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https://issues.apache.org/jira/browse/HBASE-12295?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14584222#comment-14584222
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stack commented on HBASE-12295:
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I had a nice chat with [~mbertozzi] on how to figure when we can let go of
offheap bucketcache blocks.
Would the following work?
Add a {{done}} or {{release}} method on serverside Cells. It is called when the
Cell has been used up; i.e. the Cell has been dumped out on the RPC. It is a
noop unless Cell is an instance of RefCountedDBBBackedCell. In this case, we
decrement our refcounter. How we get to our refcounter you say? Well, a
RefCountedDBBBackedCell is backed by a byte array. Let the refcounter be at
location zero in the backing byte array (In the BucketCache case, this would be
offset zero in a bucket I believe -- haven't checked). Changing refcount would
have to be threadsafe so synchronize on the backing DBB doing any access would
be a coarse first cut means of doing this. When we make Cells -- in
HFileScannerImpl -- we'd have to ask the HFileBlock, what type it was... and if
a refcounting block, we'd make RefCountedDBBBackedCell and up the refcount per
one made (could try and do bit increment/decrement so don't have to parse as an
int each time we want to increment).
> Prevent block eviction under us if reads are in progress from the BBs
> ---------------------------------------------------------------------
>
> Key: HBASE-12295
> URL: https://issues.apache.org/jira/browse/HBASE-12295
> Project: HBase
> Issue Type: Sub-task
> Components: regionserver, Scanners
> Reporter: ramkrishna.s.vasudevan
> Assignee: ramkrishna.s.vasudevan
> Fix For: 2.0.0
>
> Attachments: HBASE-12295.pdf, HBASE-12295_1.patch, HBASE-12295_1.pdf,
> HBASE-12295_2.patch, HBASE-12295_4.patch, HBASE-12295_trunk.patch
>
>
> While we try to serve the reads from the BBs directly from the block cache,
> we need to ensure that the blocks does not get evicted under us while
> reading. This JIRA is to discuss and implement a strategy for the same.
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