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https://issues.apache.org/jira/browse/BOOKKEEPER-432?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13488131#comment-13488131
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Yixue (Andrew) Zhu commented on BOOKKEEPER-432:
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The B-Tree is block based, and are consecutively stored in the (ledger-id,
entry-id) order. The index blocks are at tail of the file, will not interfere
with read-ahead.
If you mean Ledger Log by write-ahead-log, yes, we will not read them unless
during recovery. I am not proposing caching it.
By entry-log, I mean entry data (messages), which is stored in data blocks (aka
pages, chunks). These data blocks can be cached on demand, using LRU
replacement policy.
We will keep per-ledger offsets in memory, which is cached on demand from
B-tree, to speed up locating the first entry, followed by chunk read from
consecutive data blocks.
The idea of using sparse index is that we are tuning for bulk read.
We could put dense index blocks in the B-tree if performance measurement show
otherwise.
> 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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