Re: performance of O_DIRECT on md/lvm

[Arjan van de Ven <arjanv@redhat.com>]

Justin Cormack wrote:
> 
> Reading files with O_DIRECT works very nicely for me off a single drive
> (for video streaming, so I dont want cacheing), but is extremely slow on
> software raid0 devices, and striped lvm volumes. Basically a striped
> raid device reads at much the same speed as a single device with O_DIRECT,
> while reading the same file without O_DIRECT gives the expected performance
> (but with unwanted cacheing).
> 
> raw devices behave similarly (though if you are using them you can probably
> do your own raid0).
> 
> My guess is this is because of the md blocksizes being 1024, rather than
> 4096: is this the case and is there a fix (my quick hack at md.c to try
> to make this happen didnt work).

well not exactly. Raid0 is faster due to readahead (eg you read one
block and the kernel 
sets the OTHER disk also working in parallel in anticipation of you
using that). O_DIRECT
is of course directly in conflict with this as you tell the kernel that
you DON'T want
any optimisations....

Greetinsg,
  Arjan van de Ven