Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

[Avi Kivity <avi@redhat.com>]

On 03/15/2010 08:48 PM, Anthony Liguori wrote:
> On 03/15/2010 04:27 AM, Avi Kivity wrote:
>>
>> That's only beneficial if the cache is shared.  Otherwise, you could 
>> use the balloon to evict cache when memory is tight.
>>
>> Shared cache is mostly a desktop thing where users run similar 
>> workloads.  For servers, it's much less likely.  So a modified-guest 
>> doesn't help a lot here.
>
> Not really.  In many cloud environments, there's a set of common 
> images that are instantiated on each node.  Usually this is because 
> you're running a horizontally scalable application or because you're 
> supporting an ephemeral storage model.

But will these servers actually benefit from shared cache?  So the 
images are shared, they boot up, what then?

- apache really won't like serving static files from the host pagecache
- dynamic content (java, cgi) will be mostly in anonymous memory, not 
pagecache
- ditto for application servers
- what else are people doing?

> In fact, with ephemeral storage, you typically want to use 
> cache=writeback since you aren't providing data guarantees across 
> shutdown/failure.

Interesting point.

We'd need a cache=volatile for this use case to avoid the fdatasync()s 
we do now.  Also useful for -snapshot.  In fact I have a patch for this 
somewhere I can dig out.

-- 
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