Re: [RFC][DATA] re "ongoing vm suckage"

[Alan Cox]

> On Sun, 5 Aug 2001, Mike Black wrote:
> And quite frankly, if your disk can push 50MB/s through a 1kB
> non-contiguous filesystem, then my name is Bugs Bunny.

Hi Bugs 8), previously Frodo Rabbit, .. I think you watch too much kids tv
8)

[To be fair I can do this through a raid controller with write back caches
and the like ..]

> You're more likely to have a nice contiguous file, probably on a 4kB
> filesystem, and it should be able to do read-ahead of 127 pages in just a
> few requests.

One problem I saw with scsi was that non power of two readaheads were
causing lots of small I/O requests to actual hit the disk controller (which
hurt big time on hardware raid as it meant reading/rewriting chunks). I
ended up seeing 128/127/1 128/127/1 128/127/1 with a 255 block queue.

It might be worth logging the number of blocks in each request that hits
the disk layer and dumping them out in /proc. I'll see if I still have the
hack for that around.

Alan