linux memory: limiting filesystem caching

Hi all
I have several redhat blade clusters running 10.1.0.4 RAC on 
2.4.9-e.43enterprise. All database storage is OCFS, with ext3 for backups, home 
dirs etc. The servers have 12GB of RAM, of which about 2GB is allocated to the 
database, which is fine. Linux, in its wisdom, uses all free memory (10GB in 
this case) for filesystem caching for the non OCFS filesystems (since OCFS uses 
directIO); so every night when I do a backup it swallows up all available 
memory and foolishly sends itself into a swapping frenzy; and afterwards it 
sometimes cannot allocate enough free mem for background processes. This seems 
to be worse on e43; I was on e27 until recently. Does anyone know how to 
control filesystem block caching? Or how to get it to de-cache some? For 
instance, I have noticed that gziping a file, then ctrl-C'ing it can free up  a 
chunk of RAM, I assume it de-caches the original uncompressed file. But its not 
enough!

Rgds
Mark

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