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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-17910) Use separated StoreFileReader for streaming read
Date Thu, 20 Apr 2017 03:03:04 GMT

    [ https://issues.apache.org/jira/browse/HBASE-17910?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15975963#comment-15975963

stack commented on HBASE-17910:

bq. I found a problem that when opening an HFileReader, we will read a lot of data such as
trailer, index, and so on.

This stuff is not in cache ?  Maybe not the trailer but the index... Could we add trailer
etc. to cache to save on these reads?

> Use separated StoreFileReader for streaming read
> ------------------------------------------------
>                 Key: HBASE-17910
>                 URL: https://issues.apache.org/jira/browse/HBASE-17910
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Duo Zhang
> For now we have already supportted using private readers for compaction, by creating
a new StoreFile copy. I think a better way is to allow creating multiple readers from a single
StoreFile instance, thus we can avoid the ugly cloning, and the reader can also be used for
streaming scan, not only for compaction.
> The reason we want to do this is that, we found a read amplification when using short
circult read. {{BlockReaderLocal}} will use an internal buffer to read data first, the buffer
size is based on the configured buffer size and the readahead option in CachingStrategy. For
normal pread request, we should just bypass the buffer, this can be achieved by setting readahead
to 0. But for streaming read I think the buffer is somehow still useful? So we need to use
different FSDataInputStream for pread and streaming read.
> And one more thing is that, we can also remove the streamLock if streaming read always
use its own reader.

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