Re: [Moosefs-users] o_direct for chunkserver

[Corin L. <in...@co...>]

Hi Michal,

Am 30.03.2012 13:00, schrieb Michał Borychowski:
> We already thought about O_DIRECT some time ago but even Linus Torvalds
> advises against using it (citation below) and probably it would decrease the
> chunkserver performance.
>
> On the other hand it would quite easy to implement O_DIRECT, but we would
> need some strong arguments for doing this. Can you share some with us?
>

My only intent is to avoid double buffering. I'm using moosefs for 
storing (big) vserver images only. Every physical machine in my cluster 
acts as a chunkserver as well as a host for some of the vservers. I 
assume double buffering occurs here without using o_direct for 
chunkservers, since the chunkservers cache the images (and a few 
additional bytes the chunkservers use for metadata) and the hosts 
(vservers) mounting the images cache them again. I assume it'd be much 
better in terms of performance and memory usage to have the data cached 
on the client side, where the image is mounted/ used, only. This is 
especially a concern when a vserver's image is on the same host as the 
chunkserver serving it.

Is this a real concern or am I missing something? :)

Thanks,
Corin