In that case, though, we'd slow down normal operation. Maybe that can be alleviated with HDFS-1783/HBASE-6116, although as mentioned in HBASE-6116, I have not been able to measure any performance improvement from this so far.
-- Lars ________________________________ From: N Keywal <[email protected]> To: [email protected] Sent: Friday, July 13, 2012 6:27 AM Subject: Re: hbase mttr vs. hdfs I looked at this part of hdfs code, and - it's not simple to add it in a clean way, even if doing it is possible. - i was wrong the the 3s hearbeat: the hearbeat is every 5 minutes actually. So changing this would not be without a lot of side effects. - as a side note HADOOP-8144 is interesting... So not writing the WAL on the local machine could be a good medium term option, that could likely be implemented with HDFS-385 (made available recently in "branch-1". I don't know what it stands for). On Fri, Jul 13, 2012 at 9:53 AM, N Keywal <[email protected]> wrote: > Another option would be to never write the wal locally: in nearly all > cases it won't be used as it's on the dead box. And then the recovery > would be directed by the NN to a dead DN in a single box failure. And > we would have 3 copies instead of 2, increasing global reliability... > > On Fri, Jul 13, 2012 at 12:16 AM, N Keywal <[email protected]> wrote: >> Hi Todd, >> >> Do you think the change would be too intrusive for hdfs? I aggree, >> there are many less critical components in hadoop :-). I was hoping >> that this state could be internal to the NN and could remain localized >> without any interface change... >> >> Your proposal would help for sure. I see 3 points if we try to do it >> for specific functions like recovery. >> - we would then need to manage the case when all 3 nodes timeouts >> after 1s, hoping that two of them are wrong positive... >> - the writes between DN would still be with the old timeout. I didn't >> look in details at the impact. It won't be an issue for single box >> crash, but for large failure it could. >> - we would want to change it to for the ipc.Client as well. Note sure >> if the change would not be visible to all functions. >> >> What worries me about setting very low timeouts is that it's difficult >> to validate, it tends to work until it goes to production... >> >> I was also thinking of making the deadNodes list public in the client, >> so hbase could tell to the DFSClient: 'this node is dead, I know it >> because I'm recovering the RS', but it would have some false positive >> (software region server crash), and seems a little like a >> workaround... >> >> In the middle (thinking again about your proposal), we could add a >> function in hbase that would first check the DNs owning the WAL, >> trying to connect with a 1s timeout, to be able to tell the DFSClient >> who's dead. >> Or we could put this function in DFSClient, a kind of boolean to say >> fail fast on dn errors for this read... >> >> >> >> On Thu, Jul 12, 2012 at 11:24 PM, Todd Lipcon <[email protected]> wrote: >>> Hey Nicolas, >>> >>> Another idea that might be able to help this without adding an entire >>> new state to the protocol would be to just improve the HDFS client >>> side in a few ways: >>> >>> 1) change the "deadnodes" cache to be a per-DFSClient structure >>> instead of per-stream. So, after reading one block, we'd note that the >>> DN was dead, and de-prioritize it on future reads. Of course we'd need >>> to be able to re-try eventually since dead nodes do eventually >>> restart. >>> 2) when connecting to a DN, if the connection hasn't succeeded within >>> 1-2 seconds, start making a connection to another replica. If the >>> other replica succeeds first, then drop the connection to the first >>> (slow) node. >>> >>> Wouldn't this solve the problem less invasively? >>> >>> -Todd >>> >>> On Thu, Jul 12, 2012 at 2:20 PM, N Keywal <[email protected]> wrote: >>>> Hi, >>>> >>>> I have looked at the HBase MTTR scenario when we lose a full box with >>>> its datanode and its hbase region server altogether: It means a RS >>>> recovery, hence reading the logs files and writing new ones (splitting >>>> logs). >>>> >>>> By default, HDFS considers a DN as dead when there is no heartbeat for >>>> 10:30 minutes. Until this point, the NaneNode will consider it as >>>> perfectly valid and it will get involved in all read & write >>>> operations. >>>> >>>> And, as we lost a RegionServer, the recovery process will take place, >>>> so we will read the WAL & write new log files. And with the RS, we >>>> lost the replica of the WAL that was with the DN of the dead box. In >>>> other words, 33% of the DN we need are dead. So, to read the WAL, per >>>> block to read and per reader, we've got one chance out of 3 to go to >>>> the dead DN, and to get a connect or read timeout issue. With a >>>> reasonnable cluster and a distributed log split, we will have a sure >>>> winner. >>>> >>>> >>>> I looked in details at the hdfs configuration parameters and their >>>> impacts. We have the calculated values: >>>> heartbeat.interval = 3s ("dfs.heartbeat.interval"). >>>> heartbeat.recheck.interval = 300s ("heartbeat.recheck.interval") >>>> heartbeatExpireInterval = 2 * 300 + 10 * 3 = 630s => 10.30 minutes >>>> >>>> At least on 1.0.3, there is no shutdown hook to tell the NN to >>>> consider this DN as dead, for example on a software crash. >>>> >>>> So before the 10:30 minutes, the DN is considered as fully available >>>> by the NN. After this delay, HDFS is likely to start replicating the >>>> blocks contained in the dead node to get back to the right number of >>>> replica. As a consequence, if we're too aggressive we will have a side >>>> effect here, adding workload to an already damaged cluster. According >>>> to Stack: "even with this 10 minutes wait, the issue was met in real >>>> production case in the past, and the latency increased badly". May be >>>> there is some tuning to do here, but going under these 10 minutes does >>>> not seem to be an easy path. >>>> >>>> For the clients, they don't fully rely on the NN feedback, and they >>>> keep, per stream, a dead node list. So for a single file, a given >>>> client will do the error once, but if there are multiple files it will >>>> go back to the wrong DN. The settings are: >>>> >>>> connect/read: (3s (hardcoded) * NumberOfReplica) + 60s >>>> ("dfs.socket.timeout") >>>> write: (5s (hardcoded) * NumberOfReplica) + 480s >>>> ("dfs.datanode.socket.write.timeout") >>>> >>>> That will set a 69s timeout to get a "connect" error with the default >>>> config. >>>> >>>> I also had a look at larger failure scenarios, when we're loosing a >>>> 20% of a cluster. The smaller the cluster is the easier it is to get >>>> there. With the distributed log split, we're actually on a better >>>> shape from an hdfs point of view: the master could have error writing >>>> the files, because it could bet a dead DN 3 times in a row. If the >>>> split is done by the RS, this issue disappears. We will however get a >>>> lot of errors between the nodes. >>>> >>>> Finally, I had a look at the lease stuff Lease: write access lock to a >>>> file, no other client can write to the file. But another client can >>>> read it. Soft lease limit: another client can preempt the lease. >>>> Configurable. >>>> Default: 1 minute. >>>> Hard lease limit: hdfs closes the file and free the resources on >>>> behalf of the initial writer. Default: 60 minutes. >>>> >>>> => This should not impact HBase, as it does not prevent the recovery >>>> process to read the WAL or to write new files. We just need writes to >>>> be immediately available to readers, and it's possible thanks to >>>> HDFS-200. So if a RS dies we should have no waits even if the lease >>>> was not freed. This seems to be confirmed by tests. >>>> => It's interesting to note that this setting is much more aggressive >>>> than the one to declare a DN dead (1 minute vs. 10 minutes). Or, in >>>> HBase, than the default ZK timeout (3 minutes). >>>> => This said, HDFS states this: "When reading a file open for writing, >>>> the length of the last block still being written is unknown >>>> to the NameNode. In this case, the client asks one of the replicas for >>>> the latest length before starting to read its content.". This leads to >>>> an extra call to get the file length on the recovery (likely with the >>>> ipc.Client), and we may once again go to the wrong dead DN. In this >>>> case we have an extra socket timeout to consider. >>>> >>>> On paper, it would be great to set "dfs.socket.timeout" to a minimal >>>> value during a log split, as we know we will get a dead DN 33% of the >>>> time. It may be more complicated in real life as the connections are >>>> shared per process. And we could still have the issue with the >>>> ipc.Client. >>>> >>>> >>>> As a conclusion, I think it could be interesting to have a third >>>> status for DN in HDFS: between live and dead as today, we could have >>>> "sick". We would have: >>>> 1) Dead, known as such => As today: Start to replicate the blocks to >>>> other nodes. You enter this state after 10 minutes. We could even wait >>>> more. >>>> 2) Likely to be dead: don't propose it for write blocks, put it with a >>>> lower priority for read blocks. We would enter this state in two >>>> conditions: >>>> 2.1) No heartbeat for 30 seconds (configurable of course). As there >>>> is an existing heartbeat of 3 seconds, we could even be more >>>> aggressive here. >>>> 2.2) We could have a shutdown hook in hdfs such as when a DN dies >>>> 'properly' it says to the NN, and the NN can put it in this 'half dead >>>> state'. >>>> => In all cases, the node stays in the second state until the 10.30 >>>> timeout is reached or until a heartbeat is received. >>>> 3) Live. >>>> >>>> For HBase it would make life much simpler I think: >>>> - no 69s timeout on mttr path >>>> - less connection to dead nodes leading to ressources held all other >>>> the place finishing by a timeout... >>>> - and there is already a very aggressive 3s heartbeat, so we would >>>> not add any workload. >>>> >>>> Thougths? >>>> >>>> Nicolas >>> >>> >>> >>> -- >>> Todd Lipcon >>> Software Engineer, Cloudera
