Disks
Disks are referred in
volume manager by two terms - device name and disk name
. The device name specifies controller , target id and slice of the disk
. Disk name is the common name given to the device name as an easy to remember
name .
For example device name
c2t3d0s2 represents controller number 2 , target id 3 , disk
group 0 and slice 2 and disk01 may be its disk name . While device name
is system dependent based on controller and disk id the disk name
is user defined .
·
A disk group
is a collection of volume manager disks grouped together to hold the data . All
the configuration changes made to a disk group are applied to the disks
in that disk group only.
·
Volume Manager
objects cannot span disk groups i.e. all the operations on a particular
disk group remains confined to that particular group .
·
Disk groups enable
high availability as these can be shared by two or more hosts but can be
accessed by only one host at a time. In two hosts and a shared storage situation
one host can take over the ownership of the disk groups and
drives in case other host fails.
·
Adding
physical disks to the volume manager results in creation of
public and private region in the disk by the volume manager .The public region is the disk space available for volume
space and the private region stores the configuration information.
·
A Volume Manager disks are created from
the public region of a physical disk that is under Volume Manager control. Each
volume manager disk corresponds to one physical disk.
·
A volume manager disk is given a disk
media name when it is added to a disk group which can be default or unique user
defined..
·
Once a volume manager disk is assigned a disk
media name, the disk is no longer referred to by its physical address of c#t#d#. The physical address of c#t#d# becomes known as the disk
access record.
·
A subdisk is a subsection of a disk's public
region and is the smallest unit of storage in Volume
Manager.
·
A subdisk is defined by an offset and a length
in sectors on a volume manager disk.
·
A volume manager disk can contain multiple
subdisks but subdisks cannot overlap or share the same portions of a
volume manager disk.
·
volume manager disk space that is not reserved
or that is not part of a subdisk is free space. You can use free space to
create new subdisks.
A subdisk is similar to a partition but with
following differences :
·
The maximum number of partitions to a disk is
eight.
·
There is no theoretical limit to number of
subdisks that can be attached to a single plex, but it has been limited to a
default value of 4096. If required, this default can be changed, using the vol_subdisk_num tunable parameter.
·
A plex
is a structured or ordered collection of subdisks that represents one copy of
the data in a volume. A plex consists
of one or more subdisks located on one or more physical disks.
·
A plex is also called a mirror. The terms plex
and mirror can be used interchangeably, even though a plex is only one copy of
the data. The terms "mirrored" or "mirroring" imply two or
more copies of data.
·
The length of a plex is determined by the last
block that can be read or written on the last subdisk in the plex.
·
The default naming convention for plexes in a
volume is volumename-##. The default plex name consists of
the volume name, a hyphen, and a two-digit number
·
A volume is composed of one or more plexes
not restricted by the physical size of the disk.
·
A volume can span across multiple disks.
·
Volume Manager uses the default naming
convention vol## for volumes,
where ## represents a two-digit number but can be user defined as per
requirement.
1.
Creating a Volume :
1.1
Creating a Concatenated Volume
By default, vxassist creates a concatenated volume using the space
available on a disk or on the number of disks in a diskgroup if the volume size
specified is more then the one available on a single disk.
Disks can be specified from a diskgroup for a volume group but if not
mentioned available disks are selected by the volume manager.
Command syntax :
· vxassist make volume_name
volume_length
To create a new volume appvol of 100 MB in the default
disk group rootdg with available disks:
· #vxassist make appvol 100m
To create the volume appvol of 100MB on disk03
· #vxassist make appvol 100m
disk03
1.2
Creating a Striped Volume
A striped volume contains at least one plex that consists of two or more
subdisks located on two or more physical disks.
Command Syntax :
· vxassist make volume_name
length layout=stripe
To create a striped volume appvol2 with the default stripe unit size
on the default number of disks
· #vxassist make appvol2 100m
layout=stripe
To create a striped volume appvol2 100MB striped volume on three
specific disks.
· #vxassist make appvol2 100m
layout=stripe disk04 disk05 disk06
1.3
Creating a RAID-5 Volume
A RAID-5 volume contains a RAID-5 plex that consists of two or more
subdisks located on two or more physical disks. Only one RAID-5 plex can exist
per volume. A RAID-5 volume may also contain one or more RAID-5 log plexes,
which are used to log information about data and parity being written to the
volume.
Command Syntax :
· vxassist make volume_name
length layout=raid5
To create the RAID-5 volume appvol4 with the default stripe unit size
on the default number of disks with RAID-5 log,
· #vxassist make appvol4 100m
layout=raid5.
