attribute based xattrs is strongly encouraged for users of
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SELinux or Posix ACLs. Both of these features heavily rely of xattrs and
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benefit significantly from the reduced xattr access time.
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.RE
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.sp
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