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From "Daniel Leffel" <daniel.lef...@gmail.com>
Subject Advice for smaller clusters in write-heavy environments
Date Thu, 08 May 2008 04:29:40 GMT
I thought I might share back with the users my experience in getting HBase
running on a small, 4 node cluster. I ran into a lot of trouble in getting
started, some because of bugs and some specific to my use case. My learnings
I think will hopefully be valuable to new users.

First of all, let me compliment the amazing group of folks developing HBase.
Also, I'd like to say that we owe a lot to the amazing strategy Powerset has
taken as a company to propel the development of their product, both
leveraging and contributing to open source - what you guys are doing is
nothing short than amazing!

My basic use case is to persist a large (and growing) sparce dataset and
enable constant incremental re-computation. In order to test performance for
this use case, it was important to load a test initial dataset - roughly 220
million rows and 6 columns (for now, I'll say columns generically - I'll get
it to strategy of column families).

Some of my learnings

   - "Commodity Hardware" is relative. When I first heard the term, I (and
   many others I know) considered this to be on the order of desktop-grade
   machines - the machines I'd purchased were Dual Core 2+ Ghz Dell Desktops
   (purchased on eBay for $350 a piece). Well, you can definitely do certain
   tasks within the framework with these types of machines, but an ideal
   configuration consists of something much stronger - server-grade, quad core,
   8Gigs of RAM, etc. HBase (particularly if you are going to do a lot of
   writes), needs really good Machine IO . If you are going to try to use
   machines with slow drives and controllers, it might be possible if you have
   a ton of datanodes, but not as advisable on smaller clusters.
   - Ideally, you should always insure that there is one processor available
   for the region server daemon and at least 2 processors for tasktracker (or 1
   if you limit the map and reduce tasks to one each), if you are going to run
   heavy map/reduce jobs. The trouble with not doing so is that until 0.2, when
   there will be better load balancing on regionservers, it's always possible
   that a single region server can be called on to shoulder the full load of
   all tasktrackers. If you have a large write operations happening, you could
   otherwise cause splits and/or compaction to take too long (expensive
   operations) and cause your job to crawl to near halt if your lucky, or die
   completely. This means, if you're only using dual core machines, I'd suggest
   that at least during heavy data-writing periods, you consider running either
   regionservers or tasktracker, but probably not both on the same machine.
   - All machines should run the datanode - this helps the regionservers to
   distribute the IO load better.  That way, when an expensive operation like
   compaction starts, it's spread over more machines. Also, Hadoop can localize
   frequently used files, to some degree.
   - Running bin/stop-hbase.sh can sometimes take a long time. Sometimes,
   regionservers are waiting for a lease to expire. There are a few times when
   there are dead processes (especially if you didn't take the earlier
   suggestions) so check the logs (.out), but often you just need to wait
   longer and it's worth it.
   - If you are writing from a MR job, it's most beneficial to find the
   right balance of number of tasks. Too many tasks means too many splits,
   startup and commits. Too few, and your region servers don't get the benefit
   of a break (the time it takes to commit and initialize a new task) - not too
   mention less to repeat after a failure.
   - Use the new release candidate 0.1.2 #1. It has a number of fixes in it
   that help for issues related to small clusters - I don't regard prior
   releases as usable for those of us being cheap about hardware.
   - Don't be afraid to adjust which daemons you run on which machines. For
   example, for my first large (initial) load, I shutdown all but a couple of
   tasktrackers and started up more region servers, whereas in normal
   operation, that ratio will probably be flipped.
   - Watch the number of regions you have on any particular regionserver.
   I'm in the process at the moment of testing how far you can push this, but
   the big concern is OOME - and unless you're running the latest release
   candidate, you're going to have big problems after an OOME.


Hope this is helpful, St^ack, please feel free to point out where I'm wrong.
:-)

Danny


PS. Thanks again to St^ack. He went over and above the call of duty to help
me and it's bought a ton of confidence I now have in this project.

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