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From Plüm, Rüdiger, VF EITO <ruediger.pl...@vodafone.com>
Subject Re: Possible new cache architecture
Date Tue, 02 May 2006 12:21:27 GMT


> -----Ursprüngliche Nachricht-----
> Von: Graham Leggett 

> > The reason it does not work currently is that that a local file
> > usually is
> > delivered in one brigade with, depending on the size of the 
> file, one or
> > more
> > file buckets.
> 
> Hmmm - ok, this makes sense.
> 
> Something I've never checked for, do output filters support 
> asynchronous
> writes?

I don't think so. Of course this would be a nice feature. Maybe somehow
possible with Colm's ideas.
Another thing: I guess on systems with no mmap support the current implementation
of mod_disk_cache will eat up a lot of memory if you cache a large local file,
because it transforms the file bucket(s) into heap buckets in this case.
Even if mmap is present I think that mod_disk_cache causes the file buckets
to be transformed into many mmap buckets if the file is large. Thus we do not
use sendfile in the case we cache the file.
I the case that a brigade only contains file_buckets it might be possible to
"copy" this brigade, sent it up the chain and process the copy of the brigade
for disk storage afterwards. Of course this opens a race if the file gets
changed in between these operations.
This approach does not work with socket or pipe buckets for obvious reasons.
Even heap buckets seem to be a somewhat critical idea because of the added memory usage.


Regards

Rüdiger


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