[parisc-linux] Re: 53c700: tagged queueing problem confirmed

[James Bottomley]

5116@telia.com said:
> I have used these drives with i386 Linux without problems with both
> the aix7xxx driver and the sym53c8xx driver. Both these drivers use a
> default of max 8 tags per lun, so I chnaged the value in your driver
> and it works. So, maybe 8 tags per lun is the conservative default
> here ? 

That's not unreasonable.  You're going to run into problems with the new 
aic7xxx driver, though, which has a default of 253 tags.

Usually you want larger numbers of tags to allow multiple active I/Os and give 
the device the most freedom to order them efficiently.  However, in linux we 
have the elevator to consider as well:  The more tags, the less time the I/Os 
spend in the elevator queue and the less chance of coalescing adjacent I/O.  
For any given workload, there's probably an optimal tag number and I bet they 
cluster around some value for a variety of workloads, but I've no idea what it 
is.

I'd still be wary of allowing tags on this drive.  The firmware clearly has a 
point beyond which it goes haywire and you don't really know where that is.  
It could be that reducing the max tags to 8 makes this unlikely but not 
impossible, so you may still end up with insidious data corruption 
possibilities.

James