For background:

Minor compaction will bucket files (see https://github.com/apache/cassandra/blob/cassandra-0.8.6/src/java/org/apache/cassandra/db/compaction/CompactionManager.java#L989) and then compact them if they have more than min_compaction_threshold set per CF. 

It will then purge tombstones if a row is only contained in the SSTables involved in the compaction (see https://github.com/apache/cassandra/blob/cassandra-0.8.6/src/java/org/apache/cassandra/db/compaction/CompactionController.java#L84

So there are a couple of approaches you can take if you want to ensure all TTL'd data is purged ASAP. Note that Tombstones and TTL'd data will be automatically purged at some point, but if more precise control you may need to take a few steps.  

First if all the data you are storing has a 24 hour TTL you can include a manualy major compaction via node tool in your maintenance routine. We normally advise against it because it tries to create one large file, but if all your data is going to be removed it's prob ok. 

Second, play around with minor compaction to increase the chances that data is purged soon after the 

Third, monkey up a process to kick of user defined compaction runs for SSTables that are over 24 hours old. 

I know disk space can be an issue, but if you have the spare capacity you can just let cassandra manage things. Also 1.0 has some major changes in this area.

Cheers

-----------------
Aaron Morton
Freelance Cassandra Developer
@aaronmorton

On 29/09/2011, at 4:39 AM, hiroