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OS mirrored to AutoRaid 12h.

Rao Uppuluri

OS mirrored to AutoRaid 12h.

I have a few questions on my L2000 server with AutoRAID 12h. I am noticing some performance
problems with disk access (some queries take long time. ran sar -d) for some of my oracle database queries.
We have 2 intrenal drives each on its own scsi controller.
We have a AutoRAID 12 connected through 2 96MB SCSI controllers.
I have OS one internal drive (Int1) mirrored to a LUN on 12h with Mirror/UX.
12h has all the Oracle data in other LUNs.

For MY OS drive, I even mirrored the SWAP space (/dev/vg00/lvol2) to 12H, is this OK? DO I need to do it?

Few months ago, my internal OS drive failed but system was up because it was mirrored, so I was able to hot swap the internal drive. If I had not mirrored Swap space from internal drive, would I have encountered problems/issues when my internal drive failed?

Thanks and Regards
Rao Uppuluri
Zeev Schultz
Honored Contributor

Re: OS mirrored to AutoRaid 12h.

12h is a nice device while its doing raid-1/0,
after some period of time (and lack of free space) it moves to raid-5 as far as I remember.
Now,all the scsi there is at best Fast Wide Differential which gives about 20MB/s.And internal drives in L-2000 are using LVD controller,something about ultra wide lvd scsi (40-80 MBs).So why not to mirror 2 system drives (on different scsi buses),that should give performance boost.All is relative,Autoraid spreads all LUN data between all its physical drives,so while there is a free space should be good write/read performance on Autoraid also :)
Mirroring swap actually can improve read IO and
help in disaster case.If your swap wouldn't be mirrored - system panic wouldn't waith to come.

So best way about Autoraid is to keep it with enough hot spare space (which is used for raid 1/0 performance).Also good to compare 3 hot-swap ultra wide lvd mirror vs autoraid mirror.
Another thing is pvlinks (mirror + failover links),like to have 2 or more controllers connected to Autoraid.

So computers don't think yet. At least not chess computers. - Seymour Cray
Eugeny Brychkov
Honored Contributor

Re: OS mirrored to AutoRaid 12h.

I would buy another internal drive to mirror primary drive to; putting swap and/or mirror of your boot disk to autoraid of course will impact perfomance. Autoraid is not 'so fast' for current tachnologies - comparing to fibre channel - and loading it with extra I/Os is not good idea...
As I said: buy another internal disk and mirror primary internal disk to another internal disk. Leave 12h for database only.
I remember I've heard that it's no need to mirror swaps... but wait for anyone's else post who knows for sure!
Zeev Schultz
Honored Contributor

Re: OS mirrored to AutoRaid 12h.

Here's the doc with procedures:

as to swap mirror - makes sense to assume that
when system crashes all swap space should be preserved somewhere (process stacks etc).I never tried to failure (test) system disk mirrored without a swap.As Eugene says - someone must did it :).
So computers don't think yet. At least not chess computers. - Seymour Cray
Trevor Roddam_1
Valued Contributor

Re: OS mirrored to AutoRaid 12h.


12H's are a very good but very old disk system.
There is some very good reading on performace on the 12H in the ITRC.
In a nutshell:
The 12H uses a metric called a "write working set". This is a good gauge on how the 12H is performing. The 12H write to cache wich is flushed to disk. On flushing to disk if there is enough disk space data is written in RAID 1+0.
Periodically the 12H re-asses the RAID allocation to data at a block level. Data is converted to RAID 5 if it is determined that more free space is required.
This will happen when usable is at approx 46% of RAW. There is a complex set of rules for this which includes written data. Suffice to say keep the 12H under 46% allocated and it will perform at its best.

Allocating swap and boot disks to the 12H is feasable but I would reccomend keeping these to internal disks. The L2000 has four bays with two seperate channels for redundancy.

When running the 12H with space allocated at RAID 5, having swap on the 12H will cause the writing of data from cache to disk to be written at RAID 5. This will cause sevear performace issues with the 12H when compared to normal operations.
Please read the attached performace brief it will detail the why and when a lot better.
I have the admin/user documents if you can't get them from ITRC.

I would highly reccomend getting swap and the OS off the auto raid, but keep them mirrored....

Hope this helps.

Baldric, I have a plan so cunning you could pin a tail on it and call it a weasle.
Regular Advisor

Re: OS mirrored to AutoRaid 12h.

If I may...
Thanks for the AutoRaid Field Performance paper. We are moving multiple Db's from an EMC Sym to the 12h's. I have lots of 12h info but not that,...the natives (dba's) are restless...good medicine to calm excited nerves. Thanks, ~jdk
Dare to Dream