HUH???

[Lacrosse]

I had a HP support engineer tell me that there is a misunderstanding about how mirroring works. He said that when using mirror/UX on logical volumes and the primary disk fails you cannot use the image on the other disk? I can understand if the primary disk corrupts data when it fails that might cause a problem but you should be able to mount the logical volume on the other disk otherwise whats the point of mirroring. Also I was curious does HP/UX and mirroring read from the least busy image as a performance enhancement??

[Pete Randall · Accepted solution]

First, never trust a hardware guy to explain how your OS works. If what he said was true, what would be the point of mirroring.

Second, no, I don't believe there's any gain in read performance.

Pete

Go UT!

[John Poff]

Hi,

You don't get any read performance from having your disks mirrored. You'll need to use something like an XP array and the AutoPath software to get performance enhancements from multiple paths.

I think your hardware guy is a Texas A&M fan. :)

JP

[Ken Hubnik_2]

I agree. He has to be an aggie from little atm.

[Sridhar Bhaskarla]

Hi,

He was both wrong and right.

If the primary disk fails, you can still be able to access the mirror disk. I have seen many instances where we didn't even notice the failure of the mirror disk.

Yes. If the lv is mirrored, the LVM does schedule reads from the mirrors. You may get performance gain but I doubt it is noticeable as there will be degradation associated with the writes.

-Sri

[Jeff Schussele]

I thought I read in a Bill Hassell post a couple of weeks ago that IF the disk queue builds to a certain depth, that reads will go to the mirror.

And here's the thread:

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x472431ec5e34d711abdc0090277a778c,00.html

So reads will go to the mirror(s) if load requires it.

Rgds,
Jeff

[Lacrosse]

Great thanks just wanted to confirm it....ya know I never did ask him if he was from A&M that might explain it LOL! thanks for the confirmation...actually raised one more question if you are doing a lot of writes what sort of impact do you think it would have on the system being a rp5430 1 550Mhz proc we are not getting the performance we think we should be getting
Thanks for all the UT support

[John Poff]

Oops. I PV links on my brain. Never mind!

JP

[A. Clay Stephenson]

I have enough disk drives that a root disk failure is quite common and yet I have had no unscheduled downtime in over 4 1/2 years thus obviously the mirrored root works. Note that I don't even shutdown to do the replacement.

As to the mirroring read performance that depends upon the scheduling policy. If using parallel, reads are done in the order of PV's with the least outstanding I/O; if using sequential scheduling policy the reads are scheduled in PV order.

[Michael Tully]

Listen you guys.... I might be from OZ, but I have good friends that work at Texas A & M .....

ha ha