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From Jean-Daniel Cryans <jdcry...@apache.org>
Subject Re: random write slower than random read in HBase 0.20.0
Date Sun, 25 Oct 2009 16:44:41 GMT

Some answers to your remarks inline.


On Sat, Oct 24, 2009 at 10:43 PM, Jun Li <jltz922181@gmail.com> wrote:
> Hi Michael and Tatsuya,
> Thank you very much for your quick replies!
> I realized that when an HTable is initiated, it starts with one region
> from one of the region servers in the cluster and as I pump more and
> more data, the number of regions grow and the regions get splitted to
> other region servers. I did the measurement only after the regions
> fully cover the available region servers in the cluster. Currently the
> table holds at least 14 million rows (as I am writing this email, it
> is counting in the HBase shell).
> I purposely had the row keys to be generated randomly (a row key, in
> my implementation, is the concatenate of the 3 sections, each section
> has 16 bytes and is the hex representation of a random number with 8
> bytes).  I just checked the distribution of the regions to the region
> servers and the current distribution is the following (totally number
> of regions: 33):
> Region Server 1:  3
> Region Server 2:  2
> Region Server 3:  4
> Region Server 4:  1
> Region Server 5:  2
> Region Server 6:  2
> Region Server 7:  4
> Region Server 8:  2
> Region Server 9:  3
> Region Server 10:  4
> Region Server 11:  2
> Region Server 12:  4
> So the distribution of the number of the regions seems not that even.
> In fact, in my Ganglia monitor, I can see that often the CPU usage
> distribution is not even.

There is a sloppiness factor in the way the regions are distributed
since we don't want some major regions reassignment every time there's
a split (a region that becomes too big is split into two new regions).
Also you said your were doing a count while writing this? The shell
count is a scan basically, so since the rows are sequentially
partitioned in each region, the shell count only hits a region at a
time thus only a region server at a time. Do consider using the
provided RowCounter MapReduce job (see the mapreduce package in the
API javadoc).

> Although the way that I generated the row key is tied to the
> application that I am building, but from the performance evaluation
> purpose, I can do some modification in my measurement code. So I would
> like to know what will be a good way to randomly generate a row key so
> that at any certain time, statistically, the rows are evenly
> distributed across available region servers?

Well that's pretty much already the case isn't it? But take into
account that you cannot get the exact number of rows on each region
server, in your case with 14M rows on 12 regions server it seems that
a slightly uneven distribution hits you a lot but on the same setup
with 10x that number of rows the difference is lessen.

> What about HTablePool? In my measurement, I did not see performance
> improvement on write, when I used a single HTable instance, vs. a
> HTable Instance for every client request, in each of the client test
> application.  Will HTablePool only help in a Java process (such as
> Axis web service) that has multiple current threads that try to access
> the HBase?


> Regarding Tatsuya’s suggestion of packing multiple writes into one
> single RPC, I am using HTable as the database tier in a 3-tier web
> application architecture to serve client’s instantaneous data access,
> rather than batched processing.  I am not sure how I can pack multiple
> records into one single flushCommits(), as in one client invocation
> session, data read and data write are interleaving.

Using the write buffer really just makes sense in a batch job you are right,

> So in your email reply, do you mean “multiple puts” across the SAME
> table, or can it be across multiple tables? From my SQL based
> programming experience, I would guess it works only across one single
> table.


> I can have my cluster to go down to 4 region servers as Tatsuya
> suggested and I just acquired a 16-machine cluster (8 cores and 32 GB
> RAM per machine), so I  can increase the region servers to 15  to see
> the performance. And I will let you know my performance measurement
> result in the next week or so.

I think that before anything else, you should get more data. In most
of my comments and others previous replies, we all say that you
probably have more region servers than needed. It may seem weird, but
HBase performs better when loaded with at least an average of 40-50
regions per region server.

> I realized that there is the HBase 0.20.1.  Are there some new
> features inside that can help improve the write performance?

