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From Bill Farner <wfar...@apache.org>
Subject Re: [PROPOSAL] DB snapshotting
Date Mon, 29 Feb 2016 19:41:30 GMT
Thanks for the detailed write up and real-world details!  I generally
support momentum towards a single task store implementation, so +1
on dealing with that.

I anticipated there would be a performance win from straight-to-SQL
snapshots, so I am a +1 on that as well.

In summary, +1 on all fronts!

On Monday, February 29, 2016, Maxim Khutornenko <maxim@apache.org> wrote:

> (Apologies for the wordy problem statement but I feel it's really
> necessary to justify the proposal).
>
> Over the past two weeks we have been battling a nasty scheduler issue
> in production: the scheduler suddenly stops responding to any user
> requests and subsequently gets killed by our health monitoring. Upon
> restart, a leader may only function for a few seconds and almost
> immediately hangs again.
>
> The long and painful investigation pointed towards internal H2 table
> lock contention that resulted in a massive db-write starvation and a
> state where a scheduler write lock would *never* be released. This was
> relatively easy to replicate in Vagrant by creating a large update
> (~4K instances) with a large batch_size (~1K), while bombarding the
> scheduler with getJobUpdateDetails() requests for that job. The
> scheduler would enter a locked up state on the very first write op
> following the update creation (e.g. a status update for an instance
> transition from the first batch) and stay in that state for minutes
> until all getJobUpdateDetails() requests are served. This behavior is
> well explained by the following sentence from [1]:
>
>     "When a lock is released, and multiple connections are waiting for
> it, one of them is picked at random."
>
> What happens here is that in a situation when many more read requests
> are competing for a shared table lock, the H2 PageStore does not help
> write requests requiring an exclusive table lock in any way to
> succeed. This leads to db-write starvation and eventual scheduler
> native store write starvation as there is no timeout on a scheduler
> write lock.
>
> We have played with various available H2/MyBatis configuration
> settings to mitigate the above with no noticeable impact. That, until
> we switched to H2 MVStore [2], at which point we were able to
> completely eliminate the scheduler lockup without making any other
> code changes! So, the solution has finally been found? The answer
> would be YES until you try MVStore-enabled H2 with any reasonable size
> production DB on scheduler restart. There was a reason why we disabled
> MVStore in the scheduler [3] in the first place and that reason was
> poor MVStore performance with bulk inserts. Re-populating
> MVStore-enabled H2 DB took at least 2.5 times longer than normal. This
> is unacceptable in prod where every second of scheduler downtime
> counts.
>
> Back to the drawing board, we tried all relevant settings and
> approaches to speed up MVStore inserts on restart but nothing really
> helped. Finally, the only reasonable way forward was to eliminate the
> point of slowness altogether - namely remove thrift-to-sql migration
> on restart. Fortunately, H2 supports an easy to operate command to
> generate the entire DB dump with a single statement [4]. We were now
> able to bypass the lengthly DB repopulation on restart by storing the
> entire DB dump in snapshot and replaying it on scheduler restart.
>
>
> Now, the proposal. Given that MVStore vastly outperforms PageStore we
> currently use, I suggest we move our H2 to it AND adopt db
> snapshotting instead of thrift snapshotting to speed up scheduler
> restarts. The rough POC is available here [5]. We are running a
> version of this build in production since last week and were able to
> completely eliminate scheduler lockups. As a welcome side effect, we
> also observed faster scheduler restart times due to eliminating
> thrift-to-sql chattiness. Depending on the snapshot freshness the
> observed failover downtimes got reduced by ~40%.
>
> Moving to db snapshotting will require us to rethink DB schema
> versioning and thrift deprecating/removal policy. We will have to move
> to pre-/post- snapshot restore SQL migration scripts to handle any
> schema changes, which is a common industry pattern but something we
> have not tried yet. The upside though is that we can get an early
> start here as we will have to adopt strict SQL migration rules anyway
> when we move to persistent DB storage. Also, given that migrating to
> H2 TaskStore will likely further degrade scheduler restart times,
> having a better performing DB snapshotting solution in place will
> definitely aid migration.
>
> Thanks,
> Maxim
>
> [1] - http://www.h2database.com/html/advanced.html?#transaction_isolation
> [2] - http://www.h2database.com/html/mvstore.html
> [3] -
> https://github.com/apache/aurora/blob/824e396ab80874cfea98ef47829279126838a3b2/src/main/java/org/apache/aurora/scheduler/storage/db/DbModule.java#L119
> [4] - http://www.h2database.com/html/grammar.html#script
> [5] -
> https://github.com/maxim111333/incubator-aurora/blob/mv_store/src/main/java/org/apache/aurora/scheduler/storage/log/SnapshotStoreImpl.java#L317-L370
>

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