Sahil Takiar created HADOOP-16241:
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Summary: S3AInputStream PositionReadable should perform ranged
read on dedicated stream
Key: HADOOP-16241
URL: https://issues.apache.org/jira/browse/HADOOP-16241
Project: Hadoop Common
Issue Type: Improvement
Components: fs/s3
Reporter: Sahil Takiar
Assignee: Sahil Takiar
The current implementation of {{PositionReadable}} in {{S3AInputStream}} is
pretty close to the default implementation in {{FsInputStream}}.
This JIRA proposes overriding the {{read(long position, byte[] buffer, int
offset, int length)}} method and re-implementing the {{readFully(long position,
byte[] buffer, int offset, int length)}} method in S3A.
The new implementation would perform a "ranged read" on a dedicated object
stream (rather than the shared one). Prototypes have shown this to bring a
considerable performance improvement to readers who are only interested in
reading a random chunk of the file at a time (e.g. Impala, although I would
assume HBase would benefit from this as well).
Setting {{fs.s3a.experimental.input.fadvise}} to {{RANDOM}} is helpful for
clients that rely on pread, but has a few drawbacks:
* Unless the client explicitly sets fadvise to RANDOM, they will get at least
one connection reset when the backwards seek is issued (after which fadvise
automatically switches to RANDOM)
* Data is only read in 64 kb chunks, so for a large read, several GET requests
must be issued to S3 to fetch the data; while the 64 kb chunk value is
configurable, it is hard to set a reasonable value for variable length preads
* If the readahead value is too big, closing the input stream can take
considerable time because the stream has to be drained of data before it can be
closed
The new implementation of {{PositionReadable}} would issue a
{{GetObjectRequest}} with the range specified by {{position}} and the size of
the given buffer. The data would be read from the {{S3ObjectInputStream}} and
then closed at the end of the method. This stream would be independent of the
{{wrappedStream}} currently maintained by S3A.
This brings the following benefits:
* The {{PositionedReadable}} methods can be thread-safe without a
{{synchronized}} block, which allows clients to concurrently call pread methods
on the same {{S3AInputStream}} instance
* preads will request all the data at once rather than requesting it in chunks
via the readahead logic
* Avoids performing potentially expensive seeks when performing preads
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