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From Michael Segel <michael_se...@hotmail.com>
Subject Re: Is it necessary to set MD5 on rowkey?
Date Tue, 18 Dec 2012 13:52:53 GMT


First, the use of a 'Salt' is a very, very bad idea and I would really hope that the author
of that blog take it down.
While it may solve an initial problem in terms of region hot spotting, it creates another
problem when it comes to fetching data. Fetching data takes more effort.

With respect to using a hash (MD5 or SHA-1) you are creating a more random key that is unique
to the record.  Some would argue that using MD5 or SHA-1 that mathematically you could have
a collision, however you could then append the key to the hash to guarantee uniqueness. You
could also do things like take the hash and then truncate it to the first byte and then append
the record key. This should give you enough randomness to avoid hot spotting after the initial
region completion and you could pre-split out any number of regions. (First byte 0-255 for
values, so you can program the split... 

Having said that... yes, you lose the ability to perform a sequential scan of the data.  At
least to a point.  It depends on your schema. 

Note that you need to think about how you are primarily going to access the data.  You can
then determine the best way to store the data to gain the best performance. For some applications...
the region hot spotting isn't an important issue. 




On Dec 18, 2012, at 3:33 AM, Damien Hardy <dhardy@viadeoteam.com> wrote:

> Hello,
> There is middle term betwen sequecial keys (hot spoting risk) and md5
> (heavy scan):
>  * you can use composed keys with a field that can segregate data
> (hostname, productname, metric name) like OpenTSDB
>  * or use Salt with a limited number of values (example
> substr(md5(rowid),0,1) = 16 values)
>    so that a scan is a combination of 16 filters on on each salt values
>    you can base your code on HBaseWD by sematext
> http://blog.sematext.com/2012/04/09/hbasewd-avoid-regionserver-hotspotting-despite-writing-records-with-sequential-keys/
>       https://github.com/sematext/HBaseWD
> Cheers,
> 2012/12/18 bigdata <bigdatabase@outlook.com>
>> Many articles tell me that MD5 rowkey or part of it is good method to
>> balance the records stored in different parts. But If I want to search some
>> sequential rowkey records, such as date as rowkey or partially. I can not
>> use rowkey filter to scan a range of date value one time on the date by
>> MD5. How to balance this issue?
>> Thanks.
> -- 
> Damien HARDY

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