[NTLUG:Discuss] Disk performance.

[Stephen Klein]

Steve Baker wrote:
> 
> Jim Wildman wrote:
> >
> > For 100's of gigs, I would definitely look at scsi and maybe even an
> > external hardware raid cabinet.  This is not going to be cheap...
> 
> Tell me the bad news.

Well, some years ago I was administering/engineering/installing... a
Netware network and built myself a superserver (for the time) using an
Adaptec 3 channel SCSI-2 raid board.  I think I was averaging around
10MB/sec continuous r/w activity off three spindles configured in a
4GB+2GB (parity) RAID 5 array.

The bad part was that the board itself cost >$700.  A newer board costs
about $400 (at Fry's) and they also have a single channel version that
costs less, but still not cheap.  But they're not supported in Linux
anyway.  Your best bet will probably be either software RAID (which is
extremely computationally intensive), or an AMI MegaRAID board which is
supported, but I have no experience with their stuff.

> 
> > Other issues to consider:
> 
> >         1) How many drives do you need on each controller?
> 
>              I may need up to 0.6 Terabytes in the largest system,
>              as little as 50Gb in the smallest.  So - between 1 and ~12
>              drives.  You can have 16 widgets on a SCSI bus - right?

The older narrow SCSI is limited to 7 devices per buss, wide SCSI is
limited to 15 devices as long as they are all Wide SCSI.  Otherwise the
limit is reduced to 7 if a narrow SCSI device is connected to the chain.

As a general rule, I recommend keeping the number of devices on a single
SCSI chain to the bare minimum that you can afford.  Believe me, it
saves some headaches.  A 12 drive cable may be somewhat difficult to
locate as well (I've never tried though).

> 
> >         2) How much power will you need?
> >         3) How much cooling/airflow do you need
> 
>              Dunno - this can go into a custom cabinet - so I imagine
>              we can engineer that.

I had 3 7200rpm drives in mine, and it got really HOT if they were
really getting pounded.  So expect to have in excess of four 3in. fans
in each of proabably 2 external drive cabinets.  Or, just by multiple
Hermanators (saw it at Fry's) and mount your drives in them, and then
mount them in the cabinet.  That'll keep'em cool. :-)

> 
> >         4) What about fsck/reboot time?
> 
>              That could be pretty scarey with 0.6 terabytes!
>              I can mount the drives read-only though...maybe
>              that'll prevent us from needing to fsck.
> 
>              I can boot and swap from a regular $100 IDE drive - so
>              we don't have to worry about that.
> 
> --
> Steve Baker   HomeEmail: <sjbaker1 at airmail.net>
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Best advice, try and balance performance and your budget as best you
can.  This stuff can get really expensive quickly.  :)

Stephen