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[lvm-devel] [PATCH 22/35] Define vgmetadatacopies in vgchange man page.



This patch adds a vgmetadatacopies parameter for metadata balancing.
This parameter provides a simple way for users to create a policy for
placing metadata on PVs automatically by LVM.  The behavior is implemented
inside LVM by managing the 'ignore' mda bits.  We chose the name
'vgmetadatacopies' as this is a natural extension to the existing parameter
'pvmetadatacopies' / 'metadatacopies' in pvcreate.

This is a first step at VG parameter based metadata balancing.  Most users
will probably want to state that they want a certain number of PVs to contai
metadata, and they may be less concerned about a specific number of metadata
copies in the volume group.  However, for default values (pvmetadatacopies
is 1 by default), the number of metadatacopies in the volume group, and the
number of PVs with metadata are the same.  In the future we could add
vgmetadatacopiespvs to define more specifically the number of pvs in the
VG that contain metadata, but for now we start with this parameter.

Another possible future extension would be to define a specific pv tag
to mark the set of PVs that should be used for metadata balancing.  This
tag based approach could be used in conjunction with 'vgmetadatacopies'.

Signed-off-by: Dave Wysochanski <dwysocha redhat com>
---
 man/vgchange.8.in |   27 ++++++++++++++++++++-------
 1 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/man/vgchange.8.in b/man/vgchange.8.in
index 2fc0f8b..eb4e4b9 100644
--- a/man/vgchange.8.in
+++ b/man/vgchange.8.in
@@ -25,6 +25,8 @@ vgchange \- change attributes of a volume group
 .IR MaxLogicalVolumes ]
 .RB [ -p | \-\-maxphysicalvolumes
 .IR MaxPhysicalVolumes ]
+.RB [ \-\-[vg]metadatacopies ]
+.IR copies ]
 .RB [ \-P | \-\-partial]
 .RB [ \-s | \-\-physicalextentsize
 .IR PhysicalExtentSize [ \fBbBsSkKmMgGtTpPeE\fR ]]
@@ -127,13 +129,24 @@ volume group.
 Changes the maximum number of physical volumes that can belong
 to this volume group.
 For volume groups with metadata in lvm1 format, the limit is 255.
-If the metadata uses lvm2 format, the value 0
-removes this restriction: there is then no limit.
-If you have a large number of physical volumes in
-a volume group with metadata in lvm2 format,
-for tool performance reasons, you should consider
-some use of \fB--pvmetadatacopies 0\fP
-as described in \fBpvcreate(8)\fP.
+If the metadata uses lvm2 format, the value 0 removes this restriction:
+there is then no limit.  If you have a large number of physical volumes in
+a volume group with metadata in lvm2 format, for tool performance reasons,
+you should consider some use of \fB--pvmetadatacopies 0\fP as described in
+\fBpvcreate(8)\fP, and/or use \fB--vgmetadatacopies\fP.
+.TP
+.BR \-\-vgmetadatacopies " " \fI#copies\fR
+Sets the targetted number of metadata copies in the volume group.
+If set to a non-zero value, LVM will attempt to use \fB#copies\fP to
+determine the set of physical volumes on which to read and write metadata.
+LVM manages the set automatically by setting or clearing the 'metadataignore'
+flag on the physical volumes (see \fBpvchange\fP), and in this way balances
+metadata across PVs in the volume group.  Only physical volumes created
+with a non-zero value of \fBmetadatacopies\fP are used in the set of physical
+volumes.  This option is useful for volume groups containing large numbers
+of physical volumes containing metadata as it may be used to minimize metadata
+read and write overhead.
+Setting \fB--vgmetadatacopies\fP to 0 disables LVM metadata balancing.
 .TP
 .BR \-s ", " \-\-physicalextentsize " " \fIPhysicalExtentSize\fR[\fBbBsSkKmMgGtTpPeE\fR]
 Changes the physical extent size on physical volumes of this volume group.
-- 
1.6.0.6


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