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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