Re: 3x slower file reading oddity

[dean gaudet]

On Mon, 17 Jun 2002, Andrew Morton wrote:

> dean gaudet wrote:
> > what if you have a disk array with lots of spindles?  it seems at some
> > point that you need to give the array or some lower level driver a lot of
> > i/os to choose from so that it can get better parallelism out of the
> > hardware.
>
> mm.  For that particular test, you'd get nice speedups from striping
> the blockgroups across disks, so each `cat' is probably talking to
> a different disk.  I don't think I've seen anything like that proposed
> though.

heh, a 128MB stripe?  that'd be huge :)

> You could fork one `cat' per file ;)  (Not so silly, really.  But if
> you took this approach, you'd need "many" more threads than blockgroups).

i actually tried this first :)  the problem then becomes a fork()
bottleneck before you run into the disk bottlenecks.  iirc the numbers
were ~45s for the 1-file-per-cat (for any -Pn, n<=10), ~30s for
100-files-per-cat (-P1) and ~1m15s for 100-files-per-cat (-P2).

> hmm.  What else?  Physical readahead - read metadata into the block
> device's pagecache and flip pages from there into directories and
> files on-demand.  Fat chance of that happening.

one idea i had -- given that the server has a volume manager and you're
working from a snapshot volume anyhow (only sane way to do backups), it
might make a lot more sense to use userland ext2/3 libraries to read the
snapshot block device anyhow.  but this kind of makes me cringe :)

-dean