Re: paralellism of device use in md

[Francois Barre <francois.barre@gmail.com>]

2006/1/18, Mario 'BitKoenig' Holbe <Mario.Holbe@tu-ilmenau.de>:
> Mario 'BitKoenig' Holbe <Mario.Holbe@TU-Ilmenau.DE> wrote:
> > scheduled read-requests. Would it probably make sense to split one
> > single read over all mirrors that are currently idle?
>
> A I got it from the other thread - seek times :)
> Perhaps using some big (virtual) chunk size could do the trick? What
> about using chunks that big that seeking is faster than data-transfer...
> assuming a data rate of 50MB/s and 9ms average seek time would result in
> at least 500kB chunks, 14ms average seek time would result in at least
> 750kB chunks.
> However, since the blocks being read are most likely somewhat close
> together, it's not a typical average seek, so probably smaller chunks
> would also be possible.
>
>
> regards
>   Mario

Stop me if I'm wrong, but this is called... huge readahead. Instead of
reading 32k on drive0 then 32k on drive1, you read continuous 512k
from drive0 (16*32k) and 512k from drive1, resulting in a 1M read.
Maybe for a single 4k page...

So my additionnal question to this would be : how well does md fit
with linux's/fs readahead policies ?