Re: Warning - running *really* short on DMA buffers while doing file transfers

Pedro M. Rodrigues
2002-09-27T12:23:31Z

Justin T. Gibbs wrote:
>> I reported this same problem some weeks ago -
>>http://marc.theaimsgroup.com/?l=linux-kernel&m=103069116227685&w=2 .
>>2.4.20pre kernels solved the error messages flooding the console, and
>>improved things a bit, but system load still got very high and disk read
>>and write performance was lousy. Adding more memory and using a
>>completely different machine didn't help. What did? Changing the Adaptec
>>scsi driver to aic7xxx_old . The performance was up 50% for writes and
>>90% for reads, and the system load was acceptable. And i didn't even had
>>to change the RedHat kernel (2.4.18-10) for a custom one. The storage was
>>two external Arena raid boxes, btw.
>
>
> I would be interested in knowing if reducing the maximum tag depth for
> the driver improves things for you. There is a large difference in the
> defaults between the two drivers. It has only reacently come to my
> attention that the SCSI layer per-transaction overhead is so high that
> you can completely starve the kernel of resources if this setting is too
> high. For example, a 4GB system installing RedHat 7.3 could not even
> complete an install on a 20 drive system with the default of 253 commands.
> The latest version of the aic7xxx driver already sent to Marcelo drops the
> default to 32.
>
> --
> Justin
>
>
>

I have a server available to test it, but the storage in question is
already deployed. Yet, by luck (irony apart) i have a maintenance window
this weekend for tuning and other matters, i can decrease the maximum
number of TCQ commands per device in the proper aic7xxx driver to 32 and
report on the results. While trying to solve this problem i browsed
RedHat's bugzilla, and there were several people burned with this
problem. Hope this sorts it out for them.

/Pedro