Layout on Raid 5 disks

[jodie]

Okay - I know this is a pretty "newbie" question, don't pound me please!

When I've set up systems in the past, I've always subscribed to the "separate the data and indexes" theory. After reading stuff here and on Ask Tom, I'm beginning to change my tune.

We have raid5, and only raid5, not allowed to use anything different. So, would you put System, Temp, Undo, data and indexes all on one big filesystem? Or would you separate System, Temp and Undo somehow?

Any opinions appreciated.

Thanks!
Jodie

[DaPi]

Hi Jodie,

My first reaction was that the more disks you could "randomise" access across, the better.

My second thought was reliability. If you need (say) 6 disk's worth of space and can only afford 8 disks - you have two options:
a) 3+1 and 3+1
b) 6+1+HOTSWAP
I like option (b).

Both thoughts tend to the same conclusion.

[jodie]

Hi Dapi -
This is a shark Disk Array - so all the disks are hot swappable. And I have plenty of them, no real limit. Also, they tend to carve out "disks" in 10 Gig chunks.

My database needs 8 Gig for data, 17 Gig for Index and another 4 Gig for overhead (temp, undo, system). Plus 4 redo groups, 100 Meg size.

My first concern is the redo. If they carve out 10 Gig chunks, and I want to separate to 2 logs per filesystem... meaning I only need 200+ meg, chances are they are going to the same disks in the array anyway. So should I just not bother and put them all in the same filesystem?

And what about TEMP, UNDO, SYSTEM. Would you put them on the same filesystem as the data and indexes?

Thanks for all your insights!
Jodie

[adrianomp]

Originally posted by jodie
My first concern is the redo. If they carve out 10 Gig chunks, and I want to separate to 2 logs per filesystem... meaning I only need 200+ meg, chances are they are going to the same disks in the array anyway. So should I just not bother and put them all in the same filesystem?

I know you first stated that nothing but RAID-5 is allowed, but... don't use it in that case. If RAID 1 or 1+0 is not allowed, simply put the logs on a disk with no RAID and mirror it only on the database level to a different disk also with no RAID.

Anyway, I think you may have a huge cache on the array controller, so you'll get an acceptable performance no matter what way you choose. Just be careful to follow a way that you can review if needed.

[jodie]

adrianomp -
I agree with you about the no raid. But again.. I don't have a choice. I've battled this before with our sys admin group. They absolutely won't do it. It's Raid 5. Period.

I guess I think I'll go with 4 filesystems. 2 for the Redo logs (1 member from each group on each). 1 for all the datafiles, including temp, undo and system. And 1 for archive logs. I'll see how that goes.

Thanks!
Jodie

[marist89]

Originally posted by jodie
So, would you put System, Temp, Undo, data and indexes all on one big filesystem? Or would you separate System, Temp and Undo somehow?

definitely seperate filesystems.

Originally posted by adrianomp
I know you first stated that nothing but RAID-5 is allowed, but... don't use it in that case. If RAID 1 or 1+0 is not allowed, simply put the logs on a disk with no RAID and mirror it only on the database level to a different disk also with no RAID.

Stop now. You can put logfiles, temp, and undo on RAID 5 (especially if you have HW raid and plenty of cache). Sure, you will incur performance issues, but if you can't do 0+1 you can't do it.

In the case of a system with RAID 5, I like to seperate things as much as possible so that I have a balance I/O load. I'd have one filesystem for the "a" side of my redo logs and another filesystem for the "b" side (mirror). I'd use common sense and the knowlege of your application to balance the rest of the I/O over the remaining filesystems.

That being said, when I say "filesystem", I mean "physical disks dedicated to one filesystem". With SAN these days, you have to instruct your Storage Admins not to slice up one 5*72G RAID 5 into 28x10G filesystems. You basically have one big RAID 5 volume with the appearance of 28 which doesn't help anybody.

[jodie]

Thanks for your input Jeff.

That's exactly what I'm doing with my redo... 1 filesystem for my a side, 1 for my b side. The problem I have is that this db isn't really that big, and to waste a 10Gig "disk slice" for just a 2 Gig temp is kind of a waste. So, I've asked them to create one big filesystem for temp, undo, system, etc for the TEST system. I'm then going to see how it performs to see if I need to break them up for Production. I'll also have a filesystem for the archive logs.

As for instructing the sysadmins to make sure the filesystems are separate disks... I know exactly what you mean. I'm the only Oracle DBA here, and I had a heck of a time convincing them of this when I set up the first system here. (Did I mention this is mostly a db2/mainframe shop?) But they understand the reasoning behind it now.

Thanks again!

[adrianomp]

Originally posted by marist89
Stop now. You can put logfiles, temp, and undo on RAID 5 (especially if you have HW raid and plenty of cache). Sure, you will incur performance issues, but if you can't do 0+1 you can't do it.

Jeff,

I think we're agreed on that. With a HW RAID and enough cache, write penalty inherent to RAID-5 may be slightly minimized. However, what I pointed is that redo logs are write-only (apart from recovery) and it may be taken in account on the capacity planning.

Something like: "ok, the performance it's acceptable now, but we're not doing the better way and we must be extra careful with the system growth".

Anyway, the best solution is always the one that meets the requirements and is most simple.