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From Juhani Connolly <juhani_conno...@cyberagent.co.jp>
Subject Re: hdfs.idleTimeout ,what's it used for ?
Date Fri, 18 Jan 2013 03:39:50 GMT
That breaks the use case idleTimeout was originally made for: making 
sure the file is closed promptly after data stops arriving. We use this 
to make sure the files ready for our batches which run quite soon after. 
The time that rollInterval will trigger is unpredictable as it will 
reset every time any other type of roll is triggered(event count or size).

By making rollInterval behave properly all of this is a non-issue. My 
recommendation to users woudl be not to use rollInterval if they're 
bucketing by time(it's redundant behavior).

Documentation could definitely be improved. Once we sort out the 
approach we want to take I can write it up to make the difference and 
usage clearer.

On 01/18/2013 12:24 PM, Connor Woodson wrote:
> The way idleTimeout works right now is that it's another rollInterval; 
> it will work best when rollInterval is not set and so it seems that 
> it's use is best for when you don't want to use a rollInterval and 
> just want to have your bucketwriters close when no events are coming 
> through (caused by path change or something else; and you can still 
> roll reliably with either count or size)
> As such, perhaps it is more clear if idleTimeout is renamed to 
> idleRoll or such?
> And then change idleTimeout to only count seconds since it was closed; 
> if a bucketwriter is closed for long enough it will automatically 
> remove itself. This type of idle will then work well with 
> rollInterval, while the other one doesn't (idleRoll + rollInterval 
> creates two time-based rollers. There are certainly times for that, 
> but not all of the time).
> - Connor
> On Thu, Jan 17, 2013 at 6:46 PM, Juhani Connolly 
> <juhani_connolly@cyberagent.co.jp 
> <mailto:juhani_connolly@cyberagent.co.jp>> wrote:
>     It seemed neater at the time. It's only an issue because
>     rollInterval doesn't remove the entry in sfWriters. We could
>     change it so that close doesn't cancel it, and have it check
>     whether or not the writer is already closed, but that'd be kind of
>     ugly.
>     @Mohit:
>     When flume dies unexpectedly the .tmp file remains. When it
>     restarts there is some logic in HDFS sink to recover it(and
>     continue writing from there). I'm not actually sure of the
>     specifics. You may want to try and just kill -9 a running flume
>     process on a test machine and then start it up, look at the logs
>     and see what happens with the output.
>     If flume dies cleanly the file is properly closed.
>     On 01/18/2013 11:23 AM, Connor Woodson wrote:
>>     And @ my aside: I hadn't realized that the idleTimeout is
>>     canceled by the rollInterval occurring. That's annoying. So
>>     setting a lower idleTimeout, and drastically decreasing
>>     maxOpenFiles to at most 2 * possible open files, is probably
>>     necessary.
>>     On Thu, Jan 17, 2013 at 6:20 PM, Connor Woodson
>>     <cwoodson.dev@gmail.com <mailto:cwoodson.dev@gmail.com>> wrote:
>>         @Mohit:
>>         For the HDFS Sink, the tmp files are placed based on the
>>         hadoop.tmp.dir property. The default location is
>>         /tmp/hadoop-${user.name <http://user.name>} To change this
>>         you can add -Dhadoop.tmp.dir=<path> to your Flume command
>>         line call, or you can specify the property in the
>>         core-site.xml of wherever your HADOOP_HOME environment
>>         variable points to.
>>         - Connor
>>         On Thu, Jan 17, 2013 at 6:19 PM, Connor Woodson
>>         <cwoodson.dev@gmail.com <mailto:cwoodson.dev@gmail.com>> wrote:
>>             Whether idleTimeout is lower or higher than rollInterval
>>             is a preference; set it before, and assume you get one
>>             message right on the turn of the hour, then you will have
>>             some part of that hour without any bucket writers; but if
>>             you get another message at the end of the hour, you will
>>             end up with two files instead of one. Set it idleTimeout
>>             to be longer and you will get just one file, but also (at
>>             worst case) you will have twice as many bucketwriters
>>             open; so it all depends on how many files you want/how
>>             much memory you have to spare.
>>             - Connor
>>             An aside:
>>             bucketwriters, after being closed by rollInterval, aren't
>>             really a memory leak; they just are very rarely useful to
>>             keep around (your path could rely on hostname, and you
>>             could use a rollinterval, and then those bucketwriters
>>             will still remain useful). And they will get removed
>>             eventually; by default after you've created your 5001st
>>             bucketwriter, the first (or whichever was used longest
>>             ago) will be removed.
>>             And I don't think that's the cause behind 1850 as he did
>>             have an idleTimeout set at 15 minutes.
>>             On Thu, Jan 17, 2013 at 6:08 PM, Juhani Connolly
>>             <juhani_connolly@cyberagent.co.jp
>>             <mailto:juhani_connolly@cyberagent.co.jp>> wrote:
>>                 It's also useful if you want files to get promptly
>>                 closed and renamed from the .tmp or whatever.
>>                 We use it with something like 30seconds setting(we
>>                 have a constant stream of data) and hourly bucketing.
>>                 There is also the issue that files closed by
>>                 rollInterval are never removed from the internal
>>                 linkedList so it actually causes a small memory
>>                 leak(which can get big in the long term if you have a
>>                 lot of files and hourly renames). I believe this is
>>                 what is causing the OOM Mohit is getting in FLUME-1850
>>                 So I personally would recommend using it(with a
>>                 setting that will close files before rollInterval does).
>>                 On 01/18/2013 06:38 AM, Bhaskar V. Karambelkar wrote:
>>                     Ah I see. Again something useful to have in the
>>                     flume user guide.
>>                     On Thu, Jan 17, 2013 at 3:29 PM, Connor Woodson
>>                     <cwoodson.dev@gmail.com
>>                     <mailto:cwoodson.dev@gmail.com>> wrote:
>>                         the rollInterval will still cause the last
>>                         01-17 file to be closed
>>                         eventually. The way the HDFS sink works with
>>                         the different files is each
>>                         unique path is specified by a different
>>                         BucketWriter object. The sink can
>>                         hold as many objects as specified by
>>                         hdfs.maxOpenWorkers (default: 5000),
>>                         and bucketwriters are only removed when you
>>                         create the 5001th writer (5001th
>>                         unique path). However, generally once a
>>                         writer is closed it is never used
>>                         again (all of your 1-17 writers will never be
>>                         used again). To avoid keeping
>>                         them in the sink's internal list of writers,
>>                         the idleTimeout is a specified
>>                         number of seconds in which no data is
>>                         received by the BucketWriter. After
>>                         this time, the writer will try to close
>>                         itself and will then tell the sink
>>                         to remove it, thus freeing up everything used
>>                         by the bucketwriter.
>>                         So the idleTimeout is just a setting to help
>>                         limit memory usage by the hdfs
>>                         sink. The ideal time for it is longer than
>>                         the maximum time between events
>>                         (capped at the rollInterval) - if you know
>>                         you'll receive a constant stream
>>                         of events you might just set it to a minute
>>                         or something. Or if you are fine
>>                         with having multiple files open per hour, you
>>                         can set it to a lower number;
>>                         maybe just over the average time between
>>                         events. For me in just testing, I
>>                         set it >= rollInterval for the cases when no
>>                         events are received in a given
>>                         hour (I'd rather keep the object alive for an
>>                         extra hour than create files
>>                         every 30 minutes or something).
>>                         Hope that was helpful,
>>                         - Connor
>>                         On Thu, Jan 17, 2013 at 12:07 PM, Bhaskar V.
>>                         Karambelkar
>>                         <bhaskarvk@gmail.com
>>                         <mailto:bhaskarvk@gmail.com>> wrote:
>>                             Say If I have
>>                             a1.sinks.k1.hdfs.path =
>>                             /flume/events/%y-%m-%d/
>>                             hdfs.rollInterval=60
>>                             Now, if there is a file
>>                             /flume/events/2013-01-17/flume_XXXXXXXXX.tmp
>>                             This file is not ready to be rolled over
>>                             yet, i.e. 60 seconds are not
>>                             up and now it's past 12 midnight, i.e.
>>                             new day
>>                             And events start to be written to
>>                             /flume/events/2013-01-18/flume_XXXXXXXX.tmp
>>                             will the file 2013-01-17 never be rolled
>>                             over, unless I have something
>>                             like hdfs.idleTimeout=60  ?
>>                             If so how do flume sinks keep track of
>>                             files they need to rollover
>>                             after idealTimeout ?
>>                             In short what's the exact use of
>>                             idealTimeout parameter ?

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