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From AnandaVelMurugan Chandra Mohan <ananthu2...@gmail.com>
Subject Re: Rowkey hashing to avoid hotspotting
Date Thu, 19 Jul 2012 15:08:41 GMT
Hi Cristofer,

No problem... I am happy to share and learn.. :)

Regarding timestamp based column family, I haven't thought about it. But my
only concern is no of column families. I read somewhere that HBase is not
good at handling more than 100 column families.


On Wed, Jul 18, 2012 at 11:15 PM, Cristofer Weber <
cristofer.weber@neogrid.com> wrote:

> Hi Anand!
>
> I see... sorry for being so curious, but since I started studying HBase I
> am curious about how people are modeling their tables, and in what kinds of
> systems HBase is in use.
>
> Have you evaluated recording your reports in a distinct CF using
> timestamps as column qualifiers? It's my curiosity asking again!
>
> Thanks for sharing!
>
> Regards,
> Cristofer
>
> -----Mensagem original-----
> De: AnandaVelMurugan Chandra Mohan [mailto:ananthu2050@gmail.com]
> Enviada em: quarta-feira, 18 de julho de 2012 13:04
> Para: user@hbase.apache.org
> Assunto: Re: Rowkey hashing to avoid hotspotting
>
> Hi Cristofer,
>
> Data i store is test cell reports about a component. I have many test cell
> reports for each model number + serial number combination. So to make
> rowkey unique, I added timstamp.
>
>
> On Wed, Jul 18, 2012 at 3:14 AM, Cristofer Weber <
> cristofer.weber@neogrid.com> wrote:
>
> > So, Anand, there are some things that can help, but again, most of
> > them are related with the famous access patterns.
> >
> > Sometimes is not easy to get more information about them in advance,
> > but if you are replacing another system you can study its data
> > distribution, grouping for counts, mean, changes over time, etc. It is
> > possible to analyze with partial data too, but it is risky because you
> > will be subjected to the way this partial data was gathered; sample
> > data may not be representative.
> >
> > Salting your rowkey with a hash calculated over your model# will
> > probably result in an uniform distribution over a range (if using
> > modulus), and pre-spliting your table will balance your load over your
> Region Servers.
> > Also, you will be able to recalculate your hash for your model# before
> > scanning for it, allowing for a scan over specific rowkey while
> > restricting this scan by startRow and stopRow. Remember that if your
> > rowkeys shares the same prefix they will probably be located in the
> > same region and your scan will be favored by this.
> >
> > I'm still curious about your need of adding a timestamp after your
> > model#,serial#... I have some background in manufacturing systems and
> > usually a serial number is unique. But, of course, it's just
> > curiosity.  :-)
> >
> > Regards,
> > Cristofer
> >
> > -----Mensagem original-----
> > De: Alex Baranau [mailto:alex.baranov.v@gmail.com] Enviada em:
> > terça-feira, 17 de julho de 2012 12:53
> > Para: user@hbase.apache.org
> > Assunto: Re: Rowkey hashing to avoid hotspotting
> >
> > The most common reason for RS hotspotting during writing data in HBase
> > is writing rows with monotonically increasing/decreasing row keys.
> > E.g. if you put timestamp in the first part of your key, then you are
> > likely to have monotonically increasing row keys. You can find more
> > info about this issue and how to solve it here: [1] and also you may
> > want to look at already implemented salting solution [2].
> >
> > As for RS hotspotting during reading - it is hard to predict without
> > knowing what it the most common data access patterns. E.g. putting
> > model # in first part of a key may seem like a good distribution, but
> > if your web site used mostly by Mercedes owners, the majority of the
> > read load may be directed to just few regions. Again, salting can help a
> lot here.
> >
> > +1 to what Cristofer said on other things, esp: use partial key scans
> > +were
> > possible instead of filters and pre-split your table.
> >
> > Alex Baranau
> > ------
> > Sematext :: http://blog.sematext.com/ :: Hadoop - HBase -
> > ElasticSearch - Solr
> >
> > [1] http://bit.ly/HnKjbc
> > [2] https://github.com/sematext/HBaseWD
> >
> > On Tue, Jul 17, 2012 at 10:44 AM, AnandaVelMurugan Chandra Mohan <
> > ananthu2050@gmail.com> wrote:
> >
> > > Hi Cristofer,
> > >
> > > Thanks for elaborate response!!!
> > >
> > > I have no much information about production data as I work with
> > > partial data. But based on discussion with my project partners, I
> > > have some answers for you.
> > >
> > > Number of model numbers and serial numbers will be finite. Not so
> many...
> > > As far as I know,there is no predefined rule for model number or
> > > serial number creation.
> > >
> > > I have two access pattern. I count the number of rows for a specific
> > > model number. I use rowkey filter for this. Also I filter the rows
> > > based on model, serial number and some other columns. I scan the
> > > table with column value filter for this case.
> > >
