Re: ext2/3 performance regression in 2.6 vs 2.4 for small interl

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Feb 12, 2004 at 10:42:15AM +0100, Giuliano Pochini wrote:
> 
> On 12-Feb-2004 Andrea Arcangeli wrote:
> 
> > the main difference is that 2.4 isn't in function of time, it's in
> > function of requests, no matter how long it takes to write a request,
> > so it's potentially optimizing slow devices when you don't care about
> > latency (deadline can be tuned for each dev via
> > /sys/block/*/queue/iosched/).
> 
> IMHO it's the opposite. Transfer speed * seek time of some
> slow devices is lower than fast devices. For example:
> 
> Hard disk  raw speed= 40MB/s   seek time =  8ms
> MO/ZIP     raw speed=  3MB/s   seek time = 25ms
> 
> One seek of HD costs about 320KB, while on a slow drive it's
> only 75KB. 2.4 has a terrible latency on slow devices, and it
> has very small advantage in terms of speed. On CDs and DVDs
> the cost of a seek is much higher, but since the data is
> usually accessed sequentially you have the high latency
> penalty with no appreciable speed gain in this case too.

I was thinking at old slow harddisks (5M/sec), and I don't think all
data on cds is always accessed sequentially, you only need two tasks
reading two files.