[GitHub] eolivelli commented on a change in pull request #1944: BP-38: Bypass journal ledger

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eolivelli commented on a change in pull request #1944: BP-38: Bypass journal ledger
URL: https://github.com/apache/bookkeeper/pull/1944#discussion_r255841970
 
 

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 File path: site/bps/BP-38-bypass-journal-ledger.md
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+---
+title: "BP-38: bypass journal ledger"
+issue: https://github.com/apache/bookkeeper/issues/1918
+state: "Under Discussion"
+release: "N/A"
+---
+
+### Motivation
+
+To guarantee high durability, BK write journal before flush data to persistent device which will cause two write of data.
+But we may not need this level of persistence under [scenarios](https://cwiki.apache.org/confluence/display/BOOKKEEPER/BP-14+Relax+durability) which prefer weak durability guarantee.
+This proposal is aimed at providing bypass journal ledger, this feature includes these parts work:
+ - add new write flag `BYPASS_JOURNAL` to existing protocol
+ - impl the newly write flag at the client side and server side
+ 
+The details of changes are listed in Section “[Proposed Changes](#proposed-changes)”.
+ 
+### Public Interfaces
+
+This feature will introduce new 'WRITE_FLAG' to enable single write storage impl.
+Like `DEFERRED_SYNC`, when we construct `WriteHandle`, we can pass `BYPASS_JOURNAL`, then every `AddRequest` will carry this flag.
+As for monitoring, we can add metrics to record the main time interval during the io path.
+The detailed compatibility related stuff is in Section “[CDMP](#compatibility-deprecation-and-migration-plan)”.
+
+### Proposed Changes
+
+While the main approach is based on [`WRITE_FLAG` impl](https://github.com/apache/bookkeeper/pull/742),
+the actual implementation is more tricky. In my opinion, there are three different solution:
+
+1. Relax LAC protocol
+
+    Modify server side code mostly and don't change legerHandle's LAC advance logic, if the write flag is `BYPASS_JOURNAL`, after write to `LegerStorage`(the data maybe in the memTable, or the buffer of File, or the os cache),
 
 Review comment:
   > don't change legerHandle's LAC advance logic
   
   I think we should use the DEFERRED_SYNC way of handling LAC, that is that a regular write will not advance LAC, but you need a "force" (like you state below).
   

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