Mostly bug fixes. And BTW, you can set hfile.block.cache.size to 0 if
you want to disable the caching because in your case I'm sure HBase is
able to cache everything thus screwing your numbers ;)

> Regards,
> Jun
> On Sat, Oct 24, 2009 at 9:38 AM, stack <stack@duboce.net> wrote:
>> What Tatsuya said and then, how many regions in your table?  Were your
>> regions distributed out over the cluster evenly so all machines were
>> participating in the test?  Did you start over from scratch each time?   I'm
>> wondering if you had enough data for 12 servers.
>> St.Ack
>> On Fri, Oct 23, 2009 at 6:56 PM, Jun Li <jltz922181@gmail.com> wrote:
>>> I have a cluster of 13 Linux RedHat machines (each one with 4 CPU
>>> cores and 8GB RAM) and I configured it to form a HBase cluster of 1
>>> master and 12 region servers, with HBase-0.20.0 code base.  For the
>>> cluster configuration, I followed what was described in the article
>>> “HBase-0.20.0 Performance Evaluation” by Anty Rao and Schubert Zhang
>>> on August 21, 2009 (accessible from:
>>> http://www.slideshare.net/schubertzhang/hbase-0200-performance-evaluation,
>>> which was also discussed in one of the HBase  mailing list archives in
>>> the last several months). So I allocated 4GB heap for each
>>> HBase-related process, with the “hbase.regionserver.handler.count” set
>>> to be “20”.  A small difference is that I used the HBase managed
>>> Zoo-keeper to manage the Hbase Masters.
>>> I then set up an Hbase table with a row key of 48 bytes, and a column
>>> that holds about 20 Bytes data.  For a single client, I was able to
>>> get in average, the write of 0.6 milliseconds per row (random write),
>>> and the read of 0.4 milliseconds per row (random read).
>>> Then  I  had each machine in the cluster to launch 1, or 2,  or 3
>>> client test applications, with each client test application read/write
>>> 100000 rows for each test run, for throughput testing.  From my
>>> measurement results, I found that the random write will have best
>>> measured performance when each machine having 2 clients (totally
>>> 2*13=26 clients in the cluster), with 8500 rows/second; and the random
>>> read will have almost the same throughput for 2 or 3 clients, with
>>> 35000 rows/second.
>>> Since I designed the Hbase table to be accessed via the Web Service
>>> for random read or write data access, the HTable instance is created
>>> in a default mode, that is, I did not have customized settings for
>>> “setAutoFlush(false)”, “setWriteBufferSize(**)”, etc.
>>> I also incorporated the HTablePool so that each test client
>>> application will use the same HTable instance. But I observed little
>>> improvement on my random write testing from the above numbers, with or
>>> without table pooling.
>>> So the question that I have is that, following the original Google’s
>>> BigTable paper, should Random Write be always much faster than Random
>>> Read?   If that is the case, what are the tunable parameters in terms
>>> of HBase setup that I can explore to improve the Random Write speed.
>>> I also downloaded the PerformanceEvaluation.java that is patched by
>>> Schubert Zhang (the link to the code is in the article mentioned
>>> above), and used it to test my cluster’s performance as well. In my
>>> cluster, to read/write 4,194,280 rows, I will need 274 seconds for
>>> random writes (translated to15307 row/second) and 305 seconds for
>>> random reads (translated to 13751 row/second). Notice that although
>>> random write is still faster than random read, but they are almost
>>> compatible.  As a comparison, in the measurement result reported by
>>> Schubert Zhang (in the article mentioned above),  with the smaller
>>> test environment that he had (1 master and 4 slaves, 4 CPU
>>> core/machine, 8GB RAM per machine),  it was reported to obtain the
>>> random write of 11366 row/second and the random read of 4424
>>> row/second. That is, the random read does get significantly improved
>>> in my case as I had more machines in the cluster, but not the random
>>> write.
>>> Please help on making comments and suggestions, for possible
>>> performance improvement on random write.
>>> Regards,
>>> Jun Li

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