[CentOS] Re: Anaconda doesn't support raid10

Fri May 11 03:40:25 UTC 2007
Feizhou <feizhou at graffiti.net>

> SATA and SCSI/SAS should give comparable single work-load sequential
> numbers, but most SCSI/SAS have better seek times so random IO will
> be better on those. Also SCSI/SAS support tagged command queuing,
> which allows multiple overlapping IOs so you will tend to see better
> mixed workload performance compared to SATA (multi-user environment).
> 
> Having said that some of the new SATA models that support NCQ, the
> SATA version of TCQ, and that use some of the same SCSI/SAS onboard
> processing (Western Digital Raptors) can approach or equal SCSI/SAS
> mixed load performance, but their price also approaches or equals
> SCSI/SAS and their spindle speeds still do not top 10K so random
> will not be as good.

Nope. SATA drives with NCQ support are in the same price range as non 
NCQ drives, if not the same price, which is by far less costly than 
SCSI/SAS drives. Also, due to the incredible packing to get 200GB-750GB 
of space onto the platter, the lower RPM is made up for especially at 
the higher densities.

A 73GB 15K RPM SCSI/SAS drive is in the same price range as a 750GB 7.5K 
RPM NCQ capable SATA drive. Ten times the density at half the speed. I 
reckon it would give the SCSI drive a run for its money in regard to 
random access times.

The SCSI drive:
Spindle Speed	15000 rpm
Average latency	2.0 msec
Random read seek time	3.50 msec
Random write seek time	4.0 msec

The SATA drive:
Spindle Speed	7200 rpm
Native Command Queuing	Y
Average latency	4.16 msec
Random read seek time	<8.5 msec
Random write seek time	<10.0 msec
Maximum interface transfer rate	300 Mbytes/sec

Without NCQ enabled, it will take twice as long as the scsi drive. With 
NCQ enabled, the game changes.

Compare to a 10K scsi drive:
Spindle Speed	10,000 rpm
Sustained data transfer rate	80 Mbytes/sec.
Average latency	3.0 msec
Random read seek time	4.9 msec
Random write seek time	5.4 msec
Maximum interface transfer rate	320 Mbytes/sec

NCQ SATA is almost a no-brainer.