Running a simple test

How does one show the impact of long Seek times on the total I/O response time? I took a maxtor 200GB firewire harddrive (for my linux RAC test installation) and reconfigured the hard drive to have partitions of 2 GB on the outer tracks and inner tracks (accomplished with fdisk).

Disk /dev/sda: 203.9 GB, 203927060480 bytes
255 heads, 63 sectors/track, 24792 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

DeviceBoot Start End Blocks Id System
/dev/sda1 1 250 2008093+ 83 Linux
/dev/sda2 251 24542 195125490 83 Linux
/dev/sda3 24543 24792 2008125 83 Linux


Then downloaded the Reader.c program from the website of Jonathan Lewis (didn't want to write my own program). The test was done with running two processes running against the partitions. The partition size is important to overcome caching issues on the disk (and the system buffer cache). I also choose raw partitions to overcome potential locking issues in the file system(s).

Here are the results:

Test 1 (run 2 processes reading from partition 1 doing a 1000 Random Reads each)

+ ./read /dev/sda1 R 8192 262144 1000 13
File name: /dev/sda1
Random or Serial: R
Read Size: 8192
Boundary: 262144
Read Count: 1000
Rand Seed: 13
Descriptor: 3
+ ./read /dev/sda1 R 8192 262144 1000 19
[root@linrac1 tmp]# File name: /dev/sda1
Random or Serial: R
Read Size: 8192
Boundary: 262144
Read Count: 1000
Rand Seed: 19
Descriptor: 3

real 0m11.323s
user 0m0.010s
sys 0m0.030s

real 0m11.536s
user 0m0.000s
sys 0m0.040s

Now we run the same test against partition 3 (inside of the disk)
+ ./read /dev/sda3 R 8192 262144 1000 23
+ ./read /dev/sda3 R 8192 262144 1000 29
File name: /dev/sda3
Random or Serial: R
Read Size: 8192
Boundary: 262144
Read Count: 1000
Rand Seed: 29
Descriptor: 3
[root@linrac1 tmp]# File name: /dev/sda3
Random or Serial: R
Read Size: 8192
Boundary: 262144
Read Count: 1000
Rand Seed: 23
Descriptor: 3

real 0m17.398s
user 0m0.000s
sys 0m0.010s

real 0m17.482s
user 0m0.000s
sys 0m0.090s

The last test (run 1 process against partition 1 and 1 process against partition 3)
+ ./read /dev/sda1 R 8192 262144 1000 33
File name: /dev/sda1
Random or Serial: R
Read Size: 8192
Boundary: 262144
Read Count: 1000
Rand Seed: 33
Descriptor: 3
+ ./read /dev/sda3 R 8192 262144 1000 39
[root@linrac1 tmp]# File name: /dev/sda3
Random or Serial: R
Read Size: 8192
Boundary: 262144
Read Count: 1000
Rand Seed: 39
Descriptor: 3

real 0m32.673s
user 0m0.010s
sys 0m0.090s

real 0m32.926s
user 0m0.000s
sys 0m0.070s

So to summarize:

Outside Partition 11.4 seconds for 2000 8K reads
Inside Partition 17.4 seconds for 2000 8K reads (50 percent slower
Inside/Outside Partition 32.7 seconds for 2000 8K reads (200 percent slower)

Average read Outside 11.4 seconds / 2000 = 0.0057 seconds (5.7 ms)
Average read Inside 17.4 seconds / 2000 = 0.0087 seconds (8.7 ms)
Average read Inside/Out 32.7 seconds / 2000 = 0.0164 seconds (16.4 ms)

So the total number of I/Os that we can do really depends on where the data is located on the disk. According to maxtor the average seek time for this disk is around 9.3 ms. It means that the full seek time will be around 16-18 ms. That seems to fit. Subtract the average outside time from the average inside/out (16.4 - 0.5) and we get 15.9 millisecond as may be 90 percent of the full seek time.

So even for the simple ATA drives, it is important where the data is stored.