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From Giuseppe Reina <g.re...@gmail.com>
Subject Re: Disk space leak when using HBase and HDFS ShortCircuit
Date Thu, 26 Jun 2014 09:08:03 GMT
On Thu, Jun 26, 2014 at 1:02 AM, Enis Söztutar <enis@apache.org> wrote:

> Agreed, this seems like an hdfs issue unless hbase itself does not close
> the hfiles properly. But judging from the fact that you were able to
> circumvent the problem by getting reducing the cache size, it does seem
> unlikely.
>


Well, we had the same issue with two different HDP 2.1 installations (with
default parameters) and two different client applications on top of HBase.
I agree this is an HDFS issue, I posted on the HBase mailing list because I
was wondering if anyone else had the same problem. Thanks for forwarding
it.

In both applications we are injesting lots of data on one HBase table with
one column family and about 10 regions for each RS.
So the RegionServer does a lot of flushes, merges and split which involves
lots of HDFS block creations and deletions.

What was your dfs.client.read.shortcircuit.streams.cache.size ?


The default value is 4096 on HDP 2.1 (not 256 as in the vanilla
distribution).
When the shortcircuit cache is too big (like in this case), the deleted
files remain open until the cache is full. This is bad when big WAL/HFiles
are merged or split as the space on disk is still occupied and the file is
never accessed (so it's a leak). Of course any application that perform
file creation, read, and deletion of big hdfs files will be affected by the
same problem.


>
> I don't think the local block reader will be notified when a file/block is
> deleted.
>

That's what I thought too, but I would be easy for the HDFS client library
to periodically scan through all the open file descriptors verifying that
the file has not been deleted (by "stat"-ing the filename for example). It
is useless to keep it in memory as the file will never be accessed, and if
it will this could potentially lead to inconsistent states.



Giuseppe




>
> Enis
>
>
> On Wed, Jun 25, 2014 at 3:29 PM, Andrew Purtell <apurtell@apache.org>
> wrote:
>
> > Forwarded
> >
> > ---------- Forwarded message ----------
> > From: Vladimir Rodionov <vrodionov@carrieriq.com>
> > Date: Wed, Jun 25, 2014 at 12:03 PM
> > Subject: RE: Disk space leak when using HBase and HDFS ShortCircuit
> > To: "user@hbase.apache.org" <user@hbase.apache.org>
> >
> >
> > >> Apparently those file descriptors were stored by the HDFS
> > >> ShortCircuit cache.
> >
> > As far as I understand this is issue of HDFS shorty-circuit-reads
> > implementation not HBase. HBase uses HDFS API to access
> > files. Did you ask this question on hdfs dev list? This looks like a very
> > serious bug.
> >
> > Best regards,
> > Vladimir Rodionov
> > Principal Platform Engineer
> > Carrier IQ, www.carrieriq.com
> > e-mail: vrodionov@carrieriq.com
> >
> > ________________________________________
> > From: Giuseppe Reina [g.reina@gmail.com]
> > Sent: Wednesday, June 25, 2014 2:54 AM
> > To: user@hbase.apache.org
> > Subject: Disk space leak when using HBase and HDFS ShortCircuit
> >
> > Hi all,
> >    we have been experiencing the same problem with 2 of our clusters. We
> > are currently using HDP 2.1 that comes with HBase 0.98.
> >
> > The problem manifested by showing a huge differences (hundreds of GB)
> > between the output of "df" and "du" of the hdfs data directories.
> > Eventually, other systems complained for the lack of space before
> shutting
> > down. We identified the problem and discovered that all the RegionServers
> > were holding lots of open file descriptors to deleted files, which
> > prevented the OS to free the disk space occupied (hence the difference
> > between "df" and "du"). The deleted files were pointing to the local HDFS
> > blocks of old HFiles deleted from HDFS during the compaction and/or split
> > operations. Apparently those file descriptors were stored by the HDFS
> > ShortCircuit cache.
> >
> > My question is, isn't the shortcircuit feautre supposed to get "notified"
> > somehow of file deletion on a file on HDFS so it can remove the open fds
> > from the cache? This creates huge leaks whenever HBase is heavily loaded
> > and we had to restart the RegionServer periodically until before
> > identifying the problem. We solved the problem first by disabling
> > shortcircuit from HDFS and then enabling it and reducing the cache size
> so
> > to trigger often the caching policies (this leads to some performance
> > loss).
> >
> >
> > p.s. I am aware of the "
> > dfs.client.read.shortcircuit.streams.cache.expiry.ms
> > " directoparameter, but for some reason the default value (5 mins) does
> not
> > work out-of-the-box on HDP 2.1, moreover the problem persists for high
> > timeouts and big cache sizes.
> >
> > Kind Regards
> >
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> >
> >
> > --
> > Best regards,
> >
> >    - Andy
> >
> > Problems worthy of attack prove their worth by hitting back. - Piet Hein
> > (via Tom White)
> >
>

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