[Gluster-devel] Logging in a multi-brick daemon

[Ravishankar N]

On 02/16/2017 04:09 AM, Jeff Darcy wrote:
> One of the issues that has come up with multiplexing is that all of the bricks in a process end up sharing a single log file.  The reaction from both of the people who have mentioned this is that we should find a way to give each brick its own log even when they're in the same process, and make sure gf_log etc. are able to direct messages to the correct one.  I can think of ways to do this, but it doesn't seem optimal to me.  It will certainly use up a lot of file descriptors.  I think it will use more memory.  And then there's the issue of whether this would really be better for debugging.  Often it's necessary to look at multiple brick logs while trying to diagnose this problem, so it's actually kind of handy to have them all in one file.  Which would you rather do?
>
> (a) Weave together entries in multiple logs, either via a script or in your head?
>
> (b) Split or filter entries in a single log, according to which brick they're from?
>
> To me, (b) seems like a much more tractable problem.  I'd say that what we need is not multiple logs, but *marking of entries* so that everything pertaining to one brick can easily be found.  One way to do this would be to modify volgen so that a brick ID (not name because that's a path and hence too long) is appended/prepended to the name of every translator in the brick.  Grep for that brick ID, and voila!  You now have all log messages for that brick and no other.  A variant of this would be to leave the names alone and modify gf_log so that it adds the brick ID automagically (based on a thread-local variable similar to THIS).  Same effect, other than making translator names longer, so I'd kind of prefer this approach.  Before I start writing the code, does anybody else have any opinions, preferences, or alternatives I haven't mentioned yet?
>
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My vote is for having separate log files per brick. Even in separate log 
files
that we have today, I find it difficult to mentally ignore irrelevant 
messages
in a single log file as I am sifting through it to look for errors that are
related to the problem at hand. Having entries from multiple bricks and then
grepping it would only make things harder. I cannot think of a case 
where having
entries from all bricks in one file would particularly be beneficial for
debugging since what happens in one brick is independent of the other 
bricks
(at least until we move client xlators to server side and run them in 
the brick process).
As for file descriptor count/memory usage, I think we should be okay
as it is not any worse than that in the non-multiplexed approach we have
today.

On a side-note, I think the problem is not having too many log files but 
having
them in multiple nodes. Having a log-aggregation solution where all 
messages are
logged to a single machine (but still in separate files) would make it 
easier to
monitor/debug issues.
-Ravi