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From chandra kant <chandralakshmikan...@gmail.com>
Subject Re: Cache invalidation in Blockcache
Date Sun, 30 Mar 2014 06:34:11 GMT
Thanks anoop..
Here is my understanding..
basically memstores will be scanned no matter whether requested row is
already present in the Blockcache . Also, if row is changed by some write ,
then it will be  reloaded in Blockcache along with the Hfile it belongs to
,if  Blockcache is enabled on table .


On Sunday, 30 March 2014, Anoop John <anoop.hbase@gmail.com> wrote:

> In block cache data is cached not as rows..  As u know when writing HFiles,
> one HFile will logically split into blocks (With def size of 64K)  . During
> reads data is read from files as blocks. (Even if u do a single row get)
> from file HBase has to to read atleast one block.   The block cache caches
> these blocks.  So during read if we find the requested block being in the
> cache, we wont read again from HDFS.  This way the block cache helps.
> So the 1st question answer is no.
> During reading, it is not like 1st check in memstore and then in block
> cache.  It is like a Heap of scanners on the memstore and on all HFiles.
> KVs comes out of this scanner as per the result of KV comparator
> comparison.  Compare  row, cf, family, TS  and finally a memstoreTS (which
> is like increasing on every write)  So mostly a KV from memstore will
> normally comes out 1st before those from files.  But during writes one can
> always specify TS, if some one writes explicetly with TS and 1st write some
> future TS cell and it got flushed to a file and later write a past TS kv
> and it is in memstore , the above said normal case may not come
> applicable.. Hope I make it clear for u..   Again when u read from Files,
> files are read as block by block and during that time check in Cache. If
> that block of this file is already read into cache, there wont be an IO.
> -Anoop-
> On Sun, Mar 30, 2014 at 11:44 AM, chandra kant <
> chandralakshmikant90@gmail.com <javascript:;>> wrote:
> > Hi,
> > I have Blockcache enabled on my table. So, I read a row and it's stored
> in
> > Blockcache . Next, I do a write on that row and I read it again .
> > My question is -  does writing that row invalidates the entry of that row
> > in Blockcache ?
> > Also, while reading , does RegionScanner first check memstore for any
> > updates regrading that row or Blockcache ?
> > It's quite confusing from what  I have read..
> > Thanks
> > Chandra
> >

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