[Gllug] A SATA Raid card worth having?

[Andy Farnsworth]

Russell Howe wrote:

> <snip>
>
>With SCSI, the bus speed is more important, since you can have perhaps
>15 devices on a single chain (perhaps more? I'm not too up to date with
>my SCSI knowledge). If you say your SCSI interface is 300Mbyte/s (U320,
>say?), that's 20Mbyte/s per device.
>  
>
This is why you use 15 drives on a SCSI chain :)  Each drive can easily 
supply the 20+ Mb/s throughput so you can keep your 320 Mb/s pipe fully 
saturated.  Actually, if you go with 15,000 RPM drives you should get 
almost double the throughput of 7200 drives, at least as long as you 
stay on a cylinder, once you cause the head to move, it is not 
appreciably faster on the 15,000 RPM drives, however latency is halved.  
This means that in most cases you can keep 320U SCSI saturated with 
about 4 drives on the channel.

Latency is the average amount of rotation the disk has to do before the 
correct location on the disk is under the read/write head of the drive.  
This average works out to about 1/2 a revolution so a 7200 rpm drive 
would have to wait 1/2 of 1/120 or 1/240th of a second which seems dmn 
fast until you think that in that time your computer CPU has gone 
through about 8,333,333 (based on 2 Ghz cpu) clock cycles and that was 
just waiting for the disk to make 1/2 a revolution.  If you go with 
15,000 RPM drives the latency drops to 1/500th of a second and the same 
CPU has to wait for 4,000,000 clock cycles.  Latency can also be tackled 
by using Mirrored drives.  If you mirror two identical drives then which 
ever drive reaches the data first finishes the race.  Therefor, with two 
drives mirrored, you can 1/2 the wait time (i.e. only have to wait for 
1/4 turn of the platter) and with 3 you wait 1/3, 10 would let you wait 
1/10th.  This is not the case with striping however, with striping it 
gets worse not better as you have to wait for ALL the drives to reach 
r/w heads.  Good news however it never get more than twice as long a 
wait since you never have to wait longer than it takes your slowest disk 
to make one complete revolution.  So with a set of 7200 RPM drive you 
have to wait 1/120th of a second and a set of 15,000 RPM drives you wait 
1/250th of a second. 

Whew, lecture on hardware... didn't mean to digress like that, oh well...

Andy
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