I see. In HBase you have machines for both CPU (to serve requests) and storage (to hold the data).
If you only grow your cluster for CPU and you keep all RegionServers 100% busy at all times, you are correct. Maybe you need to increase replication.source.size.capacity and/or replication.source.nb.capacity (although I doubt that this will help here). Also a replication source will pick region server from the target at random (10% of them at default). That has two effects: 1. Each source will pick exactly one RS at the target: ceil (3*0.1)=1 2. With such a small cluster setup the likelihood is high that two or more RSs in the source will happen to pick the same RS at the target. Thus leading less throughput. In fact your numbers might indicate that two of your source RSs might have picked the same target (you get 2/3 of your throughput via replication). In any case, before drawing conclusions this should be tested with a larger cluster. Maybe set replication.source.ratio from 0.1 to 1 (thus the source RSs will round robin all target RSs and lead to better distribution), but that might have other side-effects, too. Did you measure the disk IO at each RS at the target? Maybe one of them is mostly idle. -- Lars ________________________________ From: Asaf Mesika <[email protected]> To: "[email protected]" <[email protected]>; lars hofhansl <[email protected]> Sent: Thursday, June 20, 2013 1:38 PM Subject: Re: Replication not suited for intensive write applications? Thanks for the answer! My responses are inline. On Thu, Jun 20, 2013 at 11:02 PM, lars hofhansl <[email protected]> wrote: > First off, this is a pretty constructed case leading to a specious general > conclusion. > > If you only have three RSs/DNs and the default replication factor of 3, > each machine will get every single write. > That is the first issue. Using HBase makes little sense with such a small > cluster. > You are correct, non the less - network as I measured, was far from its capacity thus probably not the bottleneck. > > Secondly, as you say yourself, there are only three regionservers writing > to the replicated cluster using a single thread each in order to preserve > ordering. > With more region servers your scale will tip the other way. Again more > regionservers will make this better. > > I presume, in production, I will add more region servers to accommodate growing write demand on my cluster. Hence, my clients will write with more threads. Thus proportionally I will always have a lot more client threads than the number of region servers (each has one replication thread). So, I don't see how adding more region servers will tip the scale to other side. The only way to avoid this, is to design the cluster in such a way that if I can handle the events received at the client which write them to HBase with x Threads, this is the amount of region servers I should have. If I will have a spike, then it will even out eventually, but this under utilizing my cluster hardware, no? > As for your other question, more threads can lead to better interleaving > of CPU and IO, thus leading to better throughput (this relationship is not > linear, though). > > > > -- Lars > > > > ----- Original Message ----- > From: Asaf Mesika <[email protected]> > To: "[email protected]" <[email protected]> > Cc: > Sent: Thursday, June 20, 2013 3:46 AM > Subject: Replication not suited for intensive write applications? > > Hi, > > I've been conducting lots of benchmarks to test the maximum throughput of > replication in HBase. > > I've come to the conclusion that HBase replication is not suited for write > intensive application. I hope that people here can show me where I'm wrong. > > *My setup* > *Cluster (*Master and slave are alike) > 1 Master, NameNode > 3 RS, Data Node > > All computers are the same: 8 Cores x 3.4 GHz, 8 GB Ram, 1 Gigabit ethernet > card > > I insert data into HBase from a java process (client) reading files from > disk, running on the machine running the HBase Master in the master > cluster. > > *Benchmark Results* > When the client writes with 10 Threads, then the master cluster writes at > 17 MB/sec, while the replicated cluster writes at 12 Mb/sec. The data size > I wrote is 15 GB, all Puts, to two different tables. > Both clusters when tested independently without replication, achieved write > throughput of 17-19 MB/sec, so evidently the replication process is the > bottleneck. > > I also tested connectivity between the two clusters using "netcat" and > achieved 111 MB/sec. > I've checked the usage of the network cards both on the client, master > cluster region server and slave region servers. No computer when over > 30mb/sec in Receive or Transmit. > The way I checked was rather crud but works: I've run "netstat -ie" before > HBase in the master cluster starts writing and after it finishes. The same > was done on the replicated cluster (when the replication started and > finished). I can tell the amount of bytes Received and Transmitted and I > know that duration each cluster worked, thus I can calculate the > throughput. > > *The bottleneck in my opinion* > Since we've excluded network capacity, and each cluster works at faster > rate independently, all is left is the replication process. > My client writes to the master cluster with 10 Threads, and manages to > write at 17-18 MB/sec. > Each region server has only 1 thread responsible for transmitting the data > written to the WAL to the slave cluster. Thus in my setup I effectively > have 3 threads writing to the slave cluster. Thus this is the bottleneck, > since this process can not be parallelized, since it must transmit the WAL > in a certain order. > > *Conclusion* > When writes intensively to HBase with more than 3 threads (in my setup), > you can't use replication. > > *Master throughput without replication* > On a different note, I have one thing I couldn't understand at all. > When turned off replication, and wrote with my client with 3 threads I got > throughput of 11.3 MB/sec. When I wrote with 10 Threads (any more than that > doesn't help) I achieved maximum throughput of 19 MB/sec. > The network cards showed 30MB/sec Receive and 20MB/sec Transmit on each RS, > thus the network capacity was not the bottleneck. > On the HBase master machine which ran the client, the network card again > showed Receive throughput of 0.5MB/sec and Transmit throughput of 18.28 > MB/sec. Hence it's the client machine network card creating the bottleneck. > > The only explanation I have is the synchronized writes to the WAL. Those 10 > threads have to get in line, and one by one, write their batch of Puts to > the WAL, which creates a bottleneck. > > *My question*: > The one thing I couldn't understand is: When I write with 3 Threads, > meaning I have no more than 3 concurrent RPC requests to write in each RS. > They achieved 11.3 MB/sec. > The write to the WAL is synchronized, so why increasing the number of > threads to 10 (x3 more) actually increased the throughput to 19 MB/sec? > They all get in line to write to the same location, so it seems have > concurrent write shouldn't improve throughput at all. > > > Thanks you! > > Asaf > * > * > >
