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HBase >> mail # user >> Does HBase RegionServer benefit from OS Page Cache

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Re: Does HBase RegionServer benefit from OS Page Cache
Thanks Liyin for sharing your use cases.

Related to those, I was thinking of two improvements:
 - AFAIK, MySQL keeps the compressed and uncompressed versions of the blocs
in its block cache, failing over the compressed one if decompressed one
gets evicted. With very large heaps, maybe keeping around the compressed
blocks in a secondary cache makes sense?
 - A compaction will trash the cache. But maybe we can track keyvalues
(inside cached blocks are cached) for the files in the compaction, and mark
the blocks of the resulting compacted file which contain previously cached
keyvalues to be cached after the compaction. I have to research the
feasibility of this approach.

On Sun, Mar 24, 2013 at 10:15 PM, Liyin Tang <[EMAIL PROTECTED]> wrote:

> Block cache is for uncompressed data while OS page contains the compressed
> data. Unless the request pattern is full-table sequential scan, the block
> cache is still quite useful. I think the size of the block cache should be
> the amont of hot data we want to retain within a compaction cycle, which is
> quite hard to estimate in some use cases.
> Thanks a lot
> Liyin
> ________________________________________
> From: lars hofhansl [[EMAIL PROTECTED]]
> Sent: Saturday, March 23, 2013 10:20 PM
> Subject: Re: Does HBase RegionServer benefit from OS Page Cache
> Interesting.
> > 2) The blocks in the block cache will be naturally invalid quickly after
> the compactions.
> Should one keep the block cache small in order to increase the OS page
> cache?
> Does you data suggest we should not use the block cache at all?
> Thanks.
> -- Lars
> ________________________________
>  From: Liyin Tang <[EMAIL PROTECTED]>
> Sent: Saturday, March 23, 2013 9:44 PM
> Subject: Re: Does HBase RegionServer benefit from OS Page Cache
> We (Facebook) are closely monitoring the OS page cache hit ratio in the
> production environments. My experience is if your data access pattern is
> very random, then the OS page cache won't help you so much even though the
> data locality is very high. On the other hand, if the requests are always
> against the recent data points, then the page cache hit ratio could be much
> higher.
> Actually, there are lots of optimizations could be done in HDFS. For
> example, we are working on fadvice away the 2nd/3rd replicated data from OS
> page cache so that it potentially could improve your OS page cache by 3X.
> Also, by taking advantage of the tiered-based compaction+fadvice in HDFS,
> the region server could keep more hot data in OS page cache based on the
> read access pattern.
> Another separate point is that we probably should NOT reply on the
> memstore/block cache to keep hot data. 1) The more data in the memstore,
> the more data the region server need to recovery from the server failures.
> So the tradeoff is the recovery time. 2) The blocks in the block cache will
> be naturally invalid quickly after the compactions. So region server
> probably won't be benefit from large JVM size at all.
> Thanks a lot
> Liyin
> On Sat, Mar 23, 2013 at 6:13 PM, Ted Yu <[EMAIL PROTECTED]> wrote:
> > Coming up is the following enhancement which would make MSLAB even
> better:
> >
> > HBASE-8163 MemStoreChunkPool: An improvement for JAVA GC when using MSLAB
> >
> > FYI
> >
> > On Sat, Mar 23, 2013 at 5:31 PM, Pankaj Gupta <[EMAIL PROTECTED]
> > >wrote:
> >
> > > Thanks a lot for the explanation. It's good to know that MSlab is
> stable
> > > and safe to enable (we don't have it enable right now, we're using
> 0.92).
> > > This would allow us to more freely allocate memory to HBase. I really
> > > enjoyed the depth of explanation from both Enis and J-D. I was indeed
> > > mistakenly referring to HFile as HLog, fortunately you were still able
> > > understand my question.
> > >
> > > Thanks,
> > > Pankaj
> > > On Mar 21, 2013, at 1:28 PM, Enis Söztutar <[EMAIL PROTECTED]> wrote:
> > >