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From Jonathan Gray <jl...@streamy.com>
Subject Re: Cassandra vs HBase
Date Wed, 02 Sep 2009 18:32:54 GMT
@Sylvain

If you describe your use case, perhaps we can help you to understand 
what others are doing / have done similarly.  Event logging is certainly 
something many of us have done.

If you're wondering about how much load HBase can handle, provide some 
numbers of what you expect.  How much data in bytes are associated with 
each event, how many events per hour, and what operations do you want to 
do on it?  We could help you determine how big of a cluster you might 
need and the kind of write/read throughput you might see.

@Schubert

You do not need to partition your tables by stamp.  One possibility is 
to put the stamp as the first part of your rowkeys, and in that way you 
will have the table sorted by time.  Using Scan's start/stop keys, you 
can prevent doing a full table scan.


For both of you... If you are storing massive amounts of streaming 
log-type data, do you need full random read access to it?  If you just 
need to process on subsets of time, that's easily partitioned by file. 
HBase should be used if you need to *read* from it randomly, not 
streaming.  If you have processing that HBase's inherent sorting, 
grouping, and indexing can benefit from, then it also can make sense to 
use HBase in order to avoid full-scans of data.

HBase is not the answer because of lack of HDFS append.  You could 
buffer in something outside HDFS, close files after a certain size/time 
(this his what hbase does now, we can have data loss because of no
appends as well), etc...

Reads/writes of lots of streaming data to HBase will always be slower 
than HDFS.  HBase adds additional buffering, and the compaction/split 
processes actually mean you copy the same data multiple times (probably 
3-4 times avg which lines up with the 3-4x slowdown you see).


And there is currently a patch in development (that works at least 
partially) to do direct-to-hdfs imports to HBase which would then be 
nearly as fast as a normal HDFS writing job.

Issue here:  https://issues.apache.org/jira/browse/HBASE-48


JG

Sylvain Hellegouarch wrote:
> I must admit, I'm left as puzzled as you are. Our current use case at 
> work involve large amount of small event log writing. Of course HDFS was 
> quickly out of question since it's not there yet to append to a file and 
> more generally to handle large amount of small write ops.
> 
> So we decided with HBase because we trust the Hadoop/HBase 
> infrastructure will offer us the robustness and reliability we need. 
> That being said, I'm not feeling at ease in regards to the capacity of 
> HBase to handle the potential load we are looking at inputing.
> 
> In fact, it's a common treat of such systems, they've been designed with 
> a certain use case in mind and sometimes I feel like their design and 
> implementation leak way too much on our infrastructure, leading us down 
> the path of a virtual lock-in.
> 
> Now I am not accusing anyone here, just observing that I find it really 
> hard to locate any industrial story of those systems in a similar use 
> case we have at hand.
> 
> The number of nodes this or that company has doesn't quite interest me 
> as much as the way they are actually using HBase and Hadoop.
> 
> RDBMS don't scale as well but they've got a long history and people do 
> know how to optimise, use and manage them. It seems column-oriented 
> database systems are still young :)
> 
> - Sylvain
> 
> Schubert Zhang a écrit :
>> Regardless Cassandra, I want to discuss some questions about
>> HBase/Bigtable.  Any advices are expected.
>>
>> Regards runing MapReduce to scan/analyze big data in HBase.
>>
>> Compared to sequentially reading data from HDFS files directly,
>> scan/sequential-reading data from HBase is slower. (As my test, at 
>> least 3:1
>> or 4:1).
>>
>> For the data in HBase, it is diffcult to only analyze specified part of
>> data. For example, it is diffcult to only analyze the recent one day of
>> data. In my application, I am considering partition data into different
>> HBase tables (e.g. one day - one table), then, I can only touch one table
>> for analyze via MapReduce.
>> In Google's Bigtable paper, in the "8.1 Google Analytics", they also
>> discribe this usage, but I don't know how.
>>
>> It is also slower to put flooding data into HBase table than writing to
>> files. (As my test, at least 3:1 or 4:1 too). So, maybe in the future, 
>> HBase
>> can provide a bulk-load feature, like PNUTS?
>>
>> Many people suggest us to only store metadata into HBase tables, and 
>> leave
>> data in HDFS files, because our time-series dataset is very big.  I
>> understand this idea make sense for some simple application requirements.
>> But usually, I want different indexes to the raw data. It is diffcult to
>> build such indexes if the the raw data files (which are raw or are
>> reconstructed via MapReduce  periodically on recent data ) are not 
>> totally
>> sorted.  .... HBase can provide us many expected features: sorted,
>> distributed b-tree, compact/merge.
>>
>> So, it is very difficult for me to make trade-off.
>> If I store data in HDFS files (may be partitioned), and metadata/index in
>> HBase. The metadata/index is very difficult to be build.
>> If I rely on HBase totally, the performance of ingesting-data and
>> scaning-data is not good. Is it reasonable to do MapReduce on HBase? 
>> We know
>> the goal of HBase is to provide random access over HDFS, and it is a
>> extention or adaptor over HDFS.
>>
>> ----
>> Many a time, I am thinking, maybe we need a data storage engine, which 
>> need
>> not so strong consistency, and it can provide better writing and
>> reading throughput like HDFS. Maybe, we can design another system like a
>> simpler HBase ?
>>
>> Schubert
>>
>> On Wed, Sep 2, 2009 at 8:56 AM, Andrew Purtell <apurtell@apache.org> 
>> wrote:
>>
>>  
>>> To be precise, S3. http://status.aws.amazon.com/s3-20080720.html
>>>
>>>   - Andy
>>>
>>>
>>>
>>>
>>> ________________________________
>>> From: Andrew Purtell <apurtell@apache.org>
>>> To: hbase-user@hadoop.apache.org
>>> Sent: Tuesday, September 1, 2009 5:53:09 PM
>>> Subject: Re: Cassandra vs HBase
>>>
>>>
>>> Right... I recall an incident in AWS where a malformed gossip packet 
>>> took
>>> down all of Dynamo. Seems that even P2P doesn't mitigate against corner
>>> cases.
>>>
>>>
>>> On Tue, Sep 1, 2009 at 3:12 PM, Jonathan Ellis <jbellis@gmail.com> 
>>> wrote:
>>>
>>>    
>>>> The big win for Cassandra is that its p2p distribution model -- which
>>>> drives the consistency model -- means there is no single point of
>>>> failure.  SPF can be mitigated by failover but it's really, really
>>>> hard to get all the corner cases right with that approach.  Even
>>>> Google with their 3 year head start and huge engineering resources
>>>> still has trouble with that occasionally.  (See e.g.
>>>> http://groups.google.com/group/google-appengine/msg/ba95ded980c8c179.)
>>>>       
>>>
>>>
>>>     
>>
>>   
> 

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