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Accumulo, mail # user - Re: Reverse Index Timestamp


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Re: Reverse Index Timestamp
Jim Klucar 2012-11-27, 21:45
Roshan,

Depending on what your cluster setup is and what the resolution of the time
stamp is you could do something like this to spread the data around:

<timestamp-LSBs>-<string>-<reverse timestamp>

Using the LSBs of the timestamp as a uniform hash, then splitting on all
possible hashes would spread things around a bit. If you do this, then all
scans must check all hashes for data.

On Tue, Nov 27, 2012 at 1:25 PM, Keith Turner <[EMAIL PROTECTED]> wrote:

>
>
> On Tue, Nov 27, 2012 at 1:22 PM, Roshan Punnoose <[EMAIL PROTECTED]>wrote:
>
>> Thanks!
>>
>> The fact that you are using a binary tree behind the scenes makes perfect
>> sense. Btw, what do you use in the standalone (non native) implementation?
>> Does it use a TreeMap?
>>
>
> When not using native code, ConcurrentSkipListMap is used.
>
>
>>
>>
>> On Tue, Nov 27, 2012 at 12:57 PM, Keith Turner <[EMAIL PROTECTED]> wrote:
>>
>>>
>>>
>>> On Tue, Nov 27, 2012 at 12:21 PM, Roshan Punnoose <[EMAIL PROTECTED]>wrote:
>>>
>>>> The <string> would most likely be a fixed set of strings that do not
>>>> change over time.
>>>>
>>>> My question is if it is bad to use a reverse index timestamp in the row
>>>> id? Will it cause problems with the tablet splitting, compaction, and
>>>> performance if the data is always being sent to the top of the tablet? If I
>>>> define a split as everything prefixed with <string>, then the ingest will
>>>> go to one tablet, but then I add a reverse timestamp in the row, and that
>>>> would mean I am always copying data to the top of the tablet. Will this
>>>> cause performance issues? Or is it better to append to a tablet?
>>>>
>>>
>>> I do not think it should matter. Inserts go into a C++ STL map on the
>>> tablet server if using the nativemap.   I think the implementation of that
>>> is a balanced binary tree.  So I do not think inserting at the beginning vs
>>> the end would make difference.  That being said, I do not think I have
>>> tried this so I do not know if there would be any suprises.  I would be
>>> interested in hearing about your experiences.
>>>
>>>
>>>>
>>>>
>>>> On Tue, Nov 27, 2012 at 11:51 AM, Keith Turner <[EMAIL PROTECTED]>wrote:
>>>>
>>>>>
>>>>>
>>>>> Keith
>>>>>
>>>>> On Tue, Nov 27, 2012 at 10:41 AM, Roshan Punnoose <[EMAIL PROTECTED]>wrote:
>>>>>
>>>>>> I want to have a table where the row will consist of
>>>>>> "<string>-<reverse index timestamp>". But this means that the data is
>>>>>> always being prefixed to the beginning of the row (or tablet if the row is
>>>>>> large). Will this be a problem for compaction or performance?
>>>>>
>>>>>
>>>>> Can you tell me more about what <string> is?  For example is it a hash
>>>>> or does it come from the set "foo1","foo2","foo3".   How does it change
>>>>> over time?  I think the answer to your question depends on what <string> is.
>>>>>
>>>>>
>>>>>>
>>>>>> I don't know if I heard this correctly, but someone once mentioned
>>>>>> that making the row id the direct timestamp could cause performance issues
>>>>>> because data is always going to one tablet, but also because there is
>>>>>> trouble splitting since it always appends to the tablet. Is this true, is
>>>>>> it similar to what could happen if I am always prefixing to a tablet?
>>>>>>
>>>>>
>>>>> Yes using a timestamp for a row could cause data from many clients to
>>>>> always go to the same tablet, which would be bad for performance on a
>>>>> cluster.
>>>>>
>>>>>
>>>>>>
>>>>>> Thanks!
>>>>>> Roshan
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>