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From Michael Segel <michael_se...@hotmail.com>
Subject Re: Key formats and very low cardinality leading fields
Date Tue, 04 Sep 2012 17:34:53 GMT
I think you have to understand what happens as a table splits. 
If you have a composite key where the first field has the value between 0-9 and you pre-split
your table, you will  have all of your 1's going to the single region until it splits. But
both splits will start on the same node until they eventually get balanced out. 

(Note: I'm not an expert on how hbase balances the regions across a region server so I couldn't
tell you how it choses which nodes to place each region.)

But what are you trying to do? Avoid a hot spot on the initial load, or are you looking at
the longer term picture? 

On Sep 3, 2012, at 2:58 PM, Eric Czech <eric@nextbigsound.com> wrote:

> With regards to:
>> If you have 3 region servers and your data is evenly distributed, that
>> mean all the data starting with a 1 will be on server 1, and so on.
> Assuming there are multiple regions in existence for each prefix, why
> would they not be distributed across all the machines?
> In other words, if there are many regions with keys that generally
> start with 1, why would they ALL be on server 1 like you said?  It's
> my understanding that the regions aren't placed around the cluster
> according to the range of information they contain so I'm not quite
> following that explanation.
> Putting the higher cardinality values in front of the key isn't
> entirely out of the question, but I'd like to use the low cardinality
> key out front for the sake of selecting rows for MapReduce jobs.
> Otherwise, I always have to scan the full table for each job.
> On Mon, Sep 3, 2012 at 3:20 PM, Jean-Marc Spaggiari
> <jean-marc@spaggiari.org> wrote:
>> Yes, you're right, but again, it will depend on the number of
>> regionservers and the distribution of your data.
>> If you have 3 region servers and your data is evenly distributed, that
>> mean all the data starting with a 1 will be on server 1, and so on.
>> So if you write a million of lines starting with a 1, they will all
>> land on the same server.
>> Of course, you can pre-split your table. Like 1a to 1z and assign each
>> region to one of you 3 servers. That way you will avoir hotspotting
>> even if you write million of lines starting with a 1.
>> If you have une hundred regions, you will face the same issue at the
>> beginning, but the more data your will add, the more your table will
>> be split across all the servers and the less hotspottig you will have.
>> Can't you just revert your fields and put the 1 to 30 at the end of the key?
>> 2012/9/3, Eric Czech <eric@nextbigsound.com>:
>>> Thanks for the response Jean-Marc!
>>> I understand what you're saying but in a more extreme case, let's say
>>> I'm choosing the leading number on the range 1 - 3 instead of 1 - 30.
>>> In that case, it seems like all of the data for any one prefix would
>>> already be split well across the cluster and as long as the second
>>> value isn't written sequentially, there wouldn't be an issue.
>>> Is my reasoning there flawed at all?
>>> On Mon, Sep 3, 2012 at 2:31 PM, Jean-Marc Spaggiari
>>> <jean-marc@spaggiari.org> wrote:
>>>> Hi Eric,
>>>> In HBase, data is stored sequentially based on the key alphabetical
>>>> order.
>>>> It will depend of the number of reqions and regionservers you have but
>>>> if you write data from 23AAAAAA to 23ZZZZZZ they will most probably go
>>>> to the same region even if the cardinality of the 2nd part of the key
>>>> is high.
>>>> If the first number is always changing between 1 and 30 for each
>>>> write, then you will reach multiple region/servers if you have, else,
>>>> you might have some hot-stopping.
>>>> JM
>>>> 2012/9/3, Eric Czech <eric@nextbigsound.com>:
>>>>> Hi everyone,
>>>>> I was curious whether or not I should expect any write hot spots if I
>>>>> structured my composite keys in a way such that the first field is a
>>>>> low cardinality (maybe 30 distinct values) value and the next field
>>>>> contains a very high cardinality value that would not be written
>>>>> sequentially.
>>>>> More concisely, I want to do this:
>>>>> Given one number between 1 and 30, write many millions of rows with
>>>>> keys like <number chosen> : <some generally distinct, non-sequential
>>>>> value>
>>>>> Would there be any problem with the millions of writes happening with
>>>>> the same first field key prefix even if the second field is largely
>>>>> unique?
>>>>> Thank you!

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