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From Jean-Marc Spaggiari <jean-m...@spaggiari.org>
Subject Re: Key formats and very low cardinality leading fields
Date Mon, 03 Sep 2012 20:11:01 GMT
Initially your table will contain only one region.

When you will reach its maximum size, it will split into 2 regions
will are going to be distributed over the cluster.

The 2 regions are going to be ordered by keys.So all entries starting
with 1 will be on the first region. And the middle key (let's say
25......) will start the 2nd region.

So region 1 will contain 1 to 24999. and the 2nd region will contain
keys from 25

And so on.

Since keys are ordered, all keys starting with a 1 are going to be
closeby on the same region, expect if the region is big enought to be
splitted and the servers by more region servers.

So when you will load all your entries starting with 1, or 3, they
will go on one uniq region. Only entries starting with 2 are going to
be sometime on region 1, sometime on region 2.

Of course, the more data you will load, the more regions you will
have, the less hotspoting you will have. But at the beginning, it
might be difficult for some of your servers.

2012/9/3, Eric Czech <eric@nextbigsound.com>:
> 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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