2.
Extending a Volume
Caution : If you plan to use volume size change (grow or shrink ) on a
volume with data make sure you have the good backup of the volume available
before carrying out such operation .
2.1 Extending a volume up to certain length,
Command syntax
· vxassist growto volume_name
length
To extend vol3 upto 8000 sectors, type:
· #vxassist growto vol3 8000
2.2
Extending by a Given Length
Command Syntax
· vxassist growby volume_name
length
To extend volapp by 1000 sectors, type:
· #vxassist growby volapp 1000
3.
Shrinking a Volume
Caution - Do not shrink a volume below the size of the file system. If
you have a VxFS file system, you can shrink the file system and then shrink the
volume. If you do not shrink the file system first, you risk unrecoverable data
loss.
Always make sure you have a good backup of the data volume to be
shirnked.
3.1
Shrinking to a Given Length
Shrink a volume to a specific length as follows:
· vxassist shrinkto
volume_name length
Make sure you do not shrink the volume below the current size of the file
system or database using the volume. This command can be safely used on empty
volumes.
To shrink volcat to 1300 sectors, type:
· #vxassist shrinkto volcat
1300
3.2
Shrinking by a Given Length
Shrink a volume by a specific length as follows:
· vxassist shrinkby
volume_name length
To shrink volcat by 8000 sectors, type:
· #vxassist shrinkby volapp2
8000
4. Removing
a Volume
Removing a volume requires removing all references to the volumes to be
removed like unmounting the volume if mounted and removing its reference from
/etc/vfstab .
An active volume has to be stopped first to stop all the activities to
the volume only then it can be removed
Stopping Volume :
Command Syntax
· vxvol stop volume_name
To stop appvol1
· #vxvol stop appvol1
Removing Volume
Command Syntax
· vxedit -rf rm volume_name
To remove the volume appvol1
· #vxedit -rf rm appvol11
5.
Mirroring a Volume
Mirroring allow data to be written simultaneously to two disks so that in
case of failure of one disk data is available from the other mirrored disk.
Boot disks can be mirriored to provide alternate booting disk .
5.1 Creating & Mirroring a New Volume
Command Syntax :
· vxassist make volume_name
length layout=mirror
To create the mirrored volume, appvol22 of 100mb
· #vxassist make appvol22 100m
layout=mirror
To create a volume with Dirty Region Logging enabled
· #vxassist make volume_name
length layout=mirror,log
5.2
Mirroring an Existing Volume
Command syntax
· vxassist mirror volume_name
Creating a mirror of volume vol44:
· #vxassist mirror vol44
5.3 Mirroring
All Volumes
To mirror all existing volumes to available disk space
· /etc/vx/bin/vxmirror -a
6. Removing
a Mirror
Removing a mirror involves first dissociating the plex from its volume
and then removing the plex and any associated subdisks completely.The last
valid mirror plex associated with a volume cannot be removed.
Dissociate and remove the plex from its volume as follows:
· vxplex -o rm dis plex_name
vxprint -h shows the volume and associated plexes :
Disk group:
rootdg TY NAME ASSOC KSTATE
LENGTH PLOFFS
STATE TUTIL0 PUTIL0 v
volm fsgen ENABLED 10240 - ACTIVE - - pl volm-01 volm ENABLED 10240 - ACTIVE -
- sd disk01-02 volm-01 - 10240 0 - - - pl volm-02 volm ENABLED 10240 - ACTIVE -
- sd disk02-03 volm-02 - 10240 0 - - -
To disassociate and remove volmir-02 from volmir, type:
· #vxplex -o rm dis volm-02
The output of vxprint -h for volm now shows volume volm with only one
plex::
Disk group:
rootdg TY NAME ASSOC KSTATE
LENGTH PLOFFS
STATE TUTIL0 PUTIL0 v
volm fsgen ENABLED 10240 - ACTIVE - - pl volm-01 volmir ENABLED 10240 - ACTIVE
- - sd disk01-02 volmir-01 - 10240 0 - - -
7. Adding
a RAID-5 Log
A RAID-5 log maintains a copy of the data and parity being written to the
volume at any given time. If a system failure occurs, VxVM can replay the
RAID-5 log to resynchronize the volume. This copies the data and parity that
was being written at the time of failure from the log to the appropriate areas
of the RAID-5 volume.
RAID-5 log plexes are used to log information about data and parity being
written to the raid volume. A log plex is created for the volume by default
when a raid volume is created but additional ones can be added by following
command .
· vxassist addlog
name_of_raid5_vol
8. Adding a
DRL Log
Dirty region logging (DRL) is used with mirrored volume layouts. DRL
keeps track of the regions that have changed due to I/O writes to a mirrored
volume. Prior to every write, a bitmap is written to a log to record the area
of the disk that is being changed. In case of system failure, DRL uses this
information to recover only the portions of the volume that need to be
recovered.
To put Dirty Region Logging into effect for a volume, a log subdisk must
be added to that volume and the volume must be mirrored. Only one log subdisk
can exist per plex.
The following example creates a log for the mirrored volume vol03:
· #vxassist addlog vol03
When vxassist is used to add a log subdisk to a volume, a log plex is
also created to contain the log subdisk, by default.
Once created, the plex containing a log subdisk can be treated as a
regular plex. Data subdisks can be added to the log plex. The log plex and log
subdisk can be removed using the same procedures used to remove ordinary plexes
and subdisks.
9.
Removing a RAID-5 Log
Removing a RAID-5 log involves first dissociating the log from its volume
and then removing the log and any associated subdisks completely.
Dissociate the log from its volume as follows:
· #vxplex -o rm dis plex_name
To identify the log plex, use the command:
· vxprint -ht
raid5_volume_name
To disassociate the log plex volrd-02 from volrd, type:
· #vxplex -o rm dis volrd-02
The output of vxprint -h for volrd now shows:
Disk group:
rootdg TY NAME ASSOC KSTATE
LENGTH PLOFFS
STATE TUTIL0 PUTIL0 v
volrd raid5 ENABLED 32 - ACTIVE - - pl volrd-01 volrd ENABLED 32 - ACTIVE - -
sd disk01-02 volrd-01 - 32 0 - - - sd disk02-03 volrd-01 - 32 0 - - -
10. Preparing a Volume for Online Backup
Volume manager provides an easy way to take backup of the online data by
creating an offline snapshot mirror of the volume .
The vxassist snapstart operation creates a
write-only backup mirror, which is attached to and synchronized with the volume
to be backed up and later detached and made offline by vxassist snapshot
command . The snapshot volume can be used by backup utilities while the
original volume continues to be available for applications and users.
The volume backup method described here does not apply to RAID-5 volumes.
Backing up a volume with vxassist involves the following procedure
Create a snapshot mirror
snapstart starts creating a online snapshot mirror of the volume using
the available disk space . The snapshot is completed with vxassist snapshot
command when offline snapshot volume is created with a userdefined name .
Command Syntax:
- vxassist snapstart volume_name
To create a snapshot mirror of a volume called vol8, type
Create a snapshot volume
Command
Syntax
vxassist snapshot
volume_name new_volume_name
To create a snapshot volume of vol8, type:
#vxassist
snapshot vol8 snapvol8
You can now back up the snapshot volume by whatever means you prefer. To
avoid wasting space, you can then remove the snapshot volume, which occupies as
much space as the original volume
11 Displaying Volume Configuration Information
The vxprint command can be used to display information about how a volume
is configured.
Display the volume, mirror, and subdisk record information for all
volumes as follows:
#vxprint -ht
Disk group: rootdg
DG NAME NCONFIG NLOG MINORS GROUP-ID
DM NAME DEVICE TYPE PRIVLEN PUBLEN STATE
V NAME USETYPE KSTATE STATE LENGTH READPOL PREFPLEX
PL NAME VOLUME KSTATE STATE LENGTH LAYOUT NCOL/WID MODE
SD NAME PLEX DISK DISKOFFS LENGTH [COL/]OFF DEVICE MODE
dg rootdg 759096729.1025.tweety
dm disk10 c1t0d0s2 sliced 559 1044400 -
dm disk20 c2t0d0s2 sliced 559 1044400 -
v pubs fsgen ENABLED ACTIVE 2288 SELECT -
pl pubs-01 pubs ENABLED ACTIVE 2288 CONCAT - RW
sd disk10-01 pubs-01 disk10 0 2288 0 c0t0d0 ENA
v vol8 fsgen ENABLED ACTIVE 20480 SELECT -
pl vol8-01 vol8 ENABLED ACTIVE 20480 CONCAT - RW
sd disk10-02 vol8-0 disk10 2288 20480 0 c0t1d0 ENA
|
where dg is a disk group, dm is a disk, v is a volume, pl is a plex, and
sd is a subdisk. The top few lines indicate the headers that match each type of
output line that follows. Each volume is listed along with its associated
plex(es) and subdisk(s).
Display volume-related information for a specific volume as follows:
#vxprint -t
volume_name
To display information about vol8, type:
#vxprint -t vol8
