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https://issues.apache.org/jira/browse/AMQ-7080?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16760407#comment-16760407
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Alan Protasio commented on AMQ-7080:
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Hi [~gtully]
Have you had a change to look the change and the perf numbers? I can put this
behind a feature flag if you think make more sense (enablePageMapFile or
something like that). :D
Thanks! :D
> Keep track of free pages - Update db.free file during checkpoints
> -----------------------------------------------------------------
>
> Key: AMQ-7080
> URL: https://issues.apache.org/jira/browse/AMQ-7080
> Project: ActiveMQ
> Issue Type: Improvement
> Components: KahaDB
> Affects Versions: 5.15.6
> Reporter: Alan Protasio
> Assignee: Jean-Baptiste Onofré
> Priority: Major
> Fix For: 5.16.0
>
> Attachments: AMQ-7080-freeList-update.diff
>
>
> In a event of an unclean shutdown, Activemq loses the information about the
> free pages in the index. In order to recover this information, ActiveMQ read
> the whole index during shutdown searching for free pages and then save the
> db.free file. This operation can take a long time, making the failover
> slower. (during the shutdown, activemq will still hold the lock).
> From http://activemq.apache.org/shared-file-system-master-slave.html
> {quote}"If you have a SAN or shared file system it can be used to provide
> high availability such that if a broker is killed, another broker can take
> over immediately."
> {quote}
> Is important to note if the shutdown takes more than ACTIVEMQ_KILL_MAXSECONDS
> seconds, any following shutdown will be unclean. This broker will stay in
> this state unless the index is deleted (this state means that every failover
> will take more then ACTIVEMQ_KILL_MAXSECONDS, so, if you increase this time
> to 5 minutes, you fail over can take more than 5 minutes).
>
> In order to prevent ActiveMQ reading the whole index file to search for free
> pages, we can keep track of those on every Checkpoint. In order to do that we
> need to be sure that db.data and db.free are in sync. To achieve that we can
> have a attribute in the db.free page that is referenced by the db.data.
> So during the checkpoint we have:
> 1 - Save db.free and give a freePageUniqueId
> 2 - Save this freePageUniqueId in the db.data (metadata)
> In a crash, we can see if the db.data has the same freePageUniqueId as the
> db.free. If this is the case we can safely use the free page information
> contained in the db.free
> Now, the only way to read the whole index file again is IF the crash happens
> btw step 1 and 2 (what is very unlikely).
> The drawback of this implementation is that we will have to save db.free
> during the checkpoint, what can possibly increase the checkpoint time.
> Is also important to note that we CAN (and should) have stale data in db.free
> as it is referencing stale db.data:
> Imagine the timeline:
> T0 -> P1, P2 and P3 are free.
> T1 -> Checkpoint
> T2 -> P1 got occupied.
> T3 -> Crash
> In the current scenario after the Pagefile#load the P1 will be free and then
> the replay will mark P1 as occupied or will occupied another page (now that
> the recovery of free pages is done on shutdown)
> This change only make sure that db.data and db.free are in sync and showing
> the reality in T1 (checkpoint), If they are in sync we can trust the db.free.
> This is a really fast draft of what i'm suggesting... If you guys agree, i
> can create the proper patch after:
> [https://github.com/alanprot/activemq/commit/18036ef7214ef0eaa25c8650f40644dd8b4632a5]
>
> This is related to https://issues.apache.org/jira/browse/AMQ-6590
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