Accessing SCSI-Devices >2TB

[Bruce Evans]

On Tue, 14 Jun 2005, Glenn Dawson wrote:

> This is a little OT for this thread, but I'm currently trying to solve a 
> problem which might be related to the aac driver, so I'll toss this out there 
> and see if anyone has any ideas for me.

It seems to be very OT :-).

> It trying to make the transition from 4.x to 5.x I've noticed that 
> performance of the md(4) (in 5.x) driver seems to be much worse than the 
> vn(4) driver under 4.x.
>
> Here's the scenario:
>
> A pair of mirrored disks on an adaptec 2120S using the aac(4) driver is being 
> used to house files that are the backing store for a number of virtual disks. 
> Each file is typically about 4GB.
>
> Under 4.11 using the vn(4) driver I see throughput of about 17MB/sec when 
> writing to one of the virtual disks (using dd).
>
> Under 5.4 using the md(4) driver I only see throughput of about 7MB/sec when 
> writing to the virtual disk.

This seems to be from sync writes now being the default for md.  Try using
"mdconfig -o async ...".  I will reply with more details to your message
in freebsd-performnance.

Bruce