Thanks, Erick, Ken, and Jeff.
Erick,
I thought about min_threshold. The document says it "Sets the minimum number of
SSTables to trigger a minor compaction." I thought removing those large files
would be considered a major compaction, and this parameter may not help. Am I
wrong?
I also wondered what side effect it may have by lowering min_threshold value.
Will there be more compactions? I understand it is a balance sometimes to
either have multiple small compactions or a single big compaction.
About your comment "never run nodetool compact". Is it what Cassandra does when
it finally compact those 4 files? I don't really see the difference between
what Cassandra does programatically and what if I run it once every two weeks
to reclaim the disk space.
Ken,
Interesting way to do it. I will think about it.
Jeff,
That would be an ideal solution. Actually I am planning to migrate to the
latest 2.1 version, and hopefully it will be solved then.
Thanks again, everyone, for your responses.
Dongfeng
On Monday, September 28, 2015 10:36 AM, Jeff Jirsa
<jeff.ji...@crowdstrike.com> wrote:
There’s a seldom discussed parameter called:
unchecked_tombstone_compaction
The documentation describes the option as follows:
| True enables more aggressive than normal tombstone compactions. A single
SSTable tombstone compaction runs without checking the likelihood of success.
Cassandra 2.0.9 and later.
|
You’d need to upgrade to newer than 2.0.9, but by doing so, and enabling
unchecked_tombstone_compaction, you could encourage cassandra to compact just
one single large sstable to purge tombstones.
From: <erickramirezonl...@gmail.com> on behalf of Erick Ramirez
Reply-To: "user@cassandra.apache.org"
Date: Sunday, September 27, 2015 at 11:59 PM
To: "user@cassandra.apache.org", Dongfeng Lu
Subject: Re: How to remove huge files with all expired data sooner?
Hello,
You should never run `nodetool compact` since this will result in a massive
SSTable that will almost never get compacted out or take a very long time to
get compacted out.
You are correct that there needs to be 4 similar-sized SSTables for them to get
compacted. If you want the expired data to be deleted quicker, try lowering the
STCS `min_threshold` to 3 or even 2. Good luck!
Cheers,
Erick
On Sat, Sep 26, 2015 at 4:40 AM, Dongfeng Lu <dlu66...@yahoo.com> wrote:
Hi I have a table where I set TTL to only 7 days for all records and we keep
pumping records in every day. In general, I would expect all data files for
that table to have timestamps less than, say 8 or 9 days old, giving the system
some time to work its magic. However, I see some files more than 9 days old
occationally. Last Friday, I saw 4 large files, each about 10G in size, with
timestamps about 5, 4, 3, 2 weeks old. Interestingly they are all gone this
Monday, leaving 1 new file 9 GB in size.
The compaction strategy is SizeTieredCompactionStrategy, and I can understand
why the above happened. It seems we have 10G of data every week and when
SizeTieredCompactionStrategy works to create various tiers, it just happened
the file size for the next tier is 10G, and all the data is packed into this
huge file. Then it starts the next cycle. Another week goes by, and another 10G
file is created. This process continues until the minimum number of files of
the same size is reached, which I think is 4 by default. Then it started to
compact this set of 4 10G files. At this time, all data in these 4 files have
expired so we end up with nothing or much smaller file if there is still some
records with TTL left.
I have many tables like this, and I'd like to reclaim those spaces sooner. What
would be the best way to do it? Should I run "nodetool compact" when I see two
large files that are 2 weeks old? Is there configuration parameters I can tune
to achieve the same effect? I looked through all the CQL Compaction
Subproperties for STCS, but I am not sure how they can help here. Any
suggestion is welcome.
BTW, I am using Cassandra 2.0.6.
