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From: Erick Erickson <erickerickson@gmail.com>
Date: Wed, 22 Nov 2017 17:17:03 -0800
Message-ID: <CAN4YXve0Rv6103HAzLHpzZM3RmkzqD-_EYMQQUQV1eFKGPkPtw@mail.gmail.com>
Subject: Re: Recovery Issue - Solr 6.6.1 and HDFS
To: solr-user <solr-user@lucene.apache.org>
Content-Type: text/plain; charset="UTF-8"
archived-at: Thu, 23 Nov 2017 01:17:51 -0000

Hmm. This is quite possible. Any time things take "too long" it can be
 a problem. For instance, if the leader sends docs to a replica and
the request times out, the leader throws the follower into "Leader
Initiated Recovery". The smoking gun here is that there are no errors
on the follower, just the notification that the leader put it into
recovery.

There are other variations on the theme, it all boils down to when
communications fall apart replicas go into recovery.....

Best,
Erick

On Wed, Nov 22, 2017 at 11:02 AM, Joe Obernberger
<joseph.obernberger@gmail.com> wrote:
> Hi Shawn - thank you for your reply.  The index is 29.9TBytes as reported
> by:
> hadoop fs -du -s -h /solr6.6.0
> 29.9 T  89.9 T  /solr6.6.0
>
> The 89.9TBytes is due to HDFS having 3x replication.  There are about 1.1
> billion documents indexed and we index about 2.5 million documents per day.
> Assuming an even distribution, each node is handling about 680GBytes of
> index.  So our cache size is 1.4%. Perhaps 'relatively small block cache'
> was an understatement! This is why we split the largest collection into two,
> where one is data going back 30 days, and the other is all the data.  Most
> of our searches are not longer than 30 days back.  The 30 day index is
> 2.6TBytes total.  I don't know how the HDFS block cache splits between
> collections, but the 30 day index performs acceptable for our specific
> application.
>
> If we wanted to cache 50% of the index, each of our 45 nodes would need a
> block cache of about 350GBytes.  I'm accepting offers of DIMMs!
>
> What I believe caused our 'recovery, fail, retry loop' was one of our
> servers died.  This caused HDFS to start to replicate blocks across the
> cluster and produced a lot of network activity.  When this happened, I
> believe there was high network contention for specific nodes in the cluster
> and their network interfaces became pegged and requests for HDFS blocks
> timed out.  When that happened, SolrCloud went into recovery which caused
> more network traffic.  Fun stuff.
>
> -Joe
>
>
> On 11/22/2017 11:44 AM, Shawn Heisey wrote:
>>
>> On 11/22/2017 6:44 AM, Joe Obernberger wrote:
>>>
>>> Right now, we have a relatively small block cache due to the
>>> requirements that the servers run other software.  We tried to find
>>> the best balance between block cache size, and RAM for programs, while
>>> still giving enough for local FS cache.  This came out to be 84 128M
>>> blocks - or about 10G for the cache per node (45 nodes total).
>>
>> How much data is being handled on a server with 10GB allocated for
>> caching HDFS data?
>>
>> The first message in this thread says the index size is 31TB, which is
>> *enormous*.  You have also said that the index takes 93TB of disk
>> space.  If the data is distributed somewhat evenly, then the answer to
>> my question would be that each of those 45 Solr servers would be
>> handling over 2TB of data.  A 10GB cache is *nothing* compared to 2TB.
>>
>> When index data that Solr needs to access for an operation is not in the
>> cache and Solr must actually wait for disk and/or network I/O, the
>> resulting performance usually isn't very good.  In most cases you don't
>> need to have enough memory to fully cache the index data ... but less
>> than half a percent is not going to be enough.
>>
>> Thanks,
>> Shawn
>>
>>
>> ---
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>