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From "Liu, Ming (HPIT-GADSC)" <ming.l...@hp.com>
Subject RE: how to explain read/write performance change after modifying the hfile.block.cache.size?
Date Fri, 21 Nov 2014 05:35:42 GMT
Thank you Ted,
It is a great explanation. You are always very helpful ^_^
I will study the link carefully.

Thanks,
Ming

-----Original Message-----
From: Ted Yu [mailto:yuzhihong@gmail.com] 
Sent: Friday, November 21, 2014 1:32 AM
To: user@hbase.apache.org
Subject: Re: how to explain read/write performance change after modifying the hfile.block.cache.size?

When block cache size increases from 0 to 0.4, the amount of heap given to memstore decreases.
This would slow down the writes.
Please see:
http://hbase.apache.org/book.html#store.memstore

For your second question, see this thread:
http://search-hadoop.com/m/DHED4TEvBy1/lars+hbase+hflush&subj=Re+Clarifications+on+HBase+Durability

Cheers

On Thu, Nov 20, 2014 at 8:05 AM, Liu, Ming (HPIT-GADSC) <ming.liu2@hp.com>
wrote:

> Hello, all,
>
> I am playing with YCSB to test HBase performance. I am using HBase 0.98.5.
> I tried to adjust the hfile.block.cache.size to see the difference, 
> when I set hfile.block.cache.size to 0, read performance is very bad, 
> but write performance is very very very good....; when I set 
> hfile.block.cache.size to 0.4, read is better, but write performance 
> drop dramatically. I disable the client side writebuffer already.
> This is hard to understand for me:
> The HBase guide just said hfile.block.cache.size setting is about how 
> much memory used as block cache used by StoreFile. I have no idea of 
> how HBase works internally. Typically, it is easy to understand that 
> increase the size of cache should help the read, but why it will harm 
> the write operation? The write performance down from 30,000 to 4,000 
> for your reference, just by changing the hfile.block.cache.size from 0 to 0.4.
> Could anyone give me a brief explanation about this observation or 
> give me some advices about what to study to understand what is block cache used for?
>
> Another question: HBase write will first write to WAL then to memstore.
> Will the write to WAL go to disk directly before hbase write memstore, 
> a sync operation or it is possible that write to WAL is still buffered 
> somewhere when hbase put the data into the memstore?
>
> Reading src code may cost me months, so a kindly reply will help me a 
> lot... ...
> Thanks very much!
>
> Best Regards,
> Ming
>
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