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Re: Hbase performance with HDFS
Thanks! I understand what you mean however I have little confusion.
Does it mean there are unused block sitting around? For eg:

HFile1 with 3 blocks spread accross 3 nodes Node A:(b1),b2,b3 Node
B:b1,(b2),b3 and Node C:b1,b2,(b3).

HFile2 with 3 blocks spread accross 3 nodes Node A:(b1),b2,b3 Node
B:b1,(b2),b3 and Node C:b1,b2,(b3)

I have 2 questions:

1) When compactions occur on Node A would it also include b2 and b3
which is actually a redundant copy? My guess is yes.
2) Now compaction occurs and creates HFile3 which as you said is
replicated. But what happens to HFile1 and HFile2? I am assuming it
gets deleted.

Thanks for everyones patience!

On Thu, Jul 7, 2011 at 2:43 PM, Buttler, David <[EMAIL PROTECTED]> wrote:
> The nice part of using HDFS as the file system is that the replication is taken care of by the file system.  So, when the compaction finishes, that means the replication has already taken place.
> -----Original Message-----
> From: Mohit Anchlia [mailto:[EMAIL PROTECTED]]
> Sent: Thursday, July 07, 2011 2:02 PM
> To: [EMAIL PROTECTED]; Andrew Purtell
> Subject: Re: Hbase performance with HDFS
> Thanks Andrew. Really helpful. I think I have one more question right
> now :) Underneath HDFS replicates blocks by default 3. Not sure how it
> relates to HFile and compactions. When compaction occurs is it also
> happening on the replica blocks from other nodes? If not then how does
> it work when one node fails.
> On Thu, Jul 7, 2011 at 1:53 PM, Andrew Purtell <[EMAIL PROTECTED]> wrote:
>>> You mentioned about compactions, when do those occur and what triggers
>>> them?
>> Compactions are triggered by an algorithm that monitors the number of flush files in a store and the size of them, and is configurable in several dimensions.
>>> Does it cause additional space usage when that happens
>> Yes.
>>> if it
>>> does it would mean you always need to have much more disk then you
>>> really need.
>> Not all regions are compacted at once. Each region by default is constrained to 256 MB. Not all regions will hold the full amount of data. The result is not a perfect copy (doubling) if some data has been deleted or are associated with TTLs that have expired. The merge sorted result is moved into place and the old files are deleted as soon as the compaction completes. So how much more is "much more"? You can't write to any kind of data store on a (nearly) full volume anyway, no matter HBase/HDFS, or MySQL, or...
>>> Since HDFS is mostly write once how are updates/deletes handled?
>> Not mostly, only write once.
>> From the BigTable paper, section 5.3: "A valid read operation is executed on a merged view of the sequence of SSTables and the memtable. Since the SSTables and the memtable are lexicographically sorted data structures, the merged view can be formed efficiently." So what this means is all the store files and the memstore serve effectively as change logs sorted in reverse chronological order.
>> Deletes are just another write, but one that writes tombstones "covering" data with older timestamps.
>> When serving queries, HBase searches store files back in time until it finds data at the coordinates requested or a tombstone.
>> The process of compaction not only merge sorts a bunch of accumulated store files (from flushes) into fewer store files (or one) for read efficiency, it also performs housekeeping, dropping data "covered" by the delete tombstones. Incidentally this is also how TTLs are supported: expired values are dropped as well.
>> Best regards,
>>    - Andy
>> Problems worthy of attack prove their worth by hitting back. - Piet Hein (via Tom White)
>>>From: Mohit Anchlia <[EMAIL PROTECTED]>
>>>To: Andrew Purtell <[EMAIL PROTECTED]>
>>>Sent: Thursday, July 7, 2011 12:30 PM
>>>Subject: Re: Hbase performance with HDFS
>>>Thanks that helps! Just few more questions: