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From James Taylor <jtay...@salesforce.com>
Subject Re: Questions on FuzzyRowFilter
Date Fri, 16 May 2014 18:50:15 GMT
Hi Mike,
I agree with you - the way you've outlined is exactly the way Phoenix has
implemented it. It's a bit of a problem with terminology, though. We call
it salting: http://phoenix.incubator.apache.org/salted.html. We hash the
key, mod the hash with the SALT_BUCKET value you provide, and prepend the
row key with this single byte value. Maybe you can coin a good term for
this technique?

FWIW, you don't lose the ability to do a range scan when you salt (or
hash-the-key and mod by the number of "buckets"), but you do need to run a
scan for each possible value of your salt byte (0 - SALT_BUCKET-1). Then
the client does a merge sort among these scans. It performs well.

Thanks,
James


On Fri, May 9, 2014 at 11:57 PM, Michael Segel <michael_segel@hotmail.com>wrote:

> 3+ Years on and a bad idea is being propagated again.
>
> Now repeat after me… DO NO USE A SALT.
>
> Having a low sodium diet, especially for HBase is really good for your
> health and sanity.
>
> The salt is going to be orthogonal to the row key (Key).
> There is no relationship to the specific Key.
>
> Using a salt means you now use the ability to randomly spread the
> distribution of data to avoid HOT SPOTTING.
> However you lose the ability to seek for a specific row.
>
> YOU HASH THE KEY.
>
> The hash whether you use SHA-1 or MD-5 is going to yield the same result
> each and every time you provide the key.
>
> But wait, the generated hash is 160 bits long. We don’t need that!
> Absolutely true if you just want to randomize the key to avoid hot
> spotting. There’s this concept called truncating the hash to the desired
> length.
> So to Adrien’s point, you can truncate it to a single byte which would be
> sufficient….
> Now when you want to seek for a specific row, you can find it.
>
> The downside to any solution is that you lose the ability to do a range
> scan.
> BUT BY USING A HASH AND NOT A SALT, YOU DONT LOSE THE ABILITY TO FETCH A
> SINGLE ROW VIA A get() CALL.
>
> <rant>
> This simple fact has been pointed out several years ago, yet for some
> reason, the use of a salt persists.
> I’ve actually made that part of the HBase course I wrote and use it in my
> presentation(s) on HBase.
>
> It amazes me that the committers and regulars who post here still don’t
> grok the fact that if you’re going to ‘SALT’ a row, you might as well not
> use HBase and stick with Hive.
> I remember Ed C’s rant about how preferential treatment on Hive patches
> was given to vendors’ committers… that preferential treatment seems to also
> be extended speakers at conferences. It wouldn’t be a problem if those said
> speakers actually knew the topic… ;-)
>
> Propagation of bad ideas means that you’re leaving a lot of performance on
> the table and it can kill or cripple projects.
>
> </rant>
>
> Sorry for the rant…
>
> -Mike
>
>
>
>
> On May 3, 2014, at 4:39 PM, Software Dev <static.void.dev@gmail.com>
> wrote:
>
> > Ok so there is no way around the FuzzyRowFilter checking every single
> > row in the table correct? If so, what is a valid use case for that
> > filter?
> >
> > Ok so salt to a low enough prefix that makes scanning reasonable. Our
> > client for accessing these tables is a Rails (not JRuby) application
> > so we are stuck with either the Thrift or Rails client. Can either of
> > these perform multiple gets/scans?
> >
> >
> >
> > On Sat, May 3, 2014 at 1:10 AM, Adrien Mogenet <adrien.mogenet@gmail.com>
> wrote:
> >> Using 4 random bytes you'll get 2^32 possibilities; thus your data can
> be
> >> split enough among all the possible regions, but you won't be able to
> >> easily benefit from distributed scans to gather what you want.
> >>
> >> Let say you want to split (time+login) with a salted key and you expect
> to
> >> be able to retrieve events from 20140429 pretty fast. Then I would split
> >> input data among 10 "spans", spread over 10 regions and 10 RS (ie:
> `$random
> >> % 10'). To retrieve ordered data, I would parallelize Scans over the 10
> >> span groups (<00>-20140429, <01>-20140429...) and merge-sort everything
> >> until I've got all the expected results.
> >>
> >> So in term of performances this looks "a little bit" faster than your
> 2^32
> >> randomization.
> >>
> >>
> >> On Fri, May 2, 2014 at 10:09 PM, Software Dev <
> static.void.dev@gmail.com>wrote:
> >>
> >>> I'm planning to work with FuzzyRowFilter to avoid hot spotting of our
> >>> time series data (20140501, 20140502...).  We can prefix all of the
> >>> keys with 4 random bytes and then just skip these during scanning. Is
> >>> that correct? These *seems* like it will work but Im questioning the
> >>> performance of this even if it does work.
> >>>
> >>> Also, is this available via the rest client, shell and/or thrift
> client?
> >>>
> >>> Also, is there a FuzzyColumn equivalent of this feature?
> >>>
> >>
> >>
> >>
> >> --
> >> Adrien Mogenet
> >> http://www.borntosegfault.com
> >
>
>

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