> > > I will evaluate salting as you have explained.
> > >
> > > Regards,
> > > Anand.C
> > >
> > > On Tue, Jul 17, 2012 at 12:30 AM, Cristofer Weber <
> > > cristofer.weber@neogrid.com> wrote:
> > >
> > > > Hi Anand,
> > > >
> > > > As usual, the answer is that 'it depends'  :)
> > > >
> > > > I think that the main question here is: why are you afraid that
> > > > this
> > > setup
> > > > would lead to region server hotspotting? Is because you don't know
> > > > how
> > > your
> > > > production data will seems?
> > > >
> > > > Based on what you told about your rowkey, you will query mostly by
> > > > providing model no. + serial no., but:
> > > > 1 - How is your rowkey distribution? There are tons of different
> > > > modelNumbers AND serialNumbers? Few modelNumbers and a lot of
> > > > serialNumbers? Few of both?
> > > > 2 - Putting modelNumber in front of your rowkey means that your
> > > > data will be sorted by rowkey. So, what is the rule that
> > > > determinates a modelNumber creation? Is it a sequential number
> > > > that will be increased by time? If
> > > so,
> > > > are newer members accessed a lot more than older members? If not,
> > > > what
> > > will
> > > > drive this number? Is it an encoding rule?
> > > > 3 - Do you expect more write/read load over a few of these
> > > > modelNumbers and/or serialNumbers? Will it be similar to a Pareto
> > Distribution?
> > > > Distributed over what?
> > > >
> > > > Also, two other things got my attention here...
> > > > 1 - Why are you filtering with regex? If your queries are over
> > > > model
> > no.
> > > +
> > > > serial no., why don't you just scan starting by your
> > > > modelNumber+SerialNumber, and stoping on your next SerialNumber?
> > > > modelNumber+Or is there another access pattern that doesn't
> > > > apply to your composited rowkey?
> > > > 2 - Why do you have to add a timestamp to ensure uniqueness?
> > > >
> > > > Now, answering your question without more info about your data,
> > > > you can apply hash in two ways:
> > > > 1 - Generating a hash (MD5 is the most common as far as I read
> > > > about) and using only this hash as your rowkey. Based on what you
> > > > have told, this
> > > way
> > > > doesn't fit your needs, because you would not be able to do apply
> > > > your filter anymore.
> > > > 2 - Salting, by prefixing your current rowkey with a pinch of hash.
> > > Notice
> > > > that the hash portion must be your rowkey prefix to ensure a kind
> > > > of balanced distribution over something (where something is your
> > > > region servers). I'm working with a case that is a bit similar to
> > > > yours, and
> > > what
> > > > I'm doing right now is calculating the hashValue of my rowkey and
> > > > using a Java Formatter to create a hex string to prepend to my
> > > > rowkey. Something like a String.format("%03x", hashValue)
> > > >
> > > > In both cases, you still have to split your regions in advance,
> > > > and it will be better to work your splitting before starting to
> > > > feed your table with production data.
> > > >
> > > > Also, you have to study the consequences that changing your rowkey
> > > > will bring. It's not for free.
> > > >
> > > > There's a lot of words here and a lot of questions, so by now I
> > > > feel I started to shoot in the dark. Try to understand your
> > > > production data and
> > > if
> > > > you have more to share, for sure it will help!
> > > >
> > > > Regards,
> > > > Cristofer
> > > >
> > > > -----Mensagem original-----
> > > > De: AnandaVelMurugan Chandra Mohan [mailto:ananthu2050@gmail.com]
> > > > Enviada em: segunda-feira, 16 de julho de 2012 02:30
> > > > Para: user@hbase.apache.org
> > > > Assunto: Rowkey hashing to avoid hotspotting
> > > >
> > > > Hi,
> > > >
> > > > I am using Hbase to store data about mechanical components. Each
> > > component
> > > > has model no. and serial no. and some other attributes.
> > > >
> > > > I would be querying my data mostly by model no. and serial no. So
> > > > I created a composite key with these two attributes and added
> > > > timestamp to make it unique.
> > > >
> > > > To filter the data, I use rowkey filter with regex string
> > > > comparator and it works well with sample seed data. Now I am
> > > > afraid whether this set up will lead to region server hotspotting
> > > > when we load production data in HBase. I read hashing may solve
> > > > this problem. Can some one help me in implementing hashing the row
> > > > key? Also I would want the row filter to
> > > work
> > > > as I have to display the number of components in a web page and I
> > > > use row key filter for implementing that functionality? Any
> > > > guidance would be of great help.
> > > >
> > > > --
> > > > Regards,
> > > > Anand
> > > >
> > >
> > >
> > >
> > > --
> > > Regards,
> > > Anand
> > >
> >
> >
> >
> > --
> > Alex Baranau
> > ------
> > Sematext :: http://blog.sematext.com/ :: Hadoop - HBase -
> > ElasticSearch - Solr
> >
>
>
>
> --
> Regards,
> Anand
>



-- 
Regards,
Anand

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