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From Ramzi Rabah <rra...@playdom.com>
Subject Re: Removes increasing disk space usage in Cassandra?
Date Fri, 04 Dec 2009 23:53:23 GMT
Thanks Jonathan.
Should I open a bug for this?

Ray

On Fri, Dec 4, 2009 at 3:47 PM, Jonathan Ellis <jbellis@gmail.com> wrote:
> On Fri, Dec 4, 2009 at 5:32 PM, Ramzi Rabah <rrabah@playdom.com> wrote:
>> Starting with fresh directories with no data and trying to do simple
>> inserts, I could not reproduce it *sigh*. Nothing is simple :(, so I
>> decided to dig deeper into the code.
>>
>> I was looking at the code for compaction, and this is a very noob
>> concern, so please bare with me if I'm way off, this code is all new
>> to me. When we are doing compactions during the normal course of
>> cassandra, we call:
>>
>>            for (List<SSTableReader> sstables :
>> getCompactionBuckets(ssTables_, 50L * 1024L * 1024L))
>>            {
>>                if (sstables.size() < minThreshold)
>>                {
>>                    continue;
>>                }
>>                other wise docompactions...
>>
>> where getCompactionBuckets puts in buckets very small files, or files
>> that are 0.5-1.5 of each other's sizes. It will only compact those if
>> they are >= minimum threshold which is 4 by default.
>
> Exactly right.
>
>> So far so good. Now how about this scenario, I have an old entry that
>> I inserted long time ago and that was compacted into a 75MB file.
>> There are fewer 75MB files than 4. I do many deletes, and I end with 4
>> extra sstable files filled with tombstones, each about 300 MB large.
>> These 4 files are compacted together and in the compaction code, if
>> the tombstone is there we don't copy it over to the new file. Now
>> since we did not compact the 75MB files, but we compacted the
>> tombstone files, doesn't that leave us with the tombstone gone, but
>> the data still intact in the 75MB file?
>
> Also right.  Glad you had a look! :)
>
> One relatively easy fix would be to only GC the tombstones if there
> are no SSTables left for that CF older than the ones being compacted.
> (So, a "major" compaction, which compacts all SSTables and is what
> nodeprobe invokes, would always GC eligible tombstones.)
>
> -Jonathan
>

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