I think you can take a look at the http://hbase.apache.org/book/regions.arch.html, it describes the data storage hierarchy of HBase. Due to the statement of Lars "stems from the fact that HBase flushes by region (which means all stores of that region are flushed)", you can think the limitations of column-families like this way. If the size of the region is fixed, the more column-families you create, the less space you can get for each column-family. As the flush operation is based on region, if one column-family research its size threshold even if the other column-families are empty, the flush operations will be triggered. Roughly speaking, the more column-families you create, the more I/O cost you will get. I think if the flush is based on column-family rather than region, maybe we can release this limitation.
On Sat, Aug 24, 2013 at 5:47 PM, Koert Kuipers <ko...@tresata.com> wrote: > thanks i think it makes sense. i will go through hbase architecture again > to make sure i fully understand the mapping to "regions" and "stores" > > > On Fri, Aug 23, 2013 at 10:33 AM, Michael Segel > <michael_se...@hotmail.com>wrote: > > > I think the issue which a lot of people miss is why do you want to use a > > column family in the first place. > > > > Column families are part of the same table structure, and each family is > > kept separate. > > > > So in your design, do you have tables which are related, but are not > > always used at the same time? > > > > The example that I use when I teach about HBase or do a > > lecture/presentation is an Order Entry system. > > Here you have an order being entered, then you have one or many pick > > slips being generated, same for shipping then there's the invoicing. > > All separate processes which relate back to the same order. > > > > So here it makes sense to use column families. > > > > Other areas could be metadata is surrounding a transaction. Again... few > > column families are tied together. > > > > Does this make sense? > > > > > > On Aug 23, 2013, at 12:35 AM, lars hofhansl <la...@apache.org> wrote: > > > > > You can think of it this way: Every region and column family is a > > "store" in HBase. Each store has a memstore and its own set of HFiles in > > HDFS. > > > The more stores you have, the more there is to manage. > > > > > > So you want to limit the number of stores. Also note that the word > > "Table" is somewhat a misnomer in HBase it should have better been called > > "Keyspace". > > > The extra caution for the number of column families per table stems > from > > the fact that HBase flushes by region (which means all stores of that > > region are flushed). This in turn means that unless are column families > > hold roughly the same amount of data you end up with very lopsided > > distributions of HFile sizes. > > > > > > -- Lars > > > > > > > > > > > > ________________________________ > > > From: Koert Kuipers <ko...@tresata.com> > > > To: user@hbase.apache.org; vrodio...@carrieriq.com > > > Sent: Thursday, August 22, 2013 12:30 PM > > > Subject: Re: one column family but lots of tables > > > > > > > > > if that is the case, how come people keep warning about limiting the > > number > > > of column families to only a handful (with more hbase performance will > > > degrade supposedly), yet there seems to be no similar warnings for > number > > > of tables? see for example here: > > > http://comments.gmane.org/gmane.comp.java.hadoop.hbase.user/27616 > > > > > > if a table means at least one column family then the number of tables > > > should also be kept to a minumum, no? > > > > > > > > > > > > > > > On Thu, Aug 22, 2013 at 1:58 PM, Vladimir Rodionov > > > <vrodio...@carrieriq.com>wrote: > > > > > >> Nope. Column family is per table (its sub-directory inside your table > > >> directory in HDFS). > > >> If you have N tables you will always have , at least, N distinct CFs > > (even > > >> if they have the same name). > > >> > > >> Best regards, > > >> Vladimir Rodionov > > >> Principal Platform Engineer > > >> Carrier IQ, www.carrieriq.com > > >> e-mail: vrodio...@carrieriq.com > > >> > > >> ________________________________________ > > >> From: Koert Kuipers [ko...@tresata.com] > > >> Sent: Thursday, August 22, 2013 8:06 AM > > >> To: user@hbase.apache.org > > >> Subject: one column family but lots of tables > > >> > > >> i read in multiple places that i should try to limit the number of > > column > > >> families in hbase. > > >> > > >> do i understand it correctly that when i create lots of tables, but > they > > >> all use the same column family (by name), that i am just using one > > column > > >> family and i am OK with respect to limiting number of column families > ? > > >> > > >> thanks! koert > > >> > > >> Confidentiality Notice: The information contained in this message, > > >> including any attachments hereto, may be confidential and is intended > > to be > > >> read only by the individual or entity to whom this message is > > addressed. If > > >> the reader of this message is not the intended recipient or an agent > or > > >> designee of the intended recipient, please note that any review, use, > > >> disclosure or distribution of this message or its attachments, in any > > form, > > >> is strictly prohibited. If you have received this message in error, > > please > > >> immediately notify the sender and/or notificati...@carrieriq.com and > > >> delete or destroy any copy of this message and its attachments. > > > > The opinions expressed here are mine, while they may reflect a cognitive > > thought, that is purely accidental. > > Use at your own risk. > > Michael Segel > > michael_segel (AT) hotmail.com > > > > > > > > > > > > >
