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https://issues.apache.org/jira/browse/HADOOP-11867?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14507553#comment-14507553
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Steve Loughran commented on HADOOP-11867:
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It could be good to combine with an {{openAt(Path, offset)}} operation, to
specify the initial offset for a read. Otherwise you see traces (Especially
with object stores) of open (one HTTP GET) followed by an immediate seek() and
a second GET.
# what happens if things go wrong?
# consistency & ordering guarantees must be "none": read ordering is based on
decisions by FS & OS; changes in the file may or may not be visible *during the
operation*
# any attempt to read past the EOF will trigger an EOF exception. This MAY
happen during the operation, rather than checked up front.
# is it an error if I ask for overlapping ranges?
> FS API: Add a high-performance vectored Read to FSDataInputStream API
> ---------------------------------------------------------------------
>
> Key: HADOOP-11867
> URL: https://issues.apache.org/jira/browse/HADOOP-11867
> Project: Hadoop Common
> Issue Type: New Feature
> Affects Versions: 2.8.0
> Reporter: Gopal V
>
> The most significant way to read from a filesystem in an efficient way is to
> let the FileSystem implementation handle the seek behaviour underneath the
> API to be the most efficient as possible.
> A better approach to the seek problem is to provide a sequence of read
> locations as part of a single call, while letting the system schedule/plan
> the reads ahead of time.
> This is exceedingly useful for seek-heavy readers on HDFS, since this allows
> for potentially optimizing away the seek-gaps within the FSDataInputStream
> implementation.
> For seek+read systems with even more latency than locally-attached disks,
> something like a {{readFully(long[] offsets, ByteBuffer[] chunks)}} would
> take of the seeks internally while reading chunk.remaining() bytes into each
> chunk (which may be {{slice()}}ed off a bigger buffer).
> The base implementation can stub in this as a sequence of seeks + read() into
> ByteBuffers, without forcing each FS implementation to override this in any
> way.
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