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From Erick Erickson <erickerick...@gmail.com>
Subject Re: Recovery Issue - Solr 6.6.1 and HDFS
Date Tue, 21 Nov 2017 20:43:58 GMT
bq: We are doing lots of soft commits for NRT search...

It's not surprising that this is slower than local storage, especially
if you have any autowarming going on. Opening  new searchers will need
to read data from disk for the new segments, and HDFS may be slower
here.

As far as the commit interval, an under-appreciated event is that when
RAMBufferSizeMB is exceeded (default 100M last I knew) new segments
are written _anyway_, they're just a little invisible. That is, the
segments_n file isn't updated even though they're closed IIUC at
least. So that very long interval isn't helping with that problem I
don't think....

Evidence to the contrary trumps my understanding of course.

About starting all these collections up at once and the Overseer
queue. I've seen this in similar situations. There are a _lot_ of
messages flying back and forth for each replica on startup, and the
Overseer processing was very inefficient historically so that queue
could get in the 100s of K, I've seen some pathological situations
where it's over 1M. SOLR-10524 made this a lot better. There are still
a lot of messages written in a case like yours, but at least the
Overseer has a much better chance to keep up.... Solr 6.6... At that
point bringing up Solr took a very long time.


Erick

On Tue, Nov 21, 2017 at 12:24 PM, Hendrik Haddorp
<hendrik.haddorp@gmx.net> wrote:
> We sometimes also have replicas not recovering. If one replica is left
> active the easiest is to then to delete the replica and create a new one.
> When all replicas are down it helps most of the time to restart one of the
> nodes that contains a replica in down state. If that also doesn't get the
> replica to recover I would check the logs of the node and also that of the
> overseer node. I have seen the same issue on Solr using local storage. The
> main HDFS related issues we had so far was those lock files and if you
> delete and recreate collections/cores and it sometimes happens that the data
> was not cleaned up in HDFS and then causes a conflict.
>
> Hendrik
>
>
> On 21.11.2017 21:07, Joe Obernberger wrote:
>>
>> We've never run an index this size in anything but HDFS, so I have no
>> comparison.  What we've been doing is keeping two main collections - all
>> data, and the last 30 days of data.  Then we handle queries based on date
>> range.  The 30 day index is significantly faster.
>>
>> My main concern right now is that 6 of the 100 shards are not coming back
>> because of no leader.  I've never seen this error before.  Any ideas?
>> ClusterStatus shows all three replicas with state 'down'.
>>
>> Thanks!
>>
>> -joe
>>
>>
>> On 11/21/2017 2:35 PM, Hendrik Haddorp wrote:
>>>
>>> We actually also have some performance issue with HDFS at the moment. We
>>> are doing lots of soft commits for NRT search. Those seem to be slower then
>>> with local storage. The investigation is however not really far yet.
>>>
>>> We have a setup with 2000 collections, with one shard each and a
>>> replication factor of 2 or 3. When we restart nodes too fast that causes
>>> problems with the overseer queue, which can lead to the queue getting out of
>>> control and Solr pretty much dying. We are still on Solr 6.3. 6.6 has some
>>> improvements and should handle these actions faster. I would check what you
>>> see for "/solr/admin/collections?action=OVERSEERSTATUS&wt=json". The
>>> critical part is the "overseer_queue_size" value. If this goes up to about
>>> 10000 it is pretty much game over on our setup. In that case it seems to be
>>> best to stop all nodes, clear the queue in ZK and then restart the nodes one
>>> by one with a gap of like 5min. That normally recovers pretty well.
>>>
>>> regards,
>>> Hendrik
>>>
>>> On 21.11.2017 20:12, Joe Obernberger wrote:
>>>>
>>>> We set the hard commit time long because we were having performance
>>>> issues with HDFS, and thought that since the block size is 128M, having a
>>>> longer hard commit made sense.  That was our hypothesis anyway.  Happy to
>>>> switch it back and see what happens.
>>>>
>>>> I don't know what caused the cluster to go into recovery in the first
>>>> place.  We had a server die over the weekend, but it's just one out of ~50.
>>>> Every shard is 3x replicated (and 3x replicated in HDFS...so 9 copies). 
It
>>>> was at this point that we noticed lots of network activity, and most of the
>>>> shards in this recovery, fail, retry loop.  That is when we decided to shut
>>>> it down resulting in zombie lock files.
>>>>
>>>> I tried using the FORCELEADER call, which completed, but doesn't seem to
>>>> have any effect on the shards that have no leader. Kinda out of ideas for
>>>> that problem.  If I can get the cluster back up, I'll try a lower hard
>>>> commit time.  Thanks again Erick!
>>>>
>>>> -Joe
>>>>
>>>>
>>>> On 11/21/2017 2:00 PM, Erick Erickson wrote:
>>>>>
>>>>> Frankly with HDFS I'm a bit out of my depth so listen to Hendrik ;)...
>>>>>
>>>>> I need to back up a bit. Once nodes are in this state it's not
>>>>> surprising that they need to be forcefully killed. I was more thinking
>>>>> about how they got in this situation in the first place. _Before_ you
>>>>> get into the nasty state how are the Solr nodes shut down? Forcefully?
>>>>>
>>>>> Your hard commit is far longer than it needs to be, resulting in much
>>>>> larger tlog files etc. I usually set this at 15-60 seconds with local
>>>>> disks, not quite sure whether longer intervals are helpful on HDFS.
>>>>> What this means is that you can spend up to 30 minutes when you
>>>>> restart solr _replaying the tlogs_! If Solr is killed, it may not have
>>>>> had a chance to fsync the segments and may have to replay on startup.
>>>>> If you have openSearcher set to false, the hard commit operation is
>>>>> not horribly expensive, it just fsync's the current segments and opens
>>>>> new ones. It won't be a total cure, but I bet reducing this interval
>>>>> would help a lot.
>>>>>
>>>>> Also, if you stop indexing there's no need to wait 30 minutes if you
>>>>> issue a manual commit, something like
>>>>> .../collection/update?commit=true. Just reducing the hard commit
>>>>> interval will make the wait between stopping indexing and restarting
>>>>> shorter all by itself if you don't want to issue the manual commit.
>>>>>
>>>>> Best,
>>>>> Erick
>>>>>
>>>>> On Tue, Nov 21, 2017 at 10:34 AM, Hendrik Haddorp
>>>>> <hendrik.haddorp@gmx.net> wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> the write.lock issue I see as well when Solr is not been stopped
>>>>>> gracefully.
>>>>>> The write.lock files are then left in the HDFS as they do not get
>>>>>> removed
>>>>>> automatically when the client disconnects like a ephemeral node in
>>>>>> ZooKeeper. Unfortunately Solr does also not realize that it should
be
>>>>>> owning
>>>>>> the lock as it is marked in the state stored in ZooKeeper as the
owner
>>>>>> and
>>>>>> is also not willing to retry, which is why you need to restart the
>>>>>> whole
>>>>>> Solr instance after the cleanup. I added some logic to my Solr start
>>>>>> up
>>>>>> script which scans the log files in HDFS and compares that with the
>>>>>> state in
>>>>>> ZooKeeper and then delete all lock files that belong to the node
that
>>>>>> I'm
>>>>>> starting.
>>>>>>
>>>>>> regards,
>>>>>> Hendrik
>>>>>>
>>>>>>
>>>>>> On 21.11.2017 14:07, Joe Obernberger wrote:
>>>>>>>
>>>>>>> Hi All - we have a system with 45 physical boxes running solr
6.6.1
>>>>>>> using
>>>>>>> HDFS as the index.  The current index size is about 31TBytes.
With 3x
>>>>>>> replication that takes up 93TBytes of disk. Our main collection
is
>>>>>>> split
>>>>>>> across 100 shards with 3 replicas each.  The issue that we're
running
>>>>>>> into
>>>>>>> is when restarting the solr6 cluster.  The shards go into recovery
>>>>>>> and start
>>>>>>> to utilize nearly all of their network interfaces.  If we start
too
>>>>>>> many of
>>>>>>> the nodes at once, the shards will go into a recovery, fail,
and
>>>>>>> retry loop
>>>>>>> and never come up.  The errors are related to HDFS not responding
>>>>>>> fast
>>>>>>> enough and warnings from the DFSClient.  If we stop a node when
this
>>>>>>> is
>>>>>>> happening, the script will force a stop (180 second timeout)
and upon
>>>>>>> restart, we have lock files (write.lock) inside of HDFS.
>>>>>>>
>>>>>>> The process at this point is to start one node, find out the
lock
>>>>>>> files,
>>>>>>> wait for it to come up completely (hours), stop it, delete the
>>>>>>> write.lock
>>>>>>> files, and restart.  Usually this second restart is faster, but
it
>>>>>>> still can
>>>>>>> take 20-60 minutes.
>>>>>>>
>>>>>>> The smaller indexes recover much faster (less than 5 minutes).
Should
>>>>>>> we
>>>>>>> have not used so many replicas with HDFS?  Is there a better
way we
>>>>>>> should
>>>>>>> have built the solr6 cluster?
>>>>>>>
>>>>>>> Thank you for any insight!
>>>>>>>
>>>>>>> -Joe
>>>>>>>
>>>>> ---
>>>>> This email has been checked for viruses by AVG.
>>>>> http://www.avg.com
>>>>>
>>>>
>>>
>>
>

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