[Beowulf] Re: SATA or SCSI drives - Multiple Read/write speeds.

Bill Broadley bill at cse.ucdavis.edu
Tue Dec 9 18:31:55 EST 2003


On Tue, Dec 09, 2003 at 07:03:12AM -0800, Lombard, David N wrote:
> Very big pro:  You can get much higher *sustained* bandwidth levels,
> regardless of CPU load.  ATA/PATA requires CPU involvement, and
> bandwidth tanks under moderate CPU load.

I've heard this before, I've yet to see it.  To what do you attribute
this advantage?  DMA scatter gather?  Higher bitrate at the read head?

Do you have a way to quantify this *sustained* bandwidth?  Care to share?

> The highest SCSI bandwidth rates I've seen first hand are 290 MB/S for
> IA32 and 380 MB/S for IPF. Both had two controllers on independent PCI-X
> busses, 6 disks for IA32 and 12 for IPF in a s/w RAID-0 config.

Was this RAID-5?  In Hardware?  In Software?  Which controllers?

Do you have any reason to believe you wouldn't see similar with the
same number of SATA drives on 2 independent PCI-X busses?

I've seen 250 MB/sec from a relatively vanilla single controller setup.

Check out: (no I don't really trust tom's that much):
http://www6.tomshardware.com/storage/20031114/raidcore-24.html#data_transfer_diagrams_raid_5

The RaidCore manages 250 MB/sec decaying to 180MB/sec on the slower inner
tracks of a drive.  Certainly seems like 2 of these on seperate busses
would have a good change of hitting the above numbers.

Note the very similar SCSI 8 drive setups are slower.

> Does SATA reduce the CPU requirement from ATA/PATA, or is it the same?
> Unless it's substantially lower, you still have a system best suited for
> low to moderate I/O needs.

Do you have any way to quantify this?  Care to share?  I've seen many similar
comments but when I actually go measure I get very similar numbers, often
single disks managing 40-60 MB/sec and 10% cpu, and maximum disk transfer
rates around 300-400 MB/sec at fairly high rates of cpu usage.

> BTW, http://www.iozone.org/ is a nice standard I/O benchmark.  But, as
> mentioned earlier in this thread, app-specific benchmarking is *always*
> best.

Agreed.  Iozone or bonnie++ seem to do fine on large sequential file
benchmarking I prefer postmark for replicating database like access patterns.



-- 
Bill Broadley
Information Architect
Computational Science and Engineering
UC Davis
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