I would say that the use of ZK by HBase today is very minimal. Very few
writes at all, almost exclusively reads and still not that often.
Dedicated resources would not make much of a difference.
For users who are just testing, developing, or running clusters that are
not highly loaded, it is not as important to have dedicated nodes _yet_.
The next version of HBase, 0.21, we are planning on widely expanding
what we use ZK for. At that time, we will be writing much more often,
and reading constantly. Dedicated nodes/disks would be highly
recommended at that point.
Thanks for the explanation.
Benjamin Reed wrote:
you need a dedicated disk for the logDir, but not the dataDir. the
reason is that the write to the log is in the critical path: we cannot
commit changes until they have been synced to disk, so we want to make
sure that we don't contend for the disk. the snapshots in the dataDir
are done in an asynchronous thread, so it can be written to a disk used
by other applications (usually the OS disk).
the problem with running other processes on the zookeeper server is
similar to the disk contention: if zookeeper starts contending with a
runaway app for CPU and memory, we can start timing out because of
by using a dedicated disk for logs and a dedicated machine you can get
very high deterministic performance.
Jonathan Gray wrote:
Thanks for the input.
Honestly, I'm thinking I need to have separate clusters. The version
of ZK is one thing; but also for an application like HBase, we have
had periods where we needed to patch ZK before it became part of a
release. Keeping track of that on a shared cluster will be tricky,
if not impossible.
And with a small development team and a very fast dev cycle, I'm a
little concerned about a runaway application hosing all the other
dependencies on ZK...
What are the actual reasons for wanting a separate disk for ZK?
Strictly reliability purposes? Should that disk be dedicated to the
logDir but not the dataDir, or both?
If I don't give it a dedicated disk or node, but it has 1GB of memory
and a core, what are the downsides? Are they just about reliability?
If I could run 5 or 7 zk nodes, but co-hosted with my HBase cluster,
is that really less reliable than 3 separate nodes, as long as the jvm
has sufficient resources? Or are there performance or usability
concerns as well?
Sorry for all the questions, just trying to get the story straight so
that we don't spread misinformation to HBase users. Most users start
out on very small clusters, so dedicated ZK nodes are not a realistic
assumption... How big of a deal is that?
Benjamin Reed wrote:
we designed zk to have high performance so that it can be shared by
multiple applications. the main thing is that you use dedicated zk
machines (with a dedicated disk for logging). once you have that in
place, watch the load on your cluster, as long as you aren't
saturating the cluster you should share.
as you point out running multiple clusters is a hardware investment,
plus you miss out on opportunities to improve reliability. for
example, if you have three applications that have a cluster of 3 zk
servers each, one failure will result in an outage. if instead of
using the 9 servers you have the same three applications use a zk
cluster with 7 servers you can tolerate three failures without an
the key of course is to make sure that you don't oversubscribe the
Jonathan Gray wrote:
Been using ZK indirectly for a few months now in the HBase and Katta
realms. Both of these applications make it really easy so you don't
have to be involved much with managing your ZK cluster to support it.
I'm now using ZK for a bunch of things internally, so now I'm
manually configuring, starting, and managing a cluster.
What advice is there about whether I should be sharing a single
cluster between all my applications, or running separate ones for
I've been told that it's strongly recommended to run your ZK nodes
separately from the application using them (this is actually what
we're telling new users over in HBase, though a majority of
installations will likely co-host them with DataNodes and
I don't have the resources to maintain a separate 3+ node ZK cluster
for each of my applications, so this is not really an option. I'm
trying to decide if I should have HBase running/managing it's own ZK
cluster that is co-located with some of the regionservers (there
will be ample memory, but ZK will not have a dedicated disk), or if
I should be pointing it to a dedicated 3 node ZK cluster.
I would then also have Katta pointing at this same shared cluster
(or a separate cluster would be co-located with katta nodes). Same
for my application; could share nodes with the app servers or
pointed at a single ZK cluster.
Trade-offs I should be aware of? Current best practices?
Any help would be much appreciated. Thanks.