Document that directio does not apply to cached files#259
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climagabriel wants to merge 1 commit intonginx:mainfrom
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Document that directio does not apply to cached files#259climagabriel wants to merge 1 commit intonginx:mainfrom
climagabriel wants to merge 1 commit intonginx:mainfrom
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Users commonly assume directio takes effect for files served from proxy_cache (the most widespread nginx deployment), and configure it expecting reduced page cache pressure on the cache path. It does not: cached responses are served through buffered I/O regardless of the directive. See nginx/nginx#654 for context.
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directioonly works for static content in Nginx. It's by design.So everything we serve from disk cache passes through the page cache and always has.
Motivation
People routinely assume directio applies to cache files, which it currently doesn't. They enable it expecting a decline in page cache pressure or memory pressure and are confused to see no change at all.
Related: nginx/nginx#654. freenginx has since added it — see freenginx 306b71004f29 ("Cache: directio support when reading cache files") and the follow-up 6afd081c554e ("Open file cache: correct directio handling with threads").