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https://issues.apache.org/jira/browse/AMQ-7080?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16760790#comment-16760790
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Alan Protasio edited comment on AMQ-7080 at 2/5/19 1:42 PM:
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I see... I did this way to keep it simple with the minimum performance impact.
What is your suggestion? I see some possibilities:
* create a HashIndex<int, boolean>.
* update the bit map at the end of the pages (end of the index file)
* write the bit map (or the sequenceSet) in pages (with overflow)
The first solution we will write way more bytes to keep track of the same
information. And also, we will have to read all those bytes during startup.
(key = pageFile and value = true if page is free)
The second solution, everything that we allocate a new page we will have to
write the whole map again.
The third one, we will have to always write the whole bitmap (or sequenceSet)
into pages. I thought that this could cause more performance degradation than
the proposed solution. Remember that with the proposed solution only the bytes
that changed are updated.
The last option that I can think of is create a sequenceset index. This can be
a little better than the first but not a lot in a fragmented pagefile.
Do you have any other idea [~gtully]?
was (Author: alanprot):
I see... I did this way to keep it simple with the minimum performance impact.
What is your suggestion? I see some possibilities:
* create a listindex<int>.
* update the map bit at the end of the pages (end of the index file)
The first solution we will write way more bytes to keep track of the same
information. And also, we will have to read all those bytes during startup.
The second solution, everything that we allocate a new page we will have to
write the whole map again.
The last option that I can think of is create a sequenceset index. This can be
a little better than the first but not a lot in a fragmented pagefile.
Do you have any other idea [~gtully]?
> 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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