On Wed, Aug 22, 2012 at 10:20 AM, Pamecha, Abhishek <apame...@x.com> wrote:
> So then a GET query means one needs to look in every HFile where key falls > within the min/max range of the file. > > From another parallel thread, I gather, HFile comprise of blocks which, I > think, is an atomic unit of persisted data in HDFS.(please correct if not). > > And that each block for a HFile has a range of keys. My key can satisfy > the range for the block and yet may not be present. So, all the blocks that > match the range for my key, will need to be scanned. There is one block > index per HFile which sorts blocks by key ranges. This index help in > reducing the number of blocks to scan by extracting only those blocks whose > ranges satisfy the key. > > In this case, if puts are random wrt order, each block may have similar > range and it may turn out that Hbase needs to scan every block for the > File. This may not be good for performance. > > I just want to validate my understanding. > > If you have such a use case I think best practice is to use bloom filters. I think in generaly it's a good idea to atleast enable bloom filter at row level. > Thanks, > Abhishek > > > -----Original Message----- > From: lars hofhansl [mailto:lhofha...@yahoo.com] > Sent: Tuesday, August 21, 2012 5:55 PM > To: user@hbase.apache.org > Subject: Re: HBase Put > > That is correct. > > > > ________________________________ > From: "Pamecha, Abhishek" <apame...@x.com> > To: "user@hbase.apache.org" <user@hbase.apache.org>; lars hofhansl < > lhofha...@yahoo.com> > Sent: Tuesday, August 21, 2012 4:45 PM > Subject: RE: HBase Put > > Hi Lars, > > Thanks for the explanation. I still have a little doubt: > > Based on your description, given gets do a merge sort, the data on disk is > not kept sorted across files, but just sorted within a file. > > So, basically if on two separate days, say these keys get inserted: > > Day1: File1: A B J M > Day2: File2: C D K P > > Then each file is sorted within itself, but scanning both files will > require Hbase to use merge sort to produce a sorted result. Right? > > Also, File 1 and File2 are immutable, and during compactions, File 1 and > File2 are compacted and sorted using merge sort to a bigger File3. Is that > correct too? > > Thanks, > Abhishek > > > -----Original Message----- > From: lars hofhansl [mailto:lhofha...@yahoo.com] > Sent: Tuesday, August 21, 2012 4:07 PM > To: user@hbase.apache.org > Subject: Re: HBase Put > > In a nutshell: > - Puts are collected in memory (in a sorted data structure) > - When the collected data reaches a certain size it is flushed to a new > file (which is sorted) > - Gets do a merge sort between the various files that have been created > - to contain the number of files they are periodically compacted into > fewer, larger files > > > So the data files (HFiles) are immutable once written, changes are batched > in memory first. > > -- Lars > > > > ________________________________ > From: "Pamecha, Abhishek" <apame...@x.com> > To: "user@hbase.apache.org" <user@hbase.apache.org> > Sent: Tuesday, August 21, 2012 4:00 PM > Subject: HBase Put > > Hi > > I had a question on Hbase Put call. In the scenario, where data is > inserted without any order to column qualifiers, how does Hbase maintain > sortedness wrt column qualifiers in its store files/blocks? > > I checked the code base and I can see checks< > https://github.com/apache/hbase/blob/trunk/hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV2.java#L319> > being made for lexicographic insertions for Key value pairs. But I cant > seem to find out how the key-offset is calculated in the first place? > > Also, given HDFS is by nature, append only, how do randomly ordered keys > make their way to sorted order. Is it only during minor/major compactions, > that this sortedness gets applied and that there is a small window during > which data is not sorted? > > > Thanks, > Abhishek >
