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https://issues.apache.org/jira/browse/BOOKKEEPER-432?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13488013#comment-13488013
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Ivan Kelly commented on BOOKKEEPER-432:
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Ah, got it. The skiplist is inmemory, preflush. The btree is on disk post
flush. The skiplist makes sense to me, the btree not so much. Trees are
O(logn). If the means logn seeks, then the approach aleady loses against an
approach that flushes the skiplist directly, and stores the per ledger offsets
in the index, which would be O(1).
Also, I don't think caching full blocks makes much sense for BKs usecases. BK
provides a writeahead log, which means, in the normal case, the average number
of times a block will be read is 0. Rarely, it will be read once. It would be
extremely rare that it would be read more than once.
> Improve performance of entry log range read per ledger entries
> ---------------------------------------------------------------
>
> Key: BOOKKEEPER-432
> URL: https://issues.apache.org/jira/browse/BOOKKEEPER-432
> Project: Bookkeeper
> Issue Type: Improvement
> Components: bookkeeper-server
> Affects Versions: 4.2.0
> Environment: Linux
> Reporter: Yixue (Andrew) Zhu
> Assignee: Yixue (Andrew) Zhu
> Labels: patch
> Attachments: BookieLedgerStorageProposal.pdf
>
>
> We observed random I/O reads when some subscribers fall behind (on some
> topics), as delivery needs to scan the entry logs (thru ledger index), which
> are interleaved with ledger entries across all ledgers being served.
> Essentially, the ledger index is a non-clustered index. It is not effective
> when a large number of ledger entries need to be served, which tend to be
> scattered around due to interleaving.
> Some possible improvements:
> 1. Change the ledger entries buffer to use a SkipList (or other suitable),
> sorted on (ledger, entry sequence). When the buffer is flushed, the entry log
> is written out in the already-sorted order.
> The "active" ledger index can point to the entries buffer (SkipList), and
> fixed up with entry-log position once latter is persisted.
> Or, the ledger index can be just rebuilt on demand. The entry log file tail
> can have index attached (light-weight b-tree, similar with big-table). We
> need to track per ledger which log files contribute entries to it, so that
> in-memory index can be rebuilt from the tails of corresponding log files.
> 2. Use affinity concept to make ensembles of ledgers (belonging to same
> topic) as identical as possible. This will help above 1. be more effective.
>
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