Re: Scsi I/O speed

Kevin Kostyszyn wrote:

> Hi all,
> I was measuring the i/o performance of my scsi drives and I have a quick
> question that maybe someone could shed some light upon. Currently I am
> using Ultra 2/Wide scsi conrollers, this is supposed to have an I/O of
> 80mb/s. Well, when I perform the test all of the machines seem to be
> operation at halp of the max speed. One operates at about 20mb/s read and
> write and the others are even slower than that. Now on the first one, it is
> the only HD on the controller, on the others there are two disks. Even on
> my Ultra/160 it seems to be maxing out at 40 read and write.
> Am I missing something? Am I reading this the wrong way? Help:(
>
> Sincerely,
> Kevin Kostyszyn
> DBA
> Dulcian, Inc
> www.dulcian.com
> kevin_at_dulcian.com

Kevin,

download SandraSoft's benchmarking tools - download.com is a good place to start. There is quite a difference between SCSI controller interface speeds and actual trasfer speeds between the OS and the physical hard drive. The Interface speed is a theoretical max, and is more important when configuring several drives on a single controller channel - e.g. RAID 0, 0+1, 5, etc. If you have 4 drives on a channel configured as a 4 drive RAID 0 volume, the controller channel SCSI interface speed could be the rate-limiting-factor. (e.g. 4 drives with an *average* transfer rate of 25 MB/sec = 100 MB/sec > 80 MB/sec).

As there is a cache on the hard drive (2-4 MB is customary) and could be a cache on the RAID contoller (128 MB - 4 GB?) the channel should be saturated during memory to memory transfers (after negotiation for the transfer has taken place) - short bursts which are then slowed by the subsequent access of the phyiscal media.

Typical sustained read/write speeds are on the order of 30 MB/sec on the latest and greatest 10,000 RPM drives.
The fastest sustained read/write I've seen is here - is for the 15,000 RPM Seagate Cheetah - close to 50 MB/sec on the outer tracks http://www.storagereview.com/articles/200105/20010510ST373405LW_1.html

Interface                speed (MB/sec)
SCSI
Ultra Wide     UW          40
Ultra2 Wide    U2W         80
Ultra160       U160       160
Ultra320       U320       320

IDE
UDMA-33        ATA-4       33
UDMA-66        ATA-5       66
Ultra ATA      ATA-6      100

Most likely, seek time will dominate transfer time unless you hike the operating system IO_size up from 64 KB.

this site looks like fun: http://www.storagereview.com/cgi-bin/bench_compare.pl

remember - little 'b' is bits, big 'B' is Bytes. This is extremely important if you happen to look at NAS - using Gigabit Ethernet for shared storage.

Paul

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