RAID-5 disk pool creation with AIX

RNH4
RNH4 used Ask the Experts™
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attempting to create a RAID -5 in a pSeries  9131-52A server.
Have 4 -  146GB drives, one has AIX 5.3 m O.S. loaded on it.
Is it possible ( and reccomended ) to create a RAID-5 set with 3 drives and one hot spare ?
Thus the O.S. drive being part of the RAID set.
I have mirrored the O.S. before but have not put it into a RAID set.
What would be the proper steps involved ?
Thanks Randy
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DavidPresident
Top Expert 2010

Commented:
Does your controller support RAID6?  This will give slightly better protection and performance than a RAID5, and you use the hot spare as the 4th disk.  Same amount usable as the 3-disk RAID5, it is just as if the hot spare is just no longer a spare.   The technique for building RAID-anything is a function of the specific make/model of the controller.

In general, you just can do this via smit.

Author

Commented:
I am using a 5703 Ultra320 SCSI RAID adapter,..it only supports 0, 5 , 10.
remembering that one of the drives is the O.S. boot device,...
Do I create an array candidate pdisk and format all drives to 522 byte sectors first?
Or do I create the RAID set first ?
When do I create the "hot spare"?
Thanks
Randy
DavidPresident
Top Expert 2010

Commented:
There is a nice document here that walks you through the process
http://support.bull.com/documentation/byproduct/servers/escala/platforms/escala-ctrl/g/86Y122EM00/86A122EM00.pdf/attachment_download/file

Look at pages 13,14.  

Before you start, I would make sure you have latest firmware for the controllers and patches, and power cycle, then create a mksysb tape.  Test this to, unless you have 100% confidence that all is well.

The PDF covers pdisk,  building the array, formatting, and defining a hot spare.  Pretty painless as it give examples

Author

Commented:
Excellent.
Do you foresee any issues with me using the boot drive with O.S. loaded on as part of the RAID set ?
Most Valuable Expert 2013
Top Expert 2013

Commented:
Hi,

having rootvg on a hardware RAID5 array has one big  drawback - since AIX doesn't allow for growing disks in the rootvg
you  will not be able to later add disks to that array in order to get more  space. (Well, you can add them, but AIX will not see the additional  space).
 
 Anyway, if you really want to do it this way - do you  plan to leave the OS intact, or are you goig to reinstall?
 
 If  you're going to reinstall, everything will be just straightforward.
  Just boot from the diagnostic CD, set up the RAID using the tools   contained on that CD
 according to your needs and then install your  OS on the resulting hdisk0 (which is the array in reality)
 
 Keeping the OS intact means that you must use the fourth  disk a spare disk since you can't add
data disks to the array  (see above).

The steps for the latter method should be as  follows (assuming rootvg is just hdisk0)

Use smitty "rm_ary_mgr_menu"  for the following tasks

- change hdisk1,  hdisk2 and hdisk3 to array candidates. This implies  formatting the 522 byte sectors.
You will get pdisk0, pdisk1, pdisk2

-  create a raid 5 array containing pdisk0, pdisk1 and pdisk2.
 You will get a new hdisk1
 
 Use " smitty lvm" for the following steps
 
 - add hdisk1 to rootvg
- mirror the rootvg to hdisk1
- remove the mirror on hdisk0
- remove hdisk0 from rootvg

Use smitty "rm_ary_mgr_menu"  for the following tasks

- change hdisk0 to an array candidate.
You will get pdisk3

- add this disk as a hot spare to your array

Please remember - adding pdisk3 as a data disk will have no effect (as seen from AIX).
And please remember, too, what I wrote above - you will not be able to increase the array by adding disks!

wmp

President
Top Expert 2010
Commented:
Well you certainly know RAID5 isn't exactly designed for performance, but data protection is a big plus.  If something goes wrong then the AIX CDs are bootable.   I would update all firmware, including the BIOS of the box and the RAID controller.  Seems there are never-ending fixes and patches for RAID when it comes to booting.  If drives have never been used or burned in, I would set up a shell script and let it run overnight that dumps data to the disks to give them some exercise,

No worries, don't forget to update disk drive firmware also, and get AIX all patched up.  This  is general good form not necessarily a requirement .. unless there is a critical patch that affects the RAID or disk drives and you never installed it.
Top Expert 2015

Commented:
normally you install drivers with diag -a ant then configure with diag

Author

Commented:
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