FreeBSD 9.0 hangs on heavy I/O

[Kees Jan Koster]

Dear Freddie,

>> You may want to play around with gshed, the GEOM Scheduler.
>> 
>> Matt Dillon did a bunch of tests comparing FreeBSD+UFS to
>> DragonflyBSD+HAMMER and found that FreeBSD starves read threads in
>> order to satisfy write threads (or the other way around?).  But,
>> adding gsched into the mix helped things immensely, allowing mixed
>> reads/writes to better shares disk I/O resources.
>> 
>> I'll see if I can dig up a link to his testing e-mail messages.
> 
> Here's the post, part of a thread on benchmarking RAID controllers:
> 
> http://leaf.dragonflybsd.org/mailarchive/kernel/2011-07/msg00034.html

I looked at "sysctl kern.geom.confdot" (another ridiculously useful feature) to see where the scheduler should be placed.

The way I was thinking, I should place a scheduler in such a way that writes to one physical device (ada3 in my case) do not cause reads on another device to stall (e.g. ada2, where the database lives). However, it looks like the GEOM tree is actually a GEOM bush, with a separate tree for each device.

Am I missing something? Is there a way to schedule across devices? Is the bush a tree after all, maybe?
--
Kees Jan

http://java-monitor.com/
kjkoster at kjkoster.org
+31651838192

The secret of success lies in the stability of the goal. -- Benjamin Disraeli