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From Mark <static.void....@gmail.com>
Subject Re: Region Splits
Date Mon, 21 Nov 2011 18:46:48 GMT
I was actually referring to the HBase book that states:


        6.3.2.3. Rowkey Length

Keep them as short as is reasonable such that they can still be useful 
for required data access


Didn't know if adding 16 bytes to each row key would have any 
performance implications.

As a side question having, what would be the best way of converting 
existing sequential keys to a hashed version? M/R script? Is there a 
simple dump?


On 11/21/11 8:18 AM, Nicolas Spiegelberg wrote:
> Mark: you are correct about the old_key suffix.  I'm assuming that you're
> worried about this because of keyspace size, correct?  The default
> algorithm for pre-splitting assumes a 32-bit (4 byte) hash prefix, which
> should be perfectly scalable for all use cases in the near future of
> computing.  Really, you could get away with an 8-bit hash prefix if your
> cluster is small&  you plan to auto-split after a certain size.  This is
> available if you use UniformSplit but will require a little power user
> investigation.  I don't think anybody deviates from the default, mainly
> just because current use cases aren't as finicky about the extra overhead.
>
> For the medium term, note that HBASE-4218 will also introduce key
> compression&  further reduce overhead.  This won't be available until 94
> or so, but you probably won't be worried about an extra 4 bytes until
> then.  We currently use the HexStringSplit algorithm in production, which
> is 8-bytes but is human-readable.  With preliminary investigation, we
> predict an 80%+ compression in our key size (currently ~80 bytes) with
> HBASE-4218.
>
> On 11/21/11 9:55 AM, "Mark"<static.void.dev@gmail.com>  wrote:
>
>> Damn, I was hoping my understanding was flawed.
>>
>> In your example I am guessing the addition of old_key suffix is to
>> prevent against any possible collision. Is that correct?
>>
>> On 11/20/11 9:39 PM, Nicolas Spiegelberg wrote:
>>> Sequential writes are also an argument for pre-splitting and using hash
>>> prefixing.  In other words, presplit your table into N regions instead
>>> of
>>> the default of 1&   transform your keys into:
>>>
>>> new_key = md5(old_key) + old_key
>>>
>>> Using this method your sequential writes under the old_key are now
>>> spread
>>> evenly across all regions.  There are some limitations to hash
>>> prefixing,
>>> such as non-sequential scans across row boundaries.  However, it's a
>>> tradeoff between even distribution&   advanced query options.
>>>
>>> On 11/20/11 7:54 PM, "Amandeep Khurana"<amansk@gmail.com>   wrote:
>>>
>>>> Mark,
>>>>
>>>> Yes, your understanding is correct. If your keys are sequential
>>>> (timestamps
>>>> etc), you will always be writing to the end of the table and "older"
>>>> regions will not get any writes. This is one of the arguments against
>>>> using
>>>> sequential keys.
>>>>
>>>> -ak
>>>>
>>>> On Sun, Nov 20, 2011 at 11:33 AM, Mark<static.void.dev@gmail.com>
>>>> wrote:
>>>>
>>>>> Say we have a use case that has sequential row keys and we have rows
>>>>> 0-100. Let's assume that 100 rows = the split size. Now when there is
>>>>> a
>>>>> split it will split at the halfway mark so there will be two regions
>>>>> as
>>>>> follows:
>>>>>
>>>>> Region1 [START-49]
>>>>> Region2 [50-END]
>>>>>
>>>>> So now at this point all inserts will be writing to Region2 only
>>>>> correct?
>>>>> Now at some point Region2 will need to split and it will look like the
>>>>> following before the split:
>>>>>
>>>>> Region1 [START-49]
>>>>> Region2 [50-150]
>>>>>
>>>>> After the split it will look like:
>>>>>
>>>>> Region1 [START-49]
>>>>> Region2 [50-100]
>>>>> Region3 [150-END]
>>>>>
>>>>> And this pattern will continue correct? My question is when there is
a
>>>>> use
>>>>> case that has sequential keys how would any of the older regions every
>>>>> receive anymore writes? It seems like they would always be stuck at
>>>>> MaxRegionSize/2. Can someone please confirm or clarify this issue?
>>>>>
>>>>> Thanks
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>

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