Re: READAHEAD

[Nuno Silva]

Hi!!

Andrew Morton wrote:
> age <ahuisman@xxxxxxxxxx> wrote:
>
> > I have a problem which i don`t understand and i hope that you
> > will and can  help me. The problem is that i experience strange disk
> > read performance. I have to set hdparm -m16 -u1 -c1 -d1 -a4096 /dev/hde
> > to get  timing buffered disk reads of 56 MB/SEC.
> > When i disable readahead i get 17 MB/SEC
> > When i enable readahead with -a8 i get  17 MB/SEC
> > When i enable readahead with -a16 i get 24,5 MB/SEC
> > When i enable readahead with -a32 i get 30,5 MB/SEC
> > When i enable readahead with -a64 i get 35 MB/SEC
> > When i enable readahead with -a128 i get 39 MB/SEC
> > When i enable readahead with -a256 i get 39 MB/SEC
> > When i enable readahead with -a512 i get 41 MB/SEC
> > When i enable readahead with -a1024 i get 50 MB/SEC
> > When i enable readahead with -a2048 i get 50 MB/SEC
> > When i enable readahead with -a4096 i get 56 MB/SEC
> > With -a8192,-a16384 and -a32768 i get also 56MB/SEC
> >
> > Before, i never had to set readahead so high
> > Please could you tell me, what is going on here ?
>
>
> Lots of people have been reporting this.  It's rather weird.

I know nothing about this but, FWIW, I think that what changed where the 
units. With 2.4 you specify sectors, with 2.6 you specify bytes.

So, having -a8, in 2.4, is the same as having -a$((8*512)) [it's 4096 
:)], in 2.6.

Not sure if it's the case, but makes sense :-)

Regards,
Nuno Silva

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