If you have the ability to combine disk spindles at both the SAN level and Oracle (ASM) level, which one is better?
Should you combine all your spindles on the SAN and present 1 big disk to OS and give that to ASM? Or should you present each individual disk spindle to ASM and let ASM do the mirroring ?
One item should help you decide very quickly. ASM does not offer RAID 5, and that’s what most people would like to run for it’s low cost.
Another item is performance. Modern disks are able to push 50 to 70 Mb/sec in sequential reads easily. Combine 3 drives’ output and you get 210 Mb/sec which is approximately the bandwith limitation of 2GBit fibre channel. That’s of course, under optimal disk setup.
So imagine, as a DBA, you have full freedom on how to divide your hard disk devices. Don’t you wish it was like that for all DBAs?
I happen to have both setups. One diskgroup with one big disk (array) and another disk group with 2 disks (arrays). Those are on the same machine, attached to the same database. All 3 arrays are RAID 5 with 256kb striping. To visualize:
(14 x 36 gb) => Raid 5 LUN => ORA_DISK_1 => ASM DISK GROUP A
(14x36gb) Raid5 => ORA_DISK 2 + (14x36gb) => ORA_DISK 3
Together = ASM DISK GROUP B
There’s 1 more detail that matters – the machine has 2 Fibre Channel controllers, 2gbit bandwith each (~200 Mb/sec). The LUNs are split equaly alternating the controllers. In the LUNs that I am testing, for the disk group with 2 LUNs, each LUN is on a separate controller.
So I created the same tablespace with the same table on those 2 disk groups. And ran the following tests:
Full table scan of 15 gb table:
- Disk Group A (1 disk) – 136 seconds – ~110 Mb/sec
- Disk Group B (2 disks) – 184 seconds – ~81 Mb/sec
Surprised? The disk group with 2 disks is slower! Those results are consistent, and confirmed with diagnostic output from iostat. You may start to wonder why would 2 be slower then 1. It should be twice as fast!
I will have to give an example of this. Imagine you go to the library. In this specific library, you dont get access to books directly. You go at the desk and request them. The librarian goes and fetches the books you want. You been a smart guy, ask for multiple books at the same time, since you know they are in the same area – thus you are saving time.
Now imagine if there were 2 librarians. Now you have 2 people to ask for books, but what you do is ask for one, wait for your books, then ask the other librarian, alternating them. You never ask for them at the same time, either one or the other. You won’t get your books faster, you will get them at the same speed!
In our situation we got slower with 2 “librarians”. Why? Well it happened that our “librarians” were really smart, and when they went to get the books, they decided to get an extra set, in case you asked for it. So when you had 1 “librarian”, it was working great and some of the books you were asking for were already available. But now that you have 2 “librarians” to ask for stuff, by the time you come back to the librarian who just brought you the books, he would decide that you don’t need them and return them.
Now the same test, but in parallel. My parallelism level is 8, full scan of 15 gb table:
ASM disk group A (1 disk) – 78 seconds – 192 Mb/sec
ASM disk group B (2 disks) – 41 seconds – ~365 Mb/sec
Now I am sending more requests SIMULTANEOUSLY – I get to use the fact that I have 2 LUNs on separate controllers. In addition, it helped my 1 LUN disk group by providing a constant flow of requests.
And then the final test I ran, rman backup validate tablespace: It simply reads all the data. Since it’s 1 big tablespace, no parallelism is available, but that’s not important. Unfortunately, the tablespace backup tests I did at a later point, thus their sizes are different:
ASM disk group A (1 disk) – 17’500 Mb in 96 seconds – 182 Mb/sec
ASM disk group B (2 disks) – 46’700 Mb in 135 seconds – 345 Mb/sec
Even though that speed looks amazing, it’s actually a bit higher, as RMAN takes 1-2 seconds after the copy before taking the timing estimate. According to iostat I reached 196 Mb/sec in group A, and 392 Mb/sec in group B.
This is 1 backup. Why the difference between backup and full table scan? They were both limited by disk, why is it different?
The reason is ASYNC IO.
RMAN uses ASYNC IO extensively, keeping 32 read requests of 1 mb each in the read queue. This is clearly visible in iostat. ASYNC IO allows RMAN to keep requests in the queue, while processing them as they come. This allows the Disk IO subsystem to fetch them very efficiently.
Think about it, if you go to the librarian and give him a list of all the books you need, he will get them in the most efficient way for him.
Conclusion? Discussion and feedback is open in the comments!
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