question Re: When I add a new rack some Impala queries became extremely slow! in Archives of Support Questions (Read Only)

When I add a new rack some Impala queries became extremely slow!

AcharkiMed — Fri, 16 Sep 2022 13:26:47 GMT

Hi,

I'm working on CDH v5.14.2/CM v5.14.1, I was having a cluster with 15 nodes in one rack (/my_cluster/rack1) in a US data center, and the execution time of queries was great (ex. 2.8 secs), when I decide to extand the cluster and takes into account the HA, I add 10 nodes in a europe datacenter and I assign them like a seconde rack (/my_cluster/rack2).
The problem is when I start the impala daemons of the 10 new nodes (rack2), the same queries execution time became extremely long (ex. 4.5 min).

NB: I realize that one rack must be faster than two separated racks, but in in my case the difference is huge (about x100)!! and what about the rack awareness in hadoop..

Here is the profile files of the query in two cases:
1 rack (15nodes):
query profile - 2.8 sec
2 racks (15+10 nodes):
query profile - 4.5 min

Thanks in advance.

Re: When I add a new rack some Impala queries became extremely slow!

Lars Volker — Wed, 11 Jul 2018 15:52:16 GMT

This sounds like a result of the drastically increased link latency between your two "racks". While within a single rack you should see latencies less than a millisecond, US-EU latencies will be around 150ms, depending on where in the US and EU your machines are located. Bandwidth between your locations is likely also much lower than between the racks.

Impala currently does not do any rack-aware scheduling of I/O and data exchanges. In addition it is not optimized for high variance in link latencies and throughput. HDFS itself to my knowledge also makes no optimizations for such a case.

Frankly, I don't think you will see good performance in such a scenario. If you want to increase data availability, you could explore replicating the data between your locations while running queries in only one at a time. If you want to increase service availability, you can look into using a load balancer and switching from one cluster to the other in case of failure.

Re: When I add a new rack some Impala queries became extremely slow!

AcharkiMed — Wed, 11 Jul 2018 18:14:01 GMT

Hi @Lars Volker

Thanks for your reply,

Did you mean that I have to create two clusters and synchronise the data between them? if yes wht is the best tool to do this? Peers, DistCp HDFS command or another technic ?

What if I do the second rack in different data centers but in same country (US for example), is latency will be reasonable or no!? or the unique solution is to have the two racks in the same data center!!

What about Kudu? is it have any rack awareness ?

Is there any kind of cloudera documentation about the architechture of multi datacenters/racks.. ?

Remark: In fact, I dont now why there is a HA config in all services if there is no rack awareness.

Thanks again.

Re: When I add a new rack some Impala queries became extremely slow!

Lars Volker — Wed, 11 Jul 2018 21:42:37 GMT

Yes, creating two clusters is what you could try. I'm no expert in setting this up and unfortunately I also don't have good advice on which tooling to use. distcp certainly could be worth a try.

Within a country your experience will depend on where your machines are, and you'll likely also be affected by reduced bandwidth between data centers.

I'm not sure about other services' behavior when running across racks. Impala is not (yet) rack-aware in its scheduling and exchanges. However, even once we get to adding support for rack-awareness, we might assume that the racks are within a single data-center.

Re: When I add a new rack some Impala queries became extremely slow!

alexm — Thu, 12 Jul 2018 13:16:53 GMT

Syncronizing data between clusters can be accomplished via distcp, BDR, or ingesting data into both clusters simulatenously using 3rd party tools. The best tool depends on your use case, risk tolerance, and budget.

We don't recommend spanning clusters across large geographic regions (e.g. US to EU); network latency and bandwidth are usually not suitable and could easily result in the slow query times you're experiencing.

We DO support spanning clusters across AWS Availability Zones if certain conditions are met; see Appendix A of Cloudera Enterprise Reference Architecture for AWS Deployments (PDF) details. For comparison, the latency between AWS AZs is typically sub-millisecond.

Spanning bare metal clusters across multiple data centers will be addressed in the next release of Cloudera Enterprise Reference Architecture for Bare Metal Deployments (PDF), to coincide with C6. It will look similar to the AWS guidance, but with the additional caveat that network latency between sides should not exceed 10ms.

Kudu does not support rack awareness.

Not all services provide HA.