[jira] [Commented] (CASSANDRA-1602) Commit Log archivation and rolling forward utility (AKA Retaining commit logs)

["Jonathan Ellis (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-1602?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13051432#comment-13051432 ] 

Jonathan Ellis commented on CASSANDRA-1602:
-------------------------------------------

Thanks for the update Tamara.  It looks good for the most part.  A few minor points:

- our modern hard link creation utility is CLibrary.createHardLink
- I think Oleg is right: we want to create the link when a segment is complete, not when it's created
- indentation standard is four spaces (http://wiki.apache.org/cassandra/CodeStyle)

> Commit Log archivation and rolling forward utility (AKA Retaining commit logs)
> ------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-1602
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1602
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Core, Tools
>            Reporter: Oleg Anastasyev
>         Attachments: 1602-0.6.4.txt, 1602-cassandra0.6.txt, 1602-v2.txt, 1602-v3.txt
>
>
> As couple of people from mailing list suggested me to share this patch (see discussion at http://comments.gmane.org/gmane.comp.db.cassandra.user/9423 ) to retain (archive) commit logs generated by cassandra and  restore data by rolling forward of commit logs to previously backed up or snapshotted data files.
> Here is an instruction of how to use it, which i extracted from out internal wiki:
> We rely on cassandra replication factor for disaster recovery.
> But there is another problem: there are bugs in data manipulation logic, which can lead to data destruction on the whole cluster. But the freshest backup to restore from is last snapshot, which can be up to 24h as old.
> To fight with it, we decided to implement snapshot + log archive  backup strategy - i.e. we collect commit logs and snapshotted data files. On event of data loss, either due hardware failure or logical bug, we restore last snapshot and roll forward all logs, collected since last snapshot time to it.
> Originally cassandra does not support log archive , so I implemented it by myself.
> The idea is simple:
> # As soon as commit log file is not needed anymore by cassadra, a hardlink (unix command "ln $1 $2") is created from just closed commit log file to commit log archive directory. Both commit log and commit log archive are on the same volume, of course.
> # Some script (which authoring i left to admin) then takes files from commit log archive dir and copies them over net to a backup location. As soon as file is transferred, it can be safely deleted from commit log archive dir.
> # Dont forget there must be some script (also authored by admins), which do data snapshots from time to time using "nodetool snapshot" command, available from standard cassandra distribution and copies snapshot files to backup location.
> ## Creating a snapshot is very light operation for cassandra - under the hood it is just hardlinking currently existing files to "snapshot/<timestamp-millis>" directory. So frequence is up to our ability to pull snapshotted data files over network.
> # To restore data, admin must:
> ## stop cassandra instance
> ## remove all corrupted data files from /data directory. Leave only commit logs you want to roll forward in /commitlog dir
> ## copy to /data last snapshot data files
> ## copy to /commitlog all archived commit log files (only files not older than snapshot data files could be copied. copying too old will do no harm, but will extend processing time)
> ## *not starting* cassandra instance, run roll forward utility (bin/logreplay) with option -forced or -forcedcompaction and wait for its completion
> ## then start cassandra node instance as usual.
> Log archive&nbsp; logic is activated using CommitLogArchive directive in storage-conf.xml:
> {code:xml}
> <CommitLogDirectory>/commitlog</CommitLogDirectory>
> <CommitLogArchive>true</CommitLogArchive>
> <DataFileDirectories>
> <DataFileDirectory>/data</DataFileDirectory>
> </DataFileDirectories>
> {code}Log files will be archived to <commit-dir>/.archive directory, i.e. as shown in example above, to /commitlog/.archive directory.
> Archived logs replay process is launched by running org.apache.cassandra.tools.ReplayLogs with option "-forced" locally on node (use option "-forcedcompact" to do major compaction right after log roll forward process completion). I also made a script named <cassandra-dir>/bin/logreplay.

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