[Lustre-discuss] Lustre drawback

[Oleg Drokin]

Hello!

On Dec 14, 2007, at 4:31 AM, Marco wrote:

> * Brian J. Murrell <Brian.Murrell at Sun.COM> [2007 12 13, 16:16]:
>> Consider a machine with a Lustre client on it which is experiencing
>> memory pressure.  One way to deal with that is to start flushing  
>> memory
>> pages to disk where for a Lustre client disk can mean to an OST.
> Ok, as you said, *can* mean to an OST. Let's suppose a host is running
> the OST and the client, but it has a local (not managed by Lustre)  
> swap.
> Let's say Lustre is used to provide access to a large dataset on a  
> SAN,
> but the system itself is stored on local disks on each node.
> In this case, if the client is experiencing memory pressure, it will
> just flush to the local disk, and it will continue doing so until  
> there
> is swap available... or not?

Only userspace pages could be swapped out.
Dirty fs data is not swapped out, instead it is attempted to be  
written by
calling FS.

Pathologic scenarios like "mmap a file region entire memory in size, and
dirty all of it" creates enormous amount of dirty pages in memory that
could not be swapped out and lustre need memory to flush out dirty pages
too. Of course there is some kernel reserve for those purposes, but
in some cases it might be not enough (I heard reports that 2.6 kernels
behave better in these conditions).

Bye,
     Oleg