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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-418) SSTable generation clash during compaction
Date Thu, 03 Sep 2009 02:09:32 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12750785#action_12750785
] 

Jonathan Ellis commented on CASSANDRA-418:
------------------------------------------

the compaction code relies on the bucketizer to keep files of the same compaction-count (a
bucket of sstables that have been compacted twice, one of sstables that have been compacted
3 times) so that you are never compacting sstables of consecutive generations -- all will
have even numbers, or all odd.  something has broken that invariant.

rather than try to band-aid the bucketizer i think making the generation-generator more robust
is the way to go.  this seems like a flimsy property to try to preserve.

my vote would be to simplify: just pick the next monotonically increasing int any time we
need a new tmp sstable file, whether for flush, compaction, or bootstrap.  I.e. via CFS.getTempSSTableFileName,
without the extra increment.

the reason historically that FB tried to be fancy is, they were trying to optimize away reading
older sstables at all if the data being queried was found in a newer one.  the "only new sstables
get a number from the atomic int, and the compactions fit in between" was to preserve this.
 (then you sort on the generation number and higher ones are always newer.)

but that can't work (see CASSANDRA-223) so we always do a full merge across all sstables now.
 so we can simplify this safely.

> SSTable generation clash during compaction
> ------------------------------------------
>
>                 Key: CASSANDRA-418
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-418
>             Project: Cassandra
>          Issue Type: Bug
>    Affects Versions: 0.5
>            Reporter: Sammy Yu
>            Assignee: Sammy Yu
>             Fix For: 0.5
>
>
> We found that one of our node started getting timeouts for get_slice.  Looking further
we found that the CFS.ssTables_ references a SStable doesn't exist on the file system.
> Walking down the log we see that the sstable in question 6038 is being compacted onto
itself (in terms of filename file wise it is written to -tmp):
> system.log.2009-09-01: INFO [MINOR-COMPACTION-POOL:1] 2009-09-01 23:50:07,553 ColumnFamilyStore.java
(line 1067) Compacting 
> [/mnt/var/cassandra/data/Digg/FriendActions-6037-Data.db,/mnt/var/cassandra/data/Digg/FriendActions-6038-Data.db,/mnt/var/cassandra/data/Digg/
> FriendActions-6040-Data.db,/mnt/var/cassandra/data/Digg/FriendActions-6042-Data.db]
> system.log.2009-09-01: INFO [MINOR-COMPACTION-POOL:1] 2009-09-01 23:51:43,727 ColumnFamilyStore.java
(line 1209) Compacted to
> /mnt/var/cassandra/data/Digg/FriendActions-6038-Data.db.  0/1010269806 bytes for 9482/9373
keys read/written.  Time: 96173ms.
> It appears the generation number is generated by looking at the lowest number in the
list of files to be compacted and adding 1.  In this scenario it is 6037+1=6038.
> The code in CFS.doFileCompaction will remove the key and add the key back and remove
the key again, hence the error we were seeing.
> Should the generation number be generated via another way or should we update doFileCompaction
to be smarter?

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