Rakesh R commented on ZOOKEEPER-2574:

Should we update the [zk 
 part, {code}As changes are made to the znodes these changes are appended to a 
transaction log, occasionally, when a log grows large, a snapshot of the 
current state of all znodes will be written to the filesystem. This snapshot 
supercedes all previous logs{code} It looks like conveying wrong behavior, 

> PurgeTxnLog can inadvertently delete required txn log files
> -----------------------------------------------------------
>                 Key: ZOOKEEPER-2574
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2574
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: server
>    Affects Versions: 3.4.7, 3.4.8, 3.5.0, 3.5.1, 3.5.2
>         Environment: Zookeeper 3.4.8, standalone, and 3-server quorum
>            Reporter: Abhishek Rai
>            Assignee: Abhishek Rai
>            Priority: Blocker
>             Fix For: 3.4.10, 3.5.3
>         Attachments: ZOOKEEPER-2574.2.patch, ZOOKEEPER-2574.3.patch, 
> ZOOKEEPER-2574.4.patch, ZOOKEEPER-2574.5.patch, ZOOKEEPER-2574.patch
> As part of the fix for ZOOKEEPER-1797, the call to 
> FileTxnSnapLog.getSnapshotLogs() was removed from PurgeTxnLog.java.  As a 
> result, some old-looking but required txn log files can be deleted, resulting 
> in data corruption or loss.
> For example, consider the following:
> 1. Configuration:
> autopurge.snapRetainCount=3
> 2. Following files exist:
> log.100 spans transactions from zxid=100 till zxid=140 (inclusive)
> snapshot.110 - snapshot as of zxid=110
> snapshot.120 - snapshot as of zxid=120
> snapshot.130 - snapshot as of zxid=130
> Above scenario is possible when snapshotting has happened multiple times but 
> without accompanying log rollover, which is possible if the server was 
> running as a learner.
> 3. PurgeTxnLog retains all snapshots but deletes log.100 because its zxid is 
> older than the zxid of the oldest snapshot (110).  This results in loss of 
> transactions in the range 131-140.
> Before the fix for ZOOKEEPER-1797, this was avoided by the call to 
> FileTxnSnapLog.getSnapshotLogs() which finds and retains the newest txn log 
> file with starting zxid < oldest retained snapshot's highest zxid.

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