Accessing SCSI-Devices >2TB

[Paul Mather]

On Thu, 2005-06-09 at 11:36 +0200, Raphael H. Becker wrote:

> So, just to work around the LBA64 problem, the idea was to recombine
> those two "drives" /dev/da1 and /dev/da2 into a RAID (in FreeBSD).
> 
> Two ways to do this:
> a) striping
>   Every access to the logical drive in FreeBSD (/dev/ccd0) will access
>   both partitions PRT1 and PRT2. Remember: Every single access to ONE 
>   of those two will result in physical disc action for PD2-PD12 and every 
>   access to /dev/ccd0 therefore will result in two accesses for each
>   physical disc.  --> Bad Idea.

Perhaps it's because I haven't had my coffee, yet, :-) but why would
this be so?  If you make your stripe size large enough, you can create a
hybrid effect between concatenation and striping---concatenation for
smaller files and striping for larger ones.  (If a file is smaller than
the stripe size, access will go only to the drive on which that stripe
resides.  However, if it is larger, then access will be spread over
multiple drives, improving load.)

I am using geom_stripe on two 80 GB PATA drives to create a 120 GB
stripe across both.  (I only use part of each drive, and use the other
part for a geom_mirror.)  I have my stripe size set at 1 MB.  I've been
very happy with the performance.  I used raidtest (now in ports) with
different stripe sizes beforehand to try and ascertain a good stripe
size.  In my case, performance rose steadily and noticeably up to a 1 MB
stripe size, and then was only slightly better as I increased
thereafter.  (I stopped at a 16 MB stripe.)

Of course, this depends upon the kind of workload your system is
expected to have.  Additionally, in your case, you have the interaction
with the hidden RAID 5 and whatever stripe size it is using.  That
possible interaction alone might be enough to use only concatenation,
rendering the situation the same as using a large RAID 5.

Cheers,

Paul.
-- 
e-mail: paul at gromit.dlib.vt.edu

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