OT: Torn between SCSI and SATA for RAID

[Jim Stapleton]

I've found that scsi isn't exceptionally faster given similar RPMs, or
even slightly higher RPM (ex. a 10K RPM SCSI vs. 10K RPM SATA drive
would have simlar performance). However, SCSI tends to high tighter
standards, and you get the following advantages, which in some cases
are worth the money, and in some cases arent:

(1) More reliable/accurate reads/writes
(2) Longer expected lifespan


My advice for reliability is a RAID-1 setup with the most
cost-effective disks you can find, then use the OS to do a drive
spanning so you can put them in the same mount point (when it runs to
the end of a disk, it starts on the next). I'm not sure if the drive
spanning is possible though - I've not looked into it, though given
that Windows can do it, I don't see why FreeBSD would have trouble.

If that is still too expensive, you could try RAID-5, but the problem
with that is, adding new disks wouldn't be quite as easy, you may not
be able to use the RAID set until you get the replacement disk, and
it's not quite as fast (I could be wrong on this part) as RAID1 in the
case of writes.


> with a 8 Port card... Let's look at what I can achieve:
> Ports 1+2: 750GB Seagates (Biggest available), 1.5TB <- I'm short on my 2TB
> Initial
> Ports 3+4: Mirror of 1+2

Maybe I'm missing something, where ports 4-8 (actually, 0 + 4-7)? With
8 500GB drives, and RAID1, you should be able to get 2TB out of that
(and more cost effective than 750GB drives)

Have you considered using two controller cards?


> nother thing that I read that I'm not completely sure about.  Some of the
> Adaptec SCSI Cards advertises a max of 30 devices - some even more.  Excuse the
> ignorance, but does the SCSI Bus not allow for a max of 8 devices?  Do these
> cards then feature multiple buses to connect the cables to?  If so, SATA will
> obviously not be able to provide something like this.

8 devices, 1 is the controller, I think some newer busses hold 16
devices, is is the controlelr, (so 7 or 15 drives).

Now, a card may have multiple busses. I have an A-Ha 39160 in my
machine, and if I remember correctly it has 2 busses on it (or is it
three?), I don't use it to nearly it's capacity, I just got it for the
price of a 19160, and I couldn't turn down that option.


> Now comes my question... Uhm.. Can SATA RAID Controllers be 'linked'.  Say, I
> but 4 x 8-Port Adaptec SATA RAID Controllers... 2 x 8 Port Cards = 16 Ports for
> 1 RAID 5 Array (@ 750GB Drives, 12TB Max).  The other 2 cards, to mirror.  I
> know that I can use one Controller to mirror another, but can I extend a array
> across multiple controllers... And then naturally, just HOW much slower does
> the array function?

The array will be using system cpu/memory, so quite a bit, and it'll
cause a hit on system perofrmance, however, the trick here is you can
do what I mentioned above with some trickery (I think), and just have
the OS "link" the two file systems, it's not any RAID form, and
shouldn't cost much performance.