[jira] [Commented] (CASSANDRA-13987) Multithreaded commitlog subtly changed durability

["Jason Brown (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-13987?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16235703#comment-16235703 ] 

Jason Brown commented on CASSANDRA-13987:
-----------------------------------------

Here is a branch that takes the simplest path: it updates the commit log chained markers (in periodic mode) much more quickly than it mysnc's.

||trunk||
|[branch|https://github.com/jasobrown/cassandra/tree/commitlog_mmap-more-frequent-markers]|
|[utests|https://circleci.com/gh/jasobrown/cassandra/tree/commitlog_mmap-more-frequent-markers]|
|[dtest|https://builds.apache.org/view/A-D/view/Cassandra/job/Cassandra-devbranch-dtest/407/]|

The basic idea is that if we can update the chained markers (say, (a configurable) once every 100 milliseconds), that should probably be more than enough time to survive a correlated failure between two nodes about OOM. This is *not* a silver bullet to ensure complete replayability, as in that case you should use batch commit log mode for each commit to ensure durability. There are alternatives that I discussed with others (see next comment).

This branch does not solve the problem for compressed/encrypted commitlog (in periodic mode), as those implementations do not use mmaped files. I am not sure how best (or if) to address those. Switching them to use a memory mapped file might not be too difficult, code-wise, but i'm not sure about performance implications. Apparently, [~benedict] and I had some discussion about the use of mmap and the commitlog a loooong time ago (CASSANDRA-6809), but I honestly can't remember the details beyond our comments on that ticket.

> Multithreaded commitlog subtly changed durability
> -------------------------------------------------
>
>                 Key: CASSANDRA-13987
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13987
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Jason Brown
>            Assignee: Jason Brown
>            Priority: Major
>             Fix For: 4.x
>
>
> When multithreaded commitlog was introduced in CASSANDRA-3578, we subtly changed the way that commitlog durability worked. Everything still gets written to an mmap file. However, not everything is replayable from the mmaped file after a process crash, in periodic mode.
> In brief, the reason this changesd is due to the chained markers that are required for the multithreaded commit log. At each msync, we wait for outstanding mutations to serialize into the commitlog, and update a marker before and after the commits that have accumluated since the last sync. With those markers, we can safely replay that section of the commitlog. Without the markers, we have no guarantee that the commits in that section were successfully written, thus we abandon those commits on replay.
> If you have correlated process failures of multiple nodes at "nearly" the same time (see ["There Is No Now"|http://queue.acm.org/detail.cfm?id=2745385]), it is possible to have data loss if none of the nodes msync the commitlog. For example, with RF=3, if quorum write succeeds on two nodes (and we acknowledge the write back to the client), and then the process on both nodes OOMs (say, due to reading the index for a 100GB partition), the write will be lost if neither process msync'ed the commitlog. More exactly, the commitlog cannot be fully replayed. The reason why this data is silently lost is due to the chained markers that were introduced with CASSANDRA-3578.
> The problem we are addressing with this ticket is incrementally improving 'durability' due to process crash, not host crash. (Note: operators should use batch mode to ensure greater durability, but batch mode in it's current implementation is a) borked, and b) will burn through, *very* rapidly, SSDs that don't have a non-volatile write cache sitting in front.) 
> The current default for {{commitlog_sync_period_in_ms}} is 10 seconds, which means that a node could lose up to ten seconds of data due to process crash. The unfortunate thing is that the data is still avaialble, in the mmap file, but we can't replay it due to incomplete chained markers.
> ftr, I don't believe we've ever had a stated policy about commitlog durability wrt process crash. Pre-2.0 we naturally piggy-backed off the memory mapped file and the fact that every mutation was acquired a lock and wrote into the mmap buffer, and the ability to replay everything out of it came for free. With CASSANDRA-3578, that was subtly changed. 
> Something [~jjirsa] pointed out to me is that [MySQL provides a way to adjust the durability guarantees|https://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html#sysvar_innodb_flush_log_at_trx_commit] of each commit in innodb via the {{innodb_flush_log_at_trx_commit}}. I'm using that idea as a loose springboard for what to do here.



--
This message was sent by Atlassian JIRA
(v6.4.14#64029)

---------------------------------------------------------------------
To unsubscribe, e-mail: commits-unsubscribe@cassandra.apache.org
For additional commands, e-mail: commits-help@cassandra.apache.org