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Mirroring rootvg - risks if any and tips

Posted on 2005-03-19
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I would like to mirror an existing rootvg in a 4.3.2 box which currently has a single physical volume.  I have some spare drives to add and create the new volume group to mirror with.  

Since the system has ran for a few years - I would assume that mirroring would replicate all of its data to the mirrored volume group safely..?

Any tips or recommendations with rootvg mirroring would be appreciated.
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Question by:nsome
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cpc2004 earned 2000 total points
ID: 13585333
Mirroring is a stand feature of AIX and it is no risk. Last month I mirrored 8 volume groups to migrate data from HDS 9200 to HDS 9570 and the process was very smooth.

Procedure to mirror rootvg and assume the new mirror pv is hdisk1 and your rootvg  has only  hdisk0 is the current boot disk.
The size of hdisk1 must be equal to or bigger than hdisk1.

1. extendvg rootvg hdisk1
2. mirrorvg rootvg
3. lsvg -l rootvg
rootvg:
LV NAME             TYPE       LPs   PPs   PVs  LV STATE      MOUNT POINT
hd5                 boot       1     2     2    closed/syncd  N/A
paging01            paging     10    20    2    open/syncd    N/A
hd8                 jfslog     1     2     2    open/syncd    N/A
hd4                 jfs        2     4     2    open/syncd    /

Note : the number of PPs of each filesystem is double the number of LPs. If not, the LV has one copy.
4. bosboot –a –d /dev/hdisk1  (write the bootstrap record)
5. bootlist -m normal -r  (display the current bootlist)
6. bootlist -m normal hdisk0 hdisk1
7. bootlist -m normal -r  (double checkt hdisk1 is added to bootlist)

Step 4 to 7 is optionlal but usually most system programmers writes the bootstrap record and hdisk1 to bootlist and it is the purpose of mirror rootvg.

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by:nsome
ID: 13586402
Thanks cpc2004.  I speculate that the mirror volume group within smitty would perform the same tricks (i.e. extend rootvg hdisk1?)

In the event of a failure of one of the disks within a mirrored rootvg, what would be the impact on the system?  Ideally the system would continue functioning within its hdisk0 of the mirror while the replacement drive is loaded and mirrored again (correct?)

I need to setup a DR procedure in the event a disk goes for a mirrored rootvg configuration.

Again, much thanks.
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by:cpc2004
ID: 13586490
Yes, you can use smit to do the rootvg mirroring. I use AIX command as it is more easy for you to understand as I don't have to capture the screen.

If a mirrored volume is scratch, it has no impact to AIX. However you must have a monitoring tools or a cron job to check the status of the rootvg mirroring and errpt entry relating to rootvg mirroring is broken. Hence you can call IBM CE replace the faulty hdisk. Usually IBM CE will unmirror the rootvg, reducevg and remdev the faulty hdisk.  Replace the faulty hdisk. Issue cfgmgr to add the new hdisk. Repeat the procedure of the previous post to mirror the rootvg.
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Author Comment

by:nsome
ID: 13586551
Thanks!
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Author Comment

by:nsome
ID: 13587772
One more question cpc2004

In a mirrored rootvg configuration, is management of data I/O to all pv's associated with the rootvg mirror transparent to the OS kernel and its activities?  In a failure of one PV, data is only written to the good PV - thus not affecting in any way the instance of the OS?

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by:cpc2004
ID: 13588243
Yes, this is the purpose of mirrored disk. One mirrored PV failure does not affect the data integrity of the the mirrored disk.
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by:esanchezvela
ID: 13641704


quick comment, check that your volume group QUORUM is set to off, since AIX uses it to define whether to vary on or not a volume group based on the number of surving hdisks, since the initial disk in a volume group has 2 votes and the next have only 1 if the first disk fails AIX will decide the QUORUM limit was not met and it will not be able to vary the volume group... u do this using ....

chvg -Q n

I would not recommend mirroring the paging space, unless you want to prevent the system from crashing if one of the disks fails. the problem with this is that mirroring the paging will hit you very hard if your server starts paging, if you can afford a reboot, that would be the way to go.

Also, make sure you dont define your primary dump device on the same disk as the secondary (and do not mirror such devices) I believe that AIX still uses /dev/hd6 as the default dump device, but not 100% sure since we seldom do installs from scratch anymore. you can verify which devices are defined as dump devices using the following command..

/htdocs/esv# sysdumpdev -l           <- small L
primary              /dev/hd7
secondary            /dev/sysdumpnull
copy directory       /var/adm/ras
forced copy flag     TRUE
always allow dump    FALSE
dump compression     ON

on this particular system the primary dump device is hd7 and there is no secondary dumpdevice, but this is not a production server, we do not mirror paging space on some servers and we make sure that the lv's associated as dump devices are not mirrored, since the failing componet might be the LVM manager which would be in charge or capturing the dump to disk since a mirrrored device is involved.

regards,
esv.
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