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From "Rakesh R (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2574) PurgeTxnLog can inadvertently delete required txn log files
Date Mon, 07 Nov 2016 05:29:59 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2574?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15643171#comment-15643171

Rakesh R commented on ZOOKEEPER-2574:

Thank you [~abhishekrai]. I could see there are few more corrections required in the [zookeeperAdmin.html#The+Log+Directory|https://zookeeper.apache.org/doc/r3.4.9/zookeeperAdmin.html#The+Log+Directory],
right?. I'd suggest you to read the {{ZooKeeper transaction log and snapshot}} related sections
in the ZK docs and do necessary changes. Appreciate your time & efforts.

The Log Directory:

A new log file is started each time a snapshot is begun. The log file's suffix is the first
zxid written to that log

Could you please create a pull request for the proposed patch, that will be used for code
reviews and commits.

Hi [~fpj], [~phunt], [~rgs]. As part of this jira we have came across a situation {{"where
snapshotting has happened multiple times without accompanying log rollover"}}, this is contradicting
with the ZooKeeper docs. I think, this would be a serious concern in the view of disaster
recovery scripts. If someone has written a script blindly following that "A new log file is
started each time a snapshot is begun". It would be really helpful if you could pitch in and
give your thoughts on this problem. Thanks!

> 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-2574.6.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
> 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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