[ILUG] ITX advice

[Michael Watterson]

paul at clubi.ie wrote:
> On Tue, 11 Sep 2007, Gavin McCullagh wrote:
>
>> You might be right that SCSI would be more reliable than SATA
>
> That's not true, good article (he links to a USENIX paper):
>
>     http://scalability.org/?p=211
>
> It hasn't been true in a long time either, if ever.
>
> regards,
The reason for SCSI is:
Decent HW RAID (most SATA/IDE "RAID controllers" are really SW RAID)
Scaling: Easy to have lots of drives
Cheap Clusters: Easy to share SCSI drives between two controllers if 
different Host IDs
Performance for multiuser (RAID 5, striping on a single bus works, 
IDE/SATA you need a separate controller for each drive to have 7 drives 
executing different commands at once.

HW RAID the SCSI host CPU manages the striping and parity calculation 
and you can boot from RAID5.

SW RAID the boot has to be part of RAID1 (slow writes), you then need a 
separate set of drives for RAID5

RAID1 is slower on average than single drive, 1/2 the disk space wasted 
in backup. Five drives in RAID5  the  wasted space is 1/5th and  
multiuser  random file access can be 10x or more faster than a single drive.

The rotation speed affects access time (sector latency), hence 15,000 
rpm (usually SCSI) is better than 5400rpm AV drive. Or 3,600rpm mp3 
player drive .

It's nothing to do with reliability. Never was. With 5 off 15,000rpm 
SCSI you might be 6 or 7 times more likely to get a failure than with a 
single  5,400 or 7,200 rpm AV Drive (SATA or PATA), but you loose no 
data instead of all of it. You might even have a hotspare and auto 
rebuild to make you  feel  safer while a replacement  drive is 
obtained.  I doubt SATA is yet supporting true HW RAID5 and and Hot 
spares.. I could be wrong.

I've used SW & HW RAID1, SW& HW RAID5 and on clusters RAID 10 (Striped 
Mirrors, it's awkward to implement a cheap cluster with RAID5) with SW 
RAID1 for each servers private Internal disk.


-- 
Mike