Why does performance increase when I mount?

Vlad_M · 10-10-2002, 04:34 PM

Hi all,

I am busy testing the max throughputs of some IBM Ultrastar XP (18GB) SCSI disks that I have.

I have noticed a very strange thing, hope some of you will be able to shed some light on this.

I use hdparm to test the drives. Usually I just type hdparm -t /dev/sda and wait for the results. So far, for every disk I have tested the throughput that I got was 10.71 MB/s. This was reproducible with different disks, and I have a whole lot of them.

Then I tried this: format the disk with mke2fs, and mount the partition, usually I mount them at /sda.

Do hdparm -t /dev/sda1, wait for the results, and I get 14.12 MB/s. Also reproducible, every time, and when I unmount and test again, the transfer rates go back to 10.71 MB/s.

My question is obviously this : What causes the transfer rate to increase in the second example (i.e. when the partition is mounted)? What is actually going on?

Thanks to all that answer!!!
Vlad.

tangle · 10-10-2002, 08:38 PM

My guess would be that it takes time to mount the dive. This time is included in on the test which would lower the preformance measurement.

mcleodnine · 10-10-2002, 08:44 PM

I get different results when I use /dev/hda and /dev/hda1...

Vlad_M · 10-11-2002, 03:47 AM

Yeah, mcleodnine thats exactly what I mean....different speeds depending whether you test /dev/sda or /dev/sda1. Why does this happen? I tried to search the whole night for something like that, and came up with squat.

What could be the reason?

mcleodnine · 10-11-2002, 07:35 AM

My first guess would be that each partition is on a different part of the platter. IIRC the closer the track is to the outside (larger diameter) the lower the seek time and maybe throughput.

Have a look at www.tldp.org and try to find the Multiple Disks How-To as it covers this in greater detail.