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asmlib

Do you need asmlib?

There are many questions from few of my clients about asmlib support in RHEL6, as they are gearing up to upgrade the database servers to RHEL6. There is a controversy about asmlib support in RHEL6.  As usual, I will only discuss technical details in this blog entry.

ASMLIB is applicable only to Linux platform and does not apply to any other platform.

Now, you might ask why bother and why not just use OEL and UK? Well, not every Linux server is used as a database server. In a typical company, there are hundreds of Linux servers and just few percent of those servers are used as Database servers. Linux system administrators prefer to keep one flavor of Linux distribution for management ease and so, asking clients to change the distribution from RHEL to OEL or OEL to RHEL is always not a viable option.

Do you need to use ASMLIB in Linux?

The renamdg command revisited-ASMLib

I have already written about the renamedg command, but since then fell in love with ASMLib. The use of ASMLib introduces a few caveats you should be aware of.

USAGE NOTES

This document presents research I performed with ASM on a lab environment. It should be applicable to any environment, but you should NOT use this for production-the renamedg command still is buggy, and you should not mess with ASM disk headers in an important system such as production or staging/UAT. You set the importance here!  The recommended setup for cloning disk groups is to use a data guard physical standby database on a different storage array to create a real time copy of your production database on that array. Again, do not use you production array for this!

Walking through a renamdg session

Oracle ASMLib introduces a new value to the ASM header, called the provider string as the following example shows:

[root@asmtest ~]# kfed read /dev/oracleasm/disks/VOL1 | grep prov
kfdhdb.driver.provstr:     ORCLDISKVOL1 ; 0x000: length=12

This can be verified with ASMLib:

[root@asmtest ~]# /etc/init.d/oracleasm querydisk /dev/xvdc1
Device "/dev/xvdc1" is marked an ASM disk with the label "VOL1"

The prefix “ORCLDISK” is automatically added by ASMLib and cannot easily be changed.

The problem with ASMLib is that the renamedg command does NOT update the provider string, which I’ll illustrate by walking through an example session. Disk group “DATA”, setup with external redundancy and two disks, DATA1 and DATA2, is to be cloned to “DATACLONE”.

The renamedg command requires the disk group to be cloned to be stopped. To prevent nasty surprises, you should stop the databases using that diskgroup manually.

[grid@rac11gr2drnode1 ~]$ srvctl stop database -d dev
[grid@rac11gr2drnode1 ~]$ ps -ef | grep smon
grid      3424     1  0 Aug07 ?        00:00:00 asm_smon_+ASM1
grid     17909 17619  0 15:13 pts/0    00:00:00 grep smon
[grid@rac11gr2drnode1 ~]$ srvctl stop diskgroup -g data
[grid@rac11gr2drnode1 ~]$

You can use the new “lsof” command of asmcmd to check for open files:

ASMCMD> lsof
DB_Name  Instance_Name  Path
+ASM     +ASM1          +ocrvote.255.4294967295
asmvol   +ASM1          +acfsdg/APACHEVOL.256.724157197
asmvol   +ASM1          +acfsdg/DRL.257.724157197
ASMCMD>

So apart from files from other disk groups no files are open, especially not referring to disk group DATA.

Now comes the part where you copy the LUNs, and this entirely depends on your system. The EVA series of storage arrays I worked with in this particular project offered a “snapclone” function, which used COW to create an identical copy of the source LUN, with a new WWID (which can be an input parameter to the snapclone call). When you are using device-mapper-multipath then ensure that your sys admins add the newly created LUNs to the /etc/multipath.conf file on all cluster nodes!

I am using Xen in my lab, which makes it simpler-all I need to do is to copy the disk containers on the domO and then add the new block devices to the running domU (“virtual machine” in Xen language). This can be done easily as the following example shows:

Usage: xm block-attach     

xm block-attach rac11gr2drnode1 file:/var/lib/xen/images/rac11gr2drShared/oradata1.clone xvdg w!
xm block-attach rac11gr2drnode2 file:/var/lib/xen/images/rac11gr2drShared/oradata1.clone xvdg w!

xm block-attach rac11gr2drnode1 file:/var/lib/xen/images/rac11gr2drShared/oradata2.clone xvdh w!
xm block-attach rac11gr2drnode2 file:/var/lib/xen/images/rac11gr2drShared/oradata2.clone xvdh w!

