[Hadoop-common-user] Reading Records from a Sequence File

The dfs.block.size part is partially right when applied in MapReduce
(actually, it would look for sync points for read start and read end).
And no, the reader does not load the entire data in the memory in
one-go. It buffers and reads off the stream just like any other
reader.

Could we have some more information on what your java program does,
and what exactly you are measuring? :)

Hi Harsh,

My job is for a Similarity Search application. But, my aim for now is to measure the IO overhead if my mapper.map() opened a sequence file and started to read it record by record with:

SequenceFile.Reader.next(key,value);

I want to make sure that "next" here is IO efficient. Otherwise, I will need to write it myself to be block read then parsed in my program using the "sync" hints.


So, what you meant in another words is that the reader will buffer couple of records (the ones between two sync(s)) into memory then use "next" to read from memory .. right? if yes, what parameter is used for the buffer size?

Thank you,
Maha

Hello,
You can have a look at SequenceFile.Reader class's source code perhaps
- it should clear out all doubts you're having?
Records are not loaded into the memory. Records are read using
key/value size informations off the buffered input stream.

You can specify a buffer size while constructing a Reader object for
SequenceFiles, or the "io.file.buffer.size" value is used as a
default.