[uml-user] Re: [uml-devel] O_DIRECT for ubd access possible?

[Nuno S. <nun...@vg...>]

[cross-posting to uml-user and uml-devel, sorry]

Hi!!

Marcus Blomenkamp wrote:
> Hi all,
> 
> we are using UML successfully for different purposes for some time now, 
> however there is one issue which cannot be solved either by Standalone- or 
> UM-Linux. It's also popping up more or less regularly on linux-kernel. It is 
> the problem of cache pollution on reading large files, even though we know 
> that we will not reuse this data in reasonable time. Application examples: 
> streaming media servers, fileservers for media content etc. This one bites 
> all users no matter how much physical ram the machines have.
> 
> The solutions proposed on kernel mailing list, such as O_DIRECT, fadvise(), 
> O_STREAMING, are IMHO less than half the way if used as proclaimed. For them 
> it is up to the system administrator to patch every single program involved 
> (nfsd, smbd) to use the feature/flag correctly. 
> 

I have a partial solution for this: disable host's swap. Simple and very 
wise if:

- You have enough memory to hold all your programs and a little bit extra;
- You don't want to do disk-cache above your free physical-RAM.

I like UML very much but, for this particular problem, I don't think 
that UML will help, because:

- Adding an extra layer will make things slower and add complexity;
- UML itself will use memory;
- UML's overhead is almost nothing for non-system load but for "system 
load" UML has lots of overhead (from 10% to 50% depending on the workload);
- If you want UML to do "disk" I/O it'll be much slower than the 
physical host because it's "system load".


Anyway, try disabling the host's swap and try again! You'll be pleased :)

Regards,
Nuno Silva