What is the point of SCSI in a Mac Pro

[ping141]

Hi again! Brand new Catalyst system here with a Mac Pro, and I see that they have put in a SCSI card and a SCSI drive.

What is the point of this when you can purchase 2 x s-ata drives, raid them and easily get a 100 mb/s in or out?

[samsc]

because sata discs have terrible 'random read access' performance.

scsi has much faster random read access because the discs spin faster.

[emilianomorgia]

because sata discs have terrible 'random read access' performance.

What do we need " 'random read access' " for ?

[samsc]

What do we need " 'random read access' " for ?

http://en.wikipedia.org/wiki/Hard_disk

because each the file for each layer you try to play back will be at a different physical place on the disc.
the disc read head - a mechanical actuator - has to move between these places on the rotating head to read each frame - and it has to do this 25 or 30 times a second for each layer.

For 4 layers the disc read head will be moving to different physical sector on the disc at least ( and possibly more ) 100/25fps to 120/30fps times per second.

For 8 layers its 200/25fps 240/30fps.

Discs have a very hard time moving the head 200 times a second.

With SATA discs this figure is out of their reach.
With SCSI its possible.

why?

Because a SATA disc rotates at only 7200 rpm or only 120rps.
Which means you can only really do 120 transactions per second on a sata disc - you can only read different 'random' data at 120 operations/second - even if the 'headline bandwidth' is 100MB/s it makes no difference.
You can only read the data from 1 track on the disc before it has to move to another - and in the worst case- it can only do this after another rotation of the disc.


A 15k scsi disc rotates 240rps
so it can theoretically do twice as many random accesses/second.

its not the scsi or sata interface that makes any difference - if sata drives rotated at 15000rpm they would be just as good as scsi at 15000rpm- ( given adequate bandwidth in high bandwidth uses )

This is also the reason why RAID configurations dont help that much- you can RAID as many discs as you want - get 300MB/s but the discs are still rotating at only 120 rps...
---

The capacity of the disc has increased exponentially over the last few years - and the interface speed has increased from ata to sata 1.5gps - to sata 3.0 gps-

but in reality the disc random access times have not increased at all in this time - and are now a serious problem restricting the speeds of many operating systems- particularily when virtual memory systems use disc files as a their storage mechanism.

The drive manufacturers have had to invent many clever buffering schemes to work around these things - but they still cannot nail the random access time - as this is dependant on the speed at which the disc rotates.
The performance of these caches and buffers is what can make one drive work better than another for sequential files.

[jasonrudolph]

This is also the big reason as to why solid state drives (SSD's) are on everyone's watch list. As the prices come down, they will have a huge impact, as there are no moving parts, so while pure read and write throughput doesn't go up very much (if at all) compared to today's harddrives, their random access times are much better.

[samsc]

i have just begun testing an MTRON 32GB SATA SSD drive.
SATA means it can be plugged into any G5. Intel needs a SATA extension, until i find a drive tray adaptor.

SSD have very variable performance - they arent all fast. at all.
The SAMSUNG SSDs are only available with IDE interfaces. i have one - and the IDE interface is the limit to its performance.

MTRON drives are more expensive but much faster.
http://www.dvnation.com/nand-flash-ssd.html

Tests in my quad g5 show its almost as fast as scsi on most codecs - but can playback more than 10 dv layers at once (SCSI does about 7 ) . without any problems.

[peppe]

The ultimate way should be to have layer 1 to read from disc 1, layer 2 from disc2, layer 3 also from another disc etc.etc

The when you hit layer 5 you access disc number 1, layer 6 from disc 2, etc
The you will have max 2 files going on one disc.

The discs are getting bigger and bigger, but (as always, Richard is right)
they are to slow to handle multiply files on a disc , and thats often the setup with a Catalyst system.In this case I get good results (as I don´t want to use DV codec) with multiply FW800 discs and a extra FW card....

But it is tricky to place all the files to be handled correct since everyone wants to "create on the fly", but that´s how I use to handle a system that I know need to NOT play any files under 25fps whatever.....

[RedLightning]

Kind of along the same lines...

Has anyone seen the new hardware RAID card available with new MacPro's?

-s

[samsc]

Kind of along the same lines...

Has anyone seen the new hardware RAID card available with new MacPro's?

-s

you can only buy this as a built to order option with a new machine - you cannot buy it separately - and apparently - i have been told - you cannot use non-apple hard drives either-
so its not an upgrade option.

[RedLightning]

you can only buy this as a built to order option with a new machine - you cannot buy it separately - and apparently - i have been told - you cannot use non-apple hard drives either-
so its not an upgrade option.

Interesting...I didn't notice that as a requirement...

Hardware RAID cards are available from other vendors, I was a little more curious as to whether it would improve speed.

-s