Re: Merging fails reading /dev/uba1

[Jens Axboe <axboe@suse.de>]

On Sun, Feb 20 2005, Pete Zaitcev wrote:
> Hi, Jens:
> 
> I think this question belongs to your domain, but please let me know
> if I'm mistaken, so I can pursue this elsewhere.
> 
> I encountered a strange performance anomaly. I do the following:
> 
> <----- Plug USB key
> [root@lembas ~]# time dd if=/dev/uba of=/dev/null bs=10k count=10240
> 10240+0 records in
> 10240+0 records out
> 
> real    0m22.731s
> user    0m0.004s
> sys     0m0.345s
> [root@lembas ~]#
> 
> <----- Remove and replug the USB key
> [root@lembas ~]# time dd if=/dev/uba1 of=/dev/null bs=10k count=10240
> 10240+0 records in
> 10240+0 records out
> 
> real    1m42.622s
> user    0m0.005s
> sys     0m1.518s
> [root@lembas ~]#
> 
> So, reading from a partition of the same device is 5 times slower than
> reading from the device itself. The question is, why?
> 
> To the best of my knowledge, this does not occur with SCSI (usb-storage
> and sd or sr). This hints strongly that the ub is not doing something
> right, but what that can be?
> 
> The ub takes the request processing machinery from Carmel exactly. I am
> wondering if Carmel (sx8) exhibits any similar performance anomalies
> (cc-ing to Jeff)

I can't explain why the replugging slows it down, maybe you were lucky
to get contigious pages in the first case? As far as I can see, ub
effectively disables merging by setting max hw/phys segment limit of 1.

-- 
Jens Axboe