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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
This commit is contained in:
Brian Behlendorf 2013-10-28 11:57:15 -07:00
parent fc9e0530c9
commit 7c2448a33e
1 changed files with 24 additions and 2 deletions
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26
man/man8/zfs.8
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@ -1176,11 +1176,33 @@ Controls whether regular files should be scanned for viruses when a file is open
.ne 2
.mk
.na
\fB\fBxattr\fR=\fBon\fR | \fBoff\fR\fR
\fB\fBxattr\fR=\fBon\fR | \fBoff\fR | \fBsa\fR\fR
.ad
.sp .6
.RS 4n
Controls whether extended attributes are enabled for this file system. The default value is \fBon\fR.
Controls whether extended attributes are enabled for this file system. Two
styles of extended attributes are supported either directory based or system
attribute based.
.sp
The default value of \fBon\fR enables directory based extended attributes.
This style of xattr imposes no practical limit on either the size or number of
xattrs which may be set on a file. Although under Linux the \fBgetxattr\fR(2)
and \fBsetxattr\fR(2) system calls limit the maximum xattr size to 64K. This
is the most compatible style of xattr and it is supported by the majority of
ZFS implementations.
.sp
System attribute based xattrs may be enabled by setting the value to \fBsa\fR.
The key advantage of this type of xattr is improved performance. Storing
xattrs as system attributes significantly decreases the amount of disk IO
required. Up to 64K of xattr data may be stored per file in the space reserved
for system attributes. If there is not enough space available for an xattr then
it will be automatically written as a directory based xattr. System attribute
based xattrs are not accessable on platforms which do not support the
\fBxattr=sa\fR feature.
.sp
The use of system attribute based xattrs is strongly encouraged for users of
SELinux or Posix ACLs. Both of these features heavily rely of xattrs and
benefit significantly from the reduced xattr access time.
.RE
.sp