In the example, rac11gr2drnode{1,2} are the domU, the backend device is the copied file on the file system, the front end device in the domU is xvd{g,h}, and the mode is read/write, shareable. The exclamation mark here is crucial or else the second domU can’t mount the new block device-it is already exclusively mounted to another domU.

The fdisk command in my example immediately “sees” the new LUNs, with device mapper multipathing you might have to go through iterations of restarting multipathd and discovering partitions using kpartx. It is again very important to have all disks presented to all cluster nodes!

Here’s the sample output from my system:

[root@rac11gr2drnode1 ~]# fdisk -l | grep Disk | sort
Disk /dev/xvda: 4294 MB, 4294967296 bytes
Disk /dev/xvdb: 16.1 GB, 16106127360 bytes
Disk /dev/xvdc: 5368 MB, 5368709120 bytes
Disk /dev/xvdd: 16.1 GB, 16106127360 bytes
Disk /dev/xvde: 16.1 GB, 16106127360 bytes
Disk /dev/xvdf: 10.7 GB, 10737418240 bytes
Disk /dev/xvdg: 16.1 GB, 16106127360 bytes
Disk /dev/xvdh: 16.1 GB, 16106127360 bytes

I cloned /dev/xvdd and /dev/xvde to /dev/xvdg and /dev/xvdh.

Do NOT run /etc/init.d/oracleasm scandisks yet! Otherwise the renamedg command will complain about duplicate disk names, which is entirely reasonable.

I dumped all headers for disks /dev/xvd{d,e,g,h}1 to /tmp to be able to compare.

[root@rac11gr2drnode1 ~]# kfed read /dev/xvdd1 > /tmp/xvdd1.header
# repeat with the other disks

Start with phase one of the renamedg command:

[root@rac11gr2drnode1 ~]# renamedg phase=one dgname=DATA newdgname=DATACLONE \
> confirm=true verbose=true config=/tmp/cfg

Parsing parameters..

Parameters in effect:

 Old DG name       : DATA
 New DG name          : DATACLONE
 Phases               :
 Phase 1
 Discovery str        : (null)
 Confirm            : TRUE
 Clean              : TRUE
 Raw only           : TRUE
renamedg operation: phase=one dgname=DATA newdgname=DATACLONE confirm=true
  verbose=true config=/tmp/cfg
Executing phase 1
Discovering the group
Performing discovery with string:
Identified disk ASM:/opt/oracle/extapi/64/asm/orcl/1/libasm.so:ORCL:DATA1 with
  disk number:0 and timestamp (32940276 1937075200)
Identified disk ASM:/opt/oracle/extapi/64/asm/orcl/1/libasm.so:ORCL:DATA2 with
  disk number:1 and timestamp (32940276 1937075200)
Checking for hearbeat...
Re-discovering the group
Performing discovery with string:
Identified disk ASM:/opt/oracle/extapi/64/asm/orcl/1/libasm.so:ORCL:DATA1 with
  disk number:0 and timestamp (32940276 1937075200)
Identified disk ASM:/opt/oracle/extapi/64/asm/orcl/1/libasm.so:ORCL:DATA2 with
  disk number:1 and timestamp (32940276 1937075200)
Checking if the diskgroup is mounted
Checking disk number:0
Checking disk number:1
Checking if diskgroup is used by CSS
Generating configuration file..
Completed phase 1
Terminating kgfd context 0x2b7a2fbac0a0
[root@rac11gr2drnode1 ~]#

You should always check “$?” for errors-the message “terminating kgfd context” sounds bad, but isn’t. At the end of stage 1, there is no change to the header. Only at phase two there is:

[root@rac11gr2drnode1 ~]# renamedg phase=two dgname=DATA newdgname=DATACLONE config=/tmp/cfg

