Re: aac0: COMMAND 0xffffffffxxxxxxxx TIMEOUT AFTER xx SECONDS

On Fri, 9 Jun 2006, Scott Long wrote:
> What the battery gives you is consistency of the parity data in the case of 
> power loss.  You can have a situation where a block is being modified, and 
> thus the parity also needs to be modified.  If the block
> gets written but not the parity, or the parity gets written but not the
> block, the stripe will be inconsistent.  You won't see this until you
> have a drive failure and are trying to do a rebuild from teh parity.  By
> that point, it's too late, you'll have silent data corruption due to the
> inconsistency.  For RAID-0, the battery is pointless, and for RAID-1, the 
> battery is nearly pointless; the mirror members will either agree or
> not, and if they disagree the worst that will happen is that you'll get
> old data.  This is no different than if the OS crashes without flushing
> out all buffers.  Old data is much easier to recover from than corrupt
> data, which is what you get if the parity is inconsistent.

That sounds incredibly bad.  I'm glad I've turned off write caching, but 
I'm not really glad about the performance hit.

> Software RAID will almost always be faster for trivial tasks than PCI
> RAID.  What PCI RAID gives you is task offloading from the CPU, and
> protection while the OS is not running.  If your CPU is sitting idle
> most of the time, then software RAID often is a win.

Somewhat OT, but it reminds me of the seemingly endless debate about 
minicomputer controllers in the early '90s and whether they got better 
performance with an onboard CPU or taking cycles from the "incredibly 
fast" new host RISC processors...  :)

Back to gmirror, I have to say I'm very happy with its performance, 
though, it seems to suit my workload well.

> That said, the design of a PCI RAID controller plays a huge role in how
> it performs.  Let's just say that the 2410 design is, um, "low end". There 
> are other cards out there from several vendors, especially the newer 
> generation ones that use PCI-Express and PCI-X, that perform a whole heck of 
> a lot better.  I have several cards that beat software
> RAID by a wide margin, but they are also expensive.

I'm starting to get the impression that the 2410SA's low end design is 
even lower than its fairly low-end price would suggest.  Any suggestions 
for similarly priced cards with better performance?  The best I can manage 
for slot type is 66x64 PCI unless I change my motherboard, which I can't 
quite afford to do at the moment...

In fact since I've just said I'm so happy with gmirror, I wonder if I 
wouldn't be better off just connecting the drives straight to the 
motherboard's SATA ports and running graid3...?  Although that might make 
booting an interesting experience.

Cheers,

Chris.