Yes you are right. You can see the code after the list of scanners are
formed. They are all collected in a KeyValueHeap.
Pls note that memstore is not a cache, it is only a data structure where
the data is first written and subsequently gets flushed into files. So the
data you read may or may not reside in the memstore. So it is always needed
to scan
the memstore and the files and then keep returning keys in the
lexographical sorted order for which the heap comes into place.
Regards
Ram
On Mon, Oct 17, 2016 at 11:54 AM, Xi Yang <alex.xi.y...@gmail.com> wrote:
> Got it. So you mean, actually, the result HBase return to user is from the
> Heap. And the scanners' jobs are collecting data into that Heap. So the
> order of how to arrange HFile scanners and memstore scanner is not a big
> deal?
>
> Thanks,
> Alex
>
> 2016-10-16 23:04 GMT-07:00 Anoop John <anoop.hb...@gmail.com>:
>
> > Over all these scanners we will be creating a Heap. (See in
> > StoreScanner where we make KeyValueHeap). Out of this cells come in
> > their key order. So said that, we will be opening and seeking to all
> > scanners and get cur cells from all.. Based on the comparator result
> > of all these cells emerge out from Heap. So it is not that we will
> > scan HFile scanners first and then do scan over memstore. Make sense?
> >
> > -Anoop-
> >
> > On Mon, Oct 17, 2016 at 11:04 AM, Xi Yang <alex.xi.y...@gmail.com>
> wrote:
> > > I found codes in HStore.java
> > >
> > > List<StoreFileScanner> sfScanners =
> > > StoreFileScanner.getScannersForStoreFiles(files,
> > > cacheBlocks, usePread, isCompaction, false, matcher, readPt,
> > > isPrimaryReplicaStore());
> > > List<KeyValueScanner> scanners = new
> > > ArrayList<KeyValueScanner>(sfScanners.size() + 1);
> > > scanners.addAll(sfScanners);
> > > // Then the memstore scanners
> > > if (memStoreScanners != null) {
> > > scanners.addAll(memStoreScanners);
> > > }
> > >
> > > So is it mean before scan memstore it will scan HFile first?
> > > Why not scan memstore first, because memory is always faster then hard
> > disk?
> > >
> > >
> > > Thanks,
> > > Alex
> >
>