Parsing parameters..
renamedg operation: phase=two dgname=DATA newdgname=DATACLONE config=/tmp/cfg
Executing phase 2
Completed phase 2

Now there are changes:

[root@rac11gr2drnode1 tmp]# grep DATA *header
xvdd1.header:kfdhdb.driver.provstr:    ORCLDISKDATA1 ; 0x000: length=13
xvdd1.header:kfdhdb.dskname:                   DATA1 ; 0x028: length=5
xvdd1.header:kfdhdb.grpname:               DATACLONE ; 0x048: length=9
xvdd1.header:kfdhdb.fgname:                    DATA1 ; 0x068: length=5
xvde1.header:kfdhdb.driver.provstr:    ORCLDISKDATA2 ; 0x000: length=13
xvde1.header:kfdhdb.dskname:                   DATA2 ; 0x028: length=5
xvde1.header:kfdhdb.grpname:               DATACLONE ; 0x048: length=9
xvde1.header:kfdhdb.fgname:                    DATA2 ; 0x068: length=5
xvdg1.header:kfdhdb.driver.provstr:    ORCLDISKDATA1 ; 0x000: length=13
xvdg1.header:kfdhdb.dskname:                   DATA1 ; 0x028: length=5
xvdg1.header:kfdhdb.grpname:                    DATA ; 0x048: length=4
xvdg1.header:kfdhdb.fgname:                    DATA1 ; 0x068: length=5
xvdh1.header:kfdhdb.driver.provstr:    ORCLDISKDATA2 ; 0x000: length=13
xvdh1.header:kfdhdb.dskname:                   DATA2 ; 0x028: length=5
xvdh1.header:kfdhdb.grpname:                    DATA ; 0x048: length=4
xvdh1.header:kfdhdb.fgname:                    DATA2 ; 0x068: length=5

Although the original disks (/dev/xvdd1 and /dev/xvde1) had their disk group name changed, the provider string remained untouched. So if we were to issue a scandisks command now through /etc/init.d/oracleasm, there’d still be duplicate disk names. This is a bug in my opinion, and a bad thing.

Renaming the disks is straight forward, the difficult bit is to find out which have to be renamed. Again, you can use kfed to figure that out. I knew the disks to be renamed were /dev/xvdd1 and /dev/xvde1 after consulting the header information.

[root@rac11gr2drnode1 tmp]# /etc/init.d/oracleasm force-renamedisk /dev/xvdd1 DATACLONE1
Renaming disk "/dev/xvdd1" to "DATACLONE1":                [  OK  ]
[root@rac11gr2drnode1 tmp]# /etc/init.d/oracleasm force-renamedisk /dev/xvde1 DATACLONE2
Renaming disk "/dev/xvde1" to "DATACLONE2":                [  OK  ]

I then performed a scandisks operation on all nodes just to be sure… I had corruption of the disk group before :)

[root@rac11gr2drnode1 tmp]# /etc/init.d/oracleasm scandisks
Scanning the system for Oracle ASMLib disks:               [  OK  ]
[root@rac11gr2drnode1 tmp]#

[root@rac11gr2drnode2 ~]# /etc/init.d/oracleasm scandisks
Scanning the system for Oracle ASMLib disks:               [  OK  ]
[root@rac11gr2drnode2 ~]#

The output on all cluster nodes should be identical, on my system I found the following disks:

[root@rac11gr2drnode1 tmp]# /etc/init.d/oracleasm listdisks
ACFS1
ACFS2
ACFS3
ACFS4
DATA1
DATA2
DATACLONE1
DATACLONE2
VOL1
VOL2
VOL3
VOL4
VOL5

Sure enough, the cloned disks were present. Although everything seemed ok at this point, I could not start disk group DATA and had to reboot the cluster nodes to rectify that problem. Maybe there is some not so transient information stored somewhere about ASM disks. After the reboot, CRS started my database correctly, and with all dependent resources:

[oracle@rac11gr2drnode1 ~]$ srvctl status database -d dev
Instance dev1 is running on node rac11gr2drnode1
Instance dev2 is running on node rac11gr2drnode2