Perma-Cache and PAV

Hi Willem,

As no-one's had a go at this one I thought i'd try and help out...

Firstly, I'm not a MF guy, and there could be a zillion reasons for your problem... I can talk a little on permacache though...

Permacache will only help you if you have a high %read and a low read hit rate on those volumes. Take a look at the statistics for those devices in performance manager... A low read hit rate is usually due to a very random workload, and as the symmetrix is unable to predict the track that is being requested you get a longer service time while you wait for the DA to fetch the track from disk. (ie: a read miss)

So if that is the case for you, then permacache may help as it pins all of the tracks for that volume in cache so you are guaranteed a 100% read hit rate..

If you have a low read% and/or a high read hit% already, then using permacache will just take resources un-necessarily and slow the performance down for everyone else on the array.

Permacache does not improve the write performance in any way, so if you think writes are your problem, then enabling it won't change anything.

If you suspect writes are the problem, then look at the device write pending counts - are you hitting the limit for any of the devices ?? If so, then you really need to spread the workload across more or faster drives...

If you do want to enable permacache then i would recommend that you first speak to your local EMC person to ensure that it won't effect negatively impact everyone else - depending on your configuration, you may find you need to purchase additional cache to make it work successfully in your environment.

Glen.

Hello Willem

I am a MF guy. But not a DBA. I work in the training group here. I doubt that Perma-Cache will help with your issue. It's really meant for smaller files, not entire databases. I wonder if your issue is with the IMS WADS file. This file crates many issues with IOS queue time. Database issues like this can be very complicated. I think the best approach would be to get a member of our MF Database Team to contact you. They are experts at helping resolve these kinds of problems. I am brand new to this forum so I am not sure how we can link up. So my email address is ayers_john@emc.com. I will get a DB team member to contact you if you write me. I think I will get a email if you reply to this post.

Cheers

John,

Here's my reply. I hope it will get you my email-address, but just to be sure: willem.vermeer@mail.ing.nl.

Now, about the WADS. I'm not sure how the load on the WADS could result in high IOSQTime for this particular database (which is very Write-intensive). We don't seem to have any problems with the WADS itself and there don't seem to be any problems with just about any other database.

Would striping the database be an option? Does IMS support striped databases? I couldn't find antyhing that says it doesn't, but neither could I find anything that says it does.

Willem.

Willem

I have been in contact with our DB Team. They dont seem to have access to this forum. They are reviewing your question and they will contact you directly, via email.

Cheers