Seial ATA and Ultra160 (SCSI)

daihard · 12-24-2004, 10:03 PM

I am just curious whether or not my observation is correct. I have a Maxtor Serial-ATA 60GB hard disk (7,200 RPM). I get the following numbers from the hdparm command:

Code:

hdparm -t /dev/hda

/dev/hda:
 Timing buffered disk reads:  64 MB in  1.27 seconds = 50.36 MB/sec

I take it to mean that the hard disk has a data transfer rate of about 50 MB/sec. Am I correct? If so, does it theoretically mean that an Ultra160 SCSI hard disk, with a data transfer rate of 160 MB/sec, is more than three times as fast as this Serial-ATA hard disk?

Thanks,
Dai

jtshaw · 12-24-2004, 10:40 PM

What you are seeing there is the sustained transfer rate of the disk. 150 Mbits is the peak rate of transfer for a SATA 150 device. Problem is, the speed of the drive can't keep up with 150 Mbits. The only time you'll see the peak rate is a burst read from the cache of the disk.

This, of course, is no different for SCSI disks. SCSI disk can have faster motors (10,000 rpm or 15,000 rpm in some cases) which would allow for a faster sustained transfer rate.

daihard · 12-24-2004, 10:57 PM

Quote:

Originally posted by jtshaw
What you are seeing there is the sustained transfer rate of the disk. 150 Mbits is the peak rate of transfer for a SATA 150 device. Problem is, the speed of the drive can't keep up with 150 Mbits. The only time you'll see the peak rate is a burst read from the cache of the disk.

I see what you mean. (I believe you meant to say the peak transfer rate of SATA is 150 Mbytes per second.) So what you're saying is that in theory, the maximum data transfer rate of the current Serial-ATA is about the same as that of Ultra160 SCSI, but the performance of the actual hard disk isn't keeping up with it. Did I get it right?

jtshaw · 12-24-2004, 10:59 PM

Ya, and your right, in this case I did mean MBytes

daihard · 12-24-2004, 11:21 PM

Quote:

Originally posted by jtshaw
Ya, and your right, in this case I did mean MBytes

Thanks a lot... you helped me understand a bit more about how hard disks work.

It leaves me wondering how much the maximum data transfer rate really matters, though. Sure, Ultra320 SCSI can do the max of 320 MB/sec, but if the hard disk can only do 50 MB/sec most of the time, isn't it kinda silly to spend a lot of money on an Ultra320 SCSI interface?

kak · 12-24-2004, 11:37 PM

After reading your thread, I had to see what "hparm -t" gave me on my system. And I am curious isn't SATA faster than ATA133?? Or are the Linux drivers lacking?
I have a Maxtor ATA133 8mb buffer 250GB 7200rpm hard drive(I got it for storage, not speed); Which I would think would have slower transfer rates, in theory right?

My output:

Quote:

bash-2.05b# hdparm -t /dev/hda

/dev/hda:
Timing buffered disk reads: 174 MB in 3.01 seconds = 57.81 MB/sec
bash-2.05b# hdparm -t /dev/hda

/dev/hda:
Timing buffered disk reads: 174 MB in 3.01 seconds = 57.81 MB/sec

Can anyone explain? I even ran it twice to verify.

daihard · 12-24-2004, 11:55 PM

From what I understand now, the sustained transfer rate (STR) of a hard disk has nothing to do with the (external) data transfer rate of the interface. In our case, my Serial ATA interface is capable of transferring 150MB/sec whereas your ATA133 is capable of transferring 133MB/sec.

However, the STRs of our physical hard disks are an entirely different story. My Maxtor drive, even with the Serial-ATA interface, only gets an STR of 50MB/sec whereas yours can do faster - 57 MB/sec.

My other Linux box has an ATA/133 Maxtor drive (200GB). The interface is actually ATA/100 on the motherboard. The STR comes out as 57 MB/sec, though.

kak · 12-25-2004, 09:02 AM

Thanks I got it.....in a nutshell max vs. real world and to increase use of external DTR, a raid setup would be in order.