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From Shotaro Kamio <kamios...@gmail.com>
Subject Re: Compacting single file forever
Date Thu, 21 Apr 2011 13:34:16 GMT
Hi Aaron,


Maybe, my previous description was not good. It's not a compaction
threshold problem.
In fact, Cassandra tries to compact 7 sstables in the minor
compaction. But it decreases the number of sstables one by one due to
insufficient disk space. At the end, it compacts a single file as in
the new log below.

Compactionstats on a node says:

  compaction type: Minor
  column family: foobar
  bytes compacted: 133473101929
  bytes total in progress: 170000743825
  pending tasks: 12

The disk usage reaches 78%. It's really tough situation. But I guess
the data contains a lot of duplicates. because we feed same data again
and again and do repair.


Another thing I'm wondering is a file selection algorithm.
For example, one of disks has 235G free space. It contains sstables of
61G, 159G, 191G, 196G, 197G. The one cassandra trying to compact
forever is 159G sstable. But there is smaller sstable. It should try
compacting 61G + 159G ideally.
A more intelligent algorithm is required to find optimal combination.
And if cassandra knows statistics about number of deleted data and old
data to be compacted for sstables, it should be useful to find more
efficient file combination.


Regards,
Shotaro



* Minor compaction log
-----
 WARN [CompactionExecutor:1] 2011-04-21 21:44:08,554
CompactionManager.java (line 405) insufficient space to compact all
requested files SSTableReader(path='foobar-f-773-Data.db'),
SSTableReader(path='foobar-f-1452-Data.db'),
SSTableReader(path='foobar-f-1620-Data.db'),
SSTableReader(path='foobar-f-1642-Data.db'),
SSTableReader(path='foobar-f-1643-Data.db'),
SSTableReader(path='foobar-f-1690-Data.db'),
SSTableReader(path='foobar-f-1814-Data.db')
 WARN [CompactionExecutor:1] 2011-04-21 21:44:28,565
CompactionManager.java (line 405) insufficient space to compact all
requested files SSTableReader(path='foobar-f-773-Data.db'),
SSTableReader(path='foobar-f-1452-Data.db'),
SSTableReader(path='foobar-f-1642-Data.db'),
SSTableReader(path='foobar-f-1643-Data.db'),
SSTableReader(path='foobar-f-1690-Data.db'),
SSTableReader(path='foobar-f-1814-Data.db')
 WARN [CompactionExecutor:1] 2011-04-21 21:44:48,576
CompactionManager.java (line 405) insufficient space to compact all
requested files SSTableReader(path='foobar-f-773-Data.db'),
SSTableReader(path='foobar-f-1452-Data.db'),
SSTableReader(path='foobar-f-1642-Data.db'),
SSTableReader(path='foobar-f-1643-Data.db'),
SSTableReader(path='foobar-f-1814-Data.db')
 WARN [CompactionExecutor:1] 2011-04-21 21:45:08,586
CompactionManager.java (line 405) insufficient space to compact all
requested files SSTableReader(path='foobar-f-1452-Data.db'),
SSTableReader(path='foobar-f-1642-Data.db'),
SSTableReader(path='foobar-f-1643-Data.db'),
SSTableReader(path='foobar-f-1814-Data.db')
 WARN [CompactionExecutor:1] 2011-04-21 21:45:28,596
CompactionManager.java (line 405) insufficient space to compact all
requested files SSTableReader(path='foobar-f-1642-Data.db'),
SSTableReader(path='foobar-f-1643-Data.db'),
SSTableReader(path='foobar-f-1814-Data.db')
 WARN [CompactionExecutor:1] 2011-04-21 21:45:48,607
CompactionManager.java (line 405) insufficient space to compact all
requested files SSTableReader(path='foobar-f-1642-Data.db'),
SSTableReader(path='foobar-f-1814-Data.db')
------



On Thu, Apr 21, 2011 at 7:20 PM, aaron morton <aaron@thelastpickle.com> wrote:
> Want to check if you are talking about minor compactions or major (nodetool)
> compactions.
> What settings compaction settings do you have for this CF ? You can increase
> the min compaction threshold and reduce the frequency of
> compactions http://wiki.apache.org/cassandra/StorageConfiguration
> It seems like compaction is running continually, are their pending tasks in
> the o.a.c.db.CompactionManager MBean ?
> How bad is you disk space problem ?
> For the code change, AFAIK it's not possible for cassandra to know if there
> are tombstones in the SSTable which can be purged until the rows are read.
> Perhaps the file could hold the earliest deleted at time somewhere (same for
> TTL), but I do not think we do that now.
> Hope that helps.
> Aaron
>
> On 20 Apr 2011, at 21:25, Shotaro Kamio wrote:
>
> Hi,
>
> I found that our cluster repeats compacting a single file forever
> (cassandra 0.7.5). We are wondering if compaction logic is wrong. I'd
> like to have comments from you guys.
>
> Situation:
> - After trying to repair a column family, our cluster's disk usage is
> quite high. Cassandra cannot compact all sstables at once. I think it
> repeats compacting single file at the end. (you can check the attached
> log below)
> - Our data doesn't have deletes. So, the compaction of single file
> doesn't make free disk space.
>
> We are approaching to full-disk. But I believe that the repair
> operation made a lot of duplicate data on the disk and it requires
> compaction. However, most of nodes stuck on compacting a single file.
> The only thing we can do is to restart the nodes.
>
> My question is why the compaction doesn't stop.
>
> I looked at the logic in CompactionManager.java:
> -----------------
>        String compactionFileLocation =
> table.getDataFileLocation(cfs.getExpectedCompactedFileSize(sstables));
>        // If the compaction file path is null that means we have no
> space left for this compaction.
>        // try again w/o the largest one.
>        List<SSTableReader> smallerSSTables = new
> ArrayList<SSTableReader>(sstables);
>        while (compactionFileLocation == null && smallerSSTables.size()
> 1)
>        {
>            logger.warn("insufficient space to compact all requested
> files " + StringUtils.join(smallerSSTables, ", "));
>            smallerSSTables.remove(cfs.getMaxSizeFile(smallerSSTables));
>            compactionFileLocation =
> table.getDataFileLocation(cfs.getExpectedCompactedFileSize(smallerSSTables));
>        }
>        if (compactionFileLocation == null)
>        {
>            logger.error("insufficient space to compact even the two
> smallest files, aborting");
>            return 0;
>        }
> -----------------
>
> The while condition: smallerSSTables.size() > 1
> Is this should be "smallerSSTables.size() > 2" ?
>
> In my understanding, compaction of single file makes free disk space
> only when the sstable has a lot of tombstone and only if the tombstone
> is removed in the compaction. If cassandra knows the sstable has
> tombstones to be removed, it's worth to compact it. Otherwise, it
> might makes free space if you are lucky. In worst case, it leads to
> infinite loop like our case.
>
> What do you think the code change?
>
>
> Best regards,
> Shotaro
>
>
> * Cassandra compaction log
> -------------------------
> WARN [CompactionExecutor:1] 2011-04-20 01:03:14,446
> CompactionManager.java (line 405) insufficient space to compact all
> requested files SSTableReader(
> path='foobar-f-3020-Data.db'), SSTableReader(path='foobar-f-3034-Data.db')
> INFO [CompactionExecutor:1] 2011-04-20 03:47:29,833
> CompactionManager.java (line 482) Compacted to
> foobar-tmp-f-3035-Data.db.  260,646,760,319 to 260,646,760,319 (~100%
> of original) bytes for 6,893,896 keys.  Time: 9,855,385ms.
>
> WARN [CompactionExecutor:1] 2011-04-20 03:48:11,308
> CompactionManager.java (line 405) insufficient space to compact all
> requested files SSTableReader(path='foobar-f-3020-Data.db'),
> SSTableReader(path='foobar-f-3035-Data.db')
> INFO [CompactionExecutor:1] 2011-04-20 06:31:41,193
> CompactionManager.java (line 482) Compacted to
> foobar-tmp-f-3036-Data.db.  260,646,760,319 to 260,646,760,319 (~100%
> of original) bytes for 6,893,896 keys.  Time: 9,809,882ms.
>
> WARN [CompactionExecutor:1] 2011-04-20 06:32:22,476
> CompactionManager.java (line 405) insufficient space to compact all
> requested files SSTableReader(path='foobar-f-3020-Data.db'),
> SSTableReader(path='foobar-f-3036-Data.db')
> INFO [CompactionExecutor:1] 2011-04-20 09:20:29,903
> CompactionManager.java (line 482) Compacted to
> foobar-tmp-f-3037-Data.db.  260,646,760,319 to 260,646,760,319 (~100%
> of original) bytes for 6,893,896 keys.  Time: 10,087,424ms.
> -------------------------
> You can see that compacted size is always the same. It repeats
> compacting the same single sstable.
>
>



-- 
Shotaro Kamio

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