According to MVStore docs, data is moved along the MVStore -> MVMap -> Page -> FileStore path. A quick look at the MVStore internals shows the closest possible interception point could the FileStore [1]. Implementing a custom FileStore would require running H2 in persistent (rather than in-memory) mode and our media would have to support random data access for things like "readFully(long pos, int len)" to read arbitrary Pages. This is not what our LevelDB-backed native log is normally capable of.
Another possibility could be a hybrid approach where a custom FileStore would store in-memory cache of everything but that would fully defeat the purpose of what we are trying to accomplish with this change. Thanks, Maxim [1] - http://www.atetric.com/atetric/javadoc/com.h2database/h2/1.3.174/src-html/org/h2/mvstore/FileStore.html On Wed, Mar 2, 2016 at 12:52 PM, Bill Farner <wfar...@apache.org> wrote: > Seems prudent to explore rather than write off though. For all we know it > simplifies a lot. > > On Wednesday, March 2, 2016, Maxim Khutornenko <ma...@apache.org> wrote: > >> Ah, sorry, missed that conversation on IRC. >> >> I have not looked into that. Would be interesting to explore that >> route. Given our ultimate goal is to get rid of the replicated log >> altogether it does not stand as an immediate priority to me though. >> >> On Wed, Mar 2, 2016 at 11:51 AM, Erb, Stephan >> <stephan....@blue-yonder.com <javascript:;>> wrote: >> > +1 for the plan and the ticket. >> > >> > In addition, for reference a couple of messages from IRC from yesterday: >> > >> > 23:42 <serb> mkhutornenko: interesting storage proposal on the >> mailinglist! I only wondered one thing... >> > 23:42 <serb> it feeld kind of weird that we use H2 as a non-replicated >> database and build some scaffolding around it in order to distribute its >> state via the Mesos replicated log. >> > 23:42 <serb> Have you looked into H2, if it would be possible to >> replace/subclass their in-process transaction log with a replicated Mesos >> one? >> > 23:43 <serb> Then we would not need that logic that performs a >> simultaneous inserts into the log and the taskstore, as the backend would >> handle that by itself >> > 23:44 <serb> (I know close to nothing about the storage layer, so that's >> like my perspective from 10.000 feet) >> > >> > 00:22 <wfarner> serb: that crossed my mind as well. I have only drilled >> in a bit, would love to more >> > >> > ________________________________________ >> > From: Maxim Khutornenko <ma...@apache.org <javascript:;>> >> > Sent: Wednesday, March 2, 2016 18:18 >> > To: dev@aurora.apache.org <javascript:;> >> > Subject: Re: [PROPOSAL] DB snapshotting >> > >> > Thanks Bill! Filed https://issues.apache.org/jira/browse/AURORA-1627 >> > to track it. >> > >> > On Mon, Feb 29, 2016 at 11:41 AM, Bill Farner <wfar...@apache.org >> <javascript:;>> wrote: >> >> 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 <ma...@apache.org >> <javascript:;>> 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 >> >>> >>
