http://en.wikipedia.org/wiki/Hard_diskOriginally Posted by emilianomorgia
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...
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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.
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