Systems Engineering and RDBMS

RAID Levels and IOs per disk

Posted by decipherinfosys on March 1, 2007

Here is the calculation of the I/O’s per disk for different RAID levels:

  • Raid 0 — I/Os per disk = (reads + writes) / number of disks
  • Raid 1 — I/Os per disk = [reads + (2 * writes)] / 2
  • Raid 5 — I/Os per disk = [reads + (4 * writes)] / number of disks
  • Raid 10 — I/Os per disk = [reads + (2 * writes)] / number of disks

So, as you can see from above RAID 5 incurs a higher overhead for writes as compared to RAID 10 which is why for requirements of a highly performant and transactional RDBMS, it is rarely recommended to have RAID 5 for the logs.

Also, how can you make use of this information to see whether you are running into a I/O bottleneck or not – because, what is presented above is just theory and to prove a well known point about RAID 5 vs RAID 10. Well, if we take the Windows OS for example and use perfmon (performance monitor) utility to measure these counters:

PhysicalDisk Object: Disk Reads/Sec., Disk Writes/Sec., Avg. Disk Queue Length (you can descriptions on these by hitting the explain button in perfmon when selecting the counters). Now, assume that you have put your log files on RAID 1 system which has 2 physical disks on it. Since logs are written to sequentially, RAID 1 is a good choice. Suppose, the counters measurements over a period of time yield you these values:

Disk Reads/sec = 90
Disk Writes/sec = 80
Avg. Disk Queue Length = 5

In that case, you are encountering (90 + (2 * 80))/2 = 125 I/Os per disk and your disk queue length = 5/2 = 2.5 which indicates a border line I/O bottleneck (any value over 2 is a cause of concern and should be evaluated along with the I/Os seen per disk).

From a RDBMS perspective, all three leading RDBMS: Oracle, SQL Server and DB2 LUW provide with system level information to look into the wait events for further diagnosis. You can look at the pending I/O requests, the latch waits etc. but do not (of course) provide any visibility into the physical disks experiencing the problem. In a future whitepaper on our site, we will cover in detail the different wait events in the three leading RDBMS and how they can help you to troubleshoot performance issues.

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