Thanks for the suggestion, Erick. However here what we need is not a patch, is a clarification from practice perspective.
I think solr replication is a great feature to scale reads, and kind of increase reliability. However, on HDFS it is not as useful as just sharding. Sharding can scale both reads and writes at same time, and doesn't have consistency concern along with replication. So I doubt Solr replication on HDFS has real meanings? I will try to reach out Mark Miller and will appreciate if he or anyone can provide more convincing points on this. Thanks, Mao On Sat, Apr 18, 2015 at 4:44 PM Erick Erickson <erickerick...@gmail.com> wrote: > AFAIK, the HDFS replication of Solr indexes isn't something that was > designed, it just came along for the ride given HDFS replication. > Having a shard with 1 leader and two followers have 9 copies of the > index around _is_ overkill, nobody argues that at all. > > I know the folks at Cloudera (who contributed the original HDFS > implementation) have discussed various options around this. In the > grand scheme of things, there have been other priorities without > tearing into the guts of Solr and/or HDFS since disk space is > relatively cheap. > > That said, I'm also sure that this will get some attention as > priorities change. All patches welcome of course ;), But if you're > inclined to work on this issue, I'd _really_ discuss it with Mark > Miller & etc. before investing too much effort in it. I don't quite > know the tradeoffs well enough to have an opinion on the right > implementation. > > Best > Erick > > On Sat, Apr 18, 2015 at 1:59 AM, Shalin Shekhar Mangar > <shalinman...@gmail.com> wrote: > > Some comments inline: > > > > On Sat, Apr 18, 2015 at 2:12 PM, gengmao <geng...@gmail.com> wrote: > > > >> On Sat, Apr 18, 2015 at 12:20 AM "Jürgen Wagner (DVT)" < > >> juergen.wag...@devoteam.com> wrote: > >> > >> > Replication on the storage layer will provide a reliable storage for > the > >> > index and other data of Solr. In particular, this replication does not > >> > guarantee your index files are consistent at any time as there may be > >> > intermediate states that are only partially replicated. Replication is > >> only > >> > a convergent process, not an instant, atomic operation. With frequent > >> > changes, this becomes an issue. > >> > > >> Firstly thanks for your reply. However I can't agree with you on this. > >> HDFS guarantees the consistency even with replicates - you always read > what > >> you write, no partially replicated state will be read, which is > guaranteed > >> by HDFS server and client. Hence HBase can rely on HDFS for consistency > and > >> availability, without implementing another replication mechanism - if I > >> understand correctly. > >> > >> > > Lucene index is not one file but a collection of files which are written > > independently. So if you replicate them out of order, Lucene might > consider > > the index as corrupted (because of missing files). I don't think HBase > > works in that way. > > > > > >> > >> > Replication inside SolrCloud as an application will not only maintain > the > >> > consistency of the search-level interfaces to your indexes, but also > >> scale > >> > in the sense of the application (query throughput). > >> > > >> Split one shard into two shards can increase the query throughput too. > >> > >> > >> > Imagine a database: if you change one record, this may also result in > an > >> > index change. If the record and the index are stored in different > storage > >> > blocks, one will get replicated first. However, the replication target > >> will > >> > only be consistent again when both have been replicated. So, you would > >> have > >> > to suspend all accesses until the entire replication has completed. > >> That's > >> > undesirable. If you replicate on the application (database management > >> > system) level, the application will employ a more fine-grained > approach > >> to > >> > replication, guaranteeing application consistency. > >> > > >> In HBase, a region only locates on single region server at any time, > which > >> guarantee its consistency. Because your read/write always drops in one > >> region, you won't have concern of parallel writes happens on multiple > >> replicates of same region. > >> The replication of HDFS is totally transparent to HBase. When a HDFS > write > >> call returns, HBase know the data is written and replicated so losing > one > >> copy of the data won't impact HBase at all. > >> So HDFS means consistency and reliability for HBase. However, HBase > doesn't > >> use replicates (either HBase itself or HDFS's) to scale reads. If one > >> region's is too "hot" for reads or write, you split that region into two > >> regions, so that the reads and writes of that region can be distributed > >> into two region servers. Hence HBase scales. > >> I think this is the simplicity and beauty of HBase. Again, I am curious > if > >> SolrCloud has better reason to use replication on HDFS? As I described, > >> HDFS provided consistency and reliability, meanwhile scalability can be > >> achieved via sharding, even without Solr replication. > >> > >> > > That's something that has been considered and may even be in the roadmap > > for the Cloudera guys. See > https://issues.apache.org/jira/browse/SOLR-6237 > > > > But one problem that isn't solved by HDFS replication is of > near-real-time > > indexing where you want the documents to be available for searchers as > fast > > as possible. SolrCloud replication supports that by replicating documents > > as they come in and indexing them in several replicas. A new index > searcher > > is opened on the flushed index files as well as on the internal data > > structures of the index writer. If we switch to relying on HDFS > replication > > then this will be awfully expensive. However, as Jürgen mentioned, HDFS > can > > certainly help with replicating static indexes. > > > > > >> > >> > Consequently, HDFS will allow you to scale storage and possibly even > >> > replicate static indexes that won't change, but it won't help much > with > >> > live index replication. That's where SolrCloud jumps in. > >> > > >> > >> > Cheers, > >> > --Jürgen > >> > > >> > > >> > On 18.04.2015 08:44, gengmao wrote: > >> > > >> > I wonder why need to use SolrCloud replication on HDFS at all, given > HDFS > >> > already provides replication and availability? The way to optimize > >> > performance and scalability should be tweaking shards, just like > tweaking > >> > regions on HBase - which doesn't provide "region replication" too, > isn't > >> > it? > >> > > >> > I have this question for a while and I didn't find clear answer about > it. > >> > Could some experts please explain a bit? > >> > > >> > Best regards, > >> > Mao Geng > >> > > >> > > >> > > >> > > >> > > >> > -- > >> > > >> > Mit freundlichen Grüßen/Kind regards/Cordialement vôtre/Atentamente/С > >> > уважением > >> > *i.A. Jürgen Wagner* > >> > Head of Competence Center "Intelligence" > >> > & Senior Cloud Consultant > >> > > >> > Devoteam GmbH, Industriestr. 3, 70565 Stuttgart, Germany > >> > Phone: +49 6151 868-8725, Fax: +49 711 13353-53, Mobile: +49 171 864 > >> 1543 > >> > E-Mail: juergen.wag...@devoteam.com, URL: www.devoteam.de > >> > ------------------------------ > >> > Managing Board: Jürgen Hatzipantelis (CEO) > >> > Address of Record: 64331 Weiterstadt, Germany; Commercial Register: > >> > Amtsgericht Darmstadt HRB 6450; Tax Number: DE 172 993 071 > >> > > >> > > >> > > >> > > > > > > > > -- > > Regards, > > Shalin Shekhar Mangar. >
