Very poor performance from Dell/LSI Logic SAS 3000 series SATA/SAS RAID controller FreeBSD 6.3

[Scott Long]

Warren Guy wrote:
> Scott,
> 
> Thanks a lot for that. This seems to have alleviated the problem, I'm seeing
> decent performance now in my limited benchmark. It seems quite odd to me that
> the write cache is not enabled by default, but oh well.
> 
> Thanks again for your help!
> 
> Warren

For data reliability, you really don't want it enabled by default.  The
problem is that SATA/ATA performs so poorly without it that everyone
turns it on and lives with the consequences.  The tweak that I
recommended puts it in line with what the FreeBSD ATA driver has been
doing for years.

According to your original benchmark, Linux performs better on the
sequential tests, but those simply aren't representative of most
people's workloads.  Linux indeed has some tricks to make sequential
benchmarks perform well, but they aren't tricks that I'm all that
interested in implementing in FreeBSD (though increasing the maxio size
for 64-bit platforms would help and has few detrimental effects).

The same benchmark shows that FreeBSD performs just as well, if not
better, than Linux in random tests, even without the write cache
enabled.  Those tests are more representative of typical workloads.  So,
it's up to you to analyze what kind of workload you expect, and make the
appropriate tradeoffs.

Scott