[lustre-discuss] Two fd's for a single file, 1 with O_DIRECT, 1 without O_DIRECT

[John Bauer]

I have a file where I would like to keep part of the file in system 
cache and the other part not.  So I open the file twice, once with 
O_DIRECT, and once without O_DIRECT.

I have written a simple testcase where I write the first 50 MB of the 
file without DIRECT, and the second 50M with O_DIRECT.  The file is 
striped 4x1M.  It would appear that when the second 50M of the file is 
written O_DIRECT, the first 50MB of the file is dropped from system 
cache.   Both fd remain open during the entire process.  From what I can 
tell, it seems that once a given stripe is impacted by an O_DIRECT  
write then the entire stripe is dropped from cache. Is this the expected 
behavior?

John
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