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Oracle Disk Load Capacity

Mike Ault

 

Recent research shows that the more loaded a disk drive is (volume wise) the poorer it performs, generally speaking you should shoot for only filling drives to at most 50-60 percent of their total capacity.

I guess it all boils down to you must know the expected IO rate from your database and plan the number of disk drives based on the required IO rate, not only on the amount of space required, or you could find yourself volume rich but IO capacity poor. In addition, don't plan to get exactly the volume of disks you need, to get maximum performance, double that number allowing for only a 50-60 percent by volume usage.

So, before purchasing disks for your next system upgrade consider IO rate, then capacity. And don't forget the number of access channels to your disks. Even if you aren't stressing a single channel, remember that most SCSI interfaces process commands to only a single device at a time so the more channels the better the performance for high activity systems. At one client they moved from a disk setup with 12 channels of access using an access model that balanced IO across all channels to an 8 channel setup that used a cascade access model (one channel would have to saturate before another was utilized). In preliminary testing everything seemed to work fine, then they implemented the system and got to month end processing. They got less than 70% of the performance of their old, slower, fewer CPU, slower disk system. Once they added IO channels and used a load balanced model for channel access their performance nearly doubled over what the old system was capable off.

Most UNIX based systems now use a maximum physical IO size of 1 megabyte per IO operation. So, for our disks we are looking at so far, we are talking a maximum of 58 IO/second/disk, many manufacturers claim IO rates of 100-150 or more IO per second, but if you look at the claims closely this is linear access rate. Linear access rate means the speed at which data stored such that it can be read in one large linear read (little or no head movement)can be accessed. How many Oracle systems store data this way? Generally you will get less than 60-70 percent of the reported linear IO rate for non-linear IO rate.

Let's look at some formulas that may help in these capacity verses IO rate decisions.

RAID10:

CEILING((MAX(APP_IO_RATE/(DISK_IO_RATE*RAID_FACTOR*MIRRORS)),1)),1)*MIRRORS

RAID5:

CEILING((MAX(APP_IO_RATE/(DISK_IO_DATE*RAID_FACTOR),2))+1,1)

For RAID10 the raid_factor is simply the ratio of:

ACTUAL_DISK_IO_RATE/MAX_DISK_IO_RATE

(Non-linear IO rate to Linear IO rate) if you can't get this value from the manufacturer assume a factor of 0.6 to 0.7 (in the above data, this would be 0.58!).

For RAID5 you have to account for the extra overhead of the additional parity writes, usually about 25% but may be less or more. This means the factor becomes:

(ACTUAL_DISK_IO_RATE/MAX_DISK_IO_RATE)*0.75

If you use mirrored RAID5 just use the RAID10 calculation with the RAID_FACTOR the RAID5 factor and adding one disk for each level of mirroring.



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