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Date: Thu, 3 Jul 2014 17:24:10 +0000
From: Eric Wong <e@80x24.org>
To: Cedric Maion <cedric@maion.com>
Cc: unicorn-public@bogomips.org
Subject: Re: Log reopening broken on Rails 4 with config.autoflush_log =
 false?
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References: <20140703144048.GA6674@cedric-maion.com>
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Cedric Maion <cedric@maion.com> wrote:
> On a Rails 4.0 app with 'config.autoflush_log = false', it seems to me
> that telling unicorn to reopen log files fails (e.g., after a log
> rotation and USR1 signal, Rails/unicorn still writes to the old rotated
> file instead of reopening a new one).
> 
> Without this config option, logs does properly get reopened.
> 
> Is this something known?

Yes, log buffering via File#sync=true is incompatible with reopening in
multiprocess servers.  File#sync=true does not guarantee writes are
atomic on line boundaries, so it's dangerous to assume they're logs.

Even without reopening, enabling this option on a multiprocess server
might corrupt logs (depending on the buffering implementation)
because the synchronization is within each process rather for the
entire OS.

Looking at the rails source:
==> railties/lib/rails/application/bootstrap.rb <==
          f = File.open path, 'a'
          f.binmode
          f.sync = config.autoflush_log # if true make sure every write flushes

You're not likely to notice any performance difference unless you're
logging excessively.  Keep in mind the Linux kernel also does buffering
and you can tune that via the sys.vm.dirty_* sysctls.