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From Hendrik Haddorp <>
Subject Re: Recovery Issue - Solr 6.6.1 and HDFS
Date Wed, 22 Nov 2017 10:20:59 GMT
Hi Joe,

sorry, I have not seen that problem. I would normally not delete a 
replica if the shard is down but only if there is an active shard. 
Without an active leader the replica should not be able to recover. I 
also just had a case where all replicas of a shard stayed in down state 
and restarts didn't help. This was however also caused by lock files. 
Once I cleaned them up and restarted all Solr instances that had a 
replica they recovered.

For the lock files I discovered that the index is not always in the 
"index" folder but can also be in an index.<timestamp> folder. There can 
be an "" file in the "data" directory in HDFS and this 
contains the correct index folder name.

If you are really desperate you could also delete all but one replica so 
that the leader election is quite trivial. But this does of course 
increase the risk of finally loosing the data quite a bit. So I would 
try looking into the code and figure out what the problem is here and 
maybe compare the state in HDFS and ZK with a shard that works.


On 21.11.2017 23:57, Joe Obernberger wrote:
> Hi Hendrick - the shards in question have three replicas.  I tried 
> restarting each one (one by one) - no luck.  No leader is found. I 
> deleted one of the replicas and added a new one, and the new one also 
> shows as 'down'.  I also tried the FORCELEADER call, but that had no 
> effect.  I checked the OVERSEERSTATUS, but there is nothing unusual 
> there.  I don't see anything useful in the logs except the error:
> org.apache.solr.common.SolrException: Error getting leader from zk for 
> shard shard21
>     at 
>     at
>     at
>     at 
> org.apache.solr.core.ZkContainer.lambda$registerInZk$0(
>     at 
> org.apache.solr.common.util.ExecutorUtil$MDCAwareThreadPoolExecutor.lambda$execute$0(
>     at 
> java.util.concurrent.ThreadPoolExecutor.runWorker(
>     at 
> java.util.concurrent.ThreadPoolExecutor$
>     at
> Caused by: org.apache.solr.common.SolrException: Could not get leader 
> props
>     at 
>     at 
>     at 
>     ... 7 more
> Caused by: org.apache.zookeeper.KeeperException$NoNodeException: 
> KeeperErrorCode = NoNode for /collections/UNCLASS/leaders/shard21/leader
>     at 
> org.apache.zookeeper.KeeperException.create(
>     at 
> org.apache.zookeeper.KeeperException.create(
>     at org.apache.zookeeper.ZooKeeper.getData(
>     at 
>     at 
>     at 
>     at 
>     at 
>     ... 9 more
> Can I modify zookeeper to force a leader?  Is there any other way to 
> recover from this?  Thanks very much!
> -Joe
> On 11/21/2017 3:24 PM, Hendrik Haddorp 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 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
>>>>>> 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
>>>>>> 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
>>>>>> <> 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
>>>>>>> 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
>>>>>>> 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
>>>>>>>> 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
>>>>>>>> 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
>>>>>>>> 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
>>>>>>>> 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
>>>>>>>> lock files,
>>>>>>>> wait for it to come up completely (hours), stop it, delete
>>>>>>>> write.lock
>>>>>>>> files, and restart.  Usually this second restart is faster,
>>>>>>>> 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.

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