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https://issues.apache.org/jira/browse/BOOKKEEPER-432?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13488290#comment-13488290
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Yixue (Andrew) Zhu commented on BOOKKEEPER-432:
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Only the leave level of btree are data blocks (for bookkeeper usage, we
probably ends up just one level of index block, if sparse index is used).
All data blocks and index blocks are stored on disk.
The Bookie's ReadEntryProcessor uses LedgerStorage::getEntry to retrieve
individual entries. The LRU block cache should help next calls from Hedwig to
read more entries.
The per-ledger index file just holds information which btree-file the entries
are stored. We could just look up the b-tree using its index block and data
blocks, which are all LRU cached.
To speed up entry look up, we can maintain in-memory-only per-ledger index
cache, which retrieve information from b-tree on demand, with each retrieval
caches starting offset of a batch of entries.
Once this is in place, we can enhance HedWig to batch read entries from
Bookies, which can handle it well using the design.
> 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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