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From Paul Brannan <paul.bran...@thesystech.com>
Subject Re: File descriptor limit for WAL
Date Fri, 24 Feb 2017 18:04:38 GMT
I'm using the debs from the cloudera-kudu ppa with little change to the
default configuration, so one master and one tablet server.  I set
num_replicas(1) when creating each table.  I used range partitioning with
(if I understand correctly) one large open-ended range.  So that should
have 334 tablet replicas.

I added two more tablet servers and was able to get to create 1002 tables
(exactly 3*334) before running out of file descriptors.  Using hash
partitioning instead of range partitioning, I was able to create 500 tables
(roughly half).  This is using 2 hash buckets, so it's what I expect.  I
run into the same limit when I have a single table with many range
partitions (997 partitions on a single partition column).

My goal here is to be able to keep N months of data (on the order of 100's
of GB per day) and to be able to drop a single date from the beginning of
the range.  Rows are only inserted for the current date, and rows for
previous dates are not  modified.  Partitioning seems ideal for this case
(it's mentioned as a use case in the non-covering range partitions
document).  Is there a better solution?

Does the 100-tablet limit only affect startup time?  In other words, if
multiple-minute startup time is acceptable, then is there any other reason
to limit each tablet server to 100 tablets?  Is it safe to increase ulimit
for open files past 1024 (i.e. does the tablet server ever call select(2))?


On Thu, Feb 16, 2017 at 3:33 PM, Adar Dembo <adar@cloudera.com> wrote:

> Hi Paul,
>
> As you discovered, Kudu holds WAL segments open until the tablets they
> belong to are deleted. block_manager_max_open_files won't help here;
> that just applies to files opened for accessing data blocks, not WAL
> segments.
>
> As far as WAL segments are concerned, we've previously discussed
> "queiscing" tablets that haven't been used in some time, which would
> involve halting their Raft consensus state machine and perhaps closing
> their WAL segments. I can't find a JIRA for this feature, but I'm also
> not aware of anyone working on it. If you're interested in
> contributing to Kudu, this could be a worthwhile avenue for you to
> explore further.
>
> I'm a little fuzzy on the details, but I believe that by default a
> tablet will retain anywhere from 2 to 10 WAL segments, all of them
> open. The exact number depends on how "caught up" the replication
> group is; if one peer is behind, more segments may be retained in
> order to help that peer catch up in the future. The settings that
> control these numbers are log_min_segments_to_retain and
> log_max_segments_to_retain.
>
> Out of curiosity, how many tablet replicas did your 334 tables
> generate in total? You can deduce that by calculating, for each table,
> the total number of partitions multiplied by the table's replication
> factor. And across how many tservers were they all distributed? By
> design, tservers can handle many tablets, but as usual, the
> implementation lags the design, and at the moment we're recommending
> no more than 100 tablets per tserver
> (http://kudu.apache.org/docs/known_issues.html#_other_known_issues).
>
>
> On Thu, Feb 16, 2017 at 8:42 AM, Paul Brannan
> <paul.brannan@thesystech.com> wrote:
> > I wrote a quick script today to see how kudu behaves if I create many
> > tables.  After creating 334 tables, I started getting timeouts.  I see
> this
> > in the master log file:
> >
> > W0216 11:37:48.961221 49810 catalog_manager.cc:2490] CreateTablet RPC for
> > tablet 9b259d5c5ff74f04820240f2159bc1a0 on TS
> > faaf4e9b6e5945d7a14953c4cc34f164 (telx-sb-dev2:7050) failed: IO error:
> > Couldn't create tablet metadata: Failed to write tablet metadata
> > 9b259d5c5ff74f04820240f2159bc1a0: Call to mkstemp() failed on name
> template
> > /var/lib/kudu/tserver/tablet-meta/9b259d5c5ff74f04820240f2159bc1
> a0.tmp.XXXXXX:
> > Too many open files (error 24)
> >
> > I decreased block_manager_max_open_files, but still got the same result.
> > Lsof shows that the open files are for the WAL:
> >
> > kudu-tser 49648 kudu 1021u   REG        8,5 67108864   16385457
> > /var/lib/kudu/tserver/wals/62b73d1b7f7a4e61a0a30a551e6623
> 0b/wal-000000001
> > kudu-tser 49648 kudu 1022r   REG        8,5 67108864   16385457
> > /var/lib/kudu/tserver/wals/62b73d1b7f7a4e61a0a30a551e6623
> 0b/wal-000000001
> > kudu-tser 49648 kudu 1023u   REG        8,5 24000000   16385458
> > /var/lib/kudu/tserver/wals/62b73d1b7f7a4e61a0a30a551e6623
> 0b/index.000000000
> >
> > The files do not get closed until the tables are deleted, even though no
> > running process has any of those tables open.
> >
> > Is there a setting that will reduce the number of WAL files that get
> created
> > or held open at any given point in time?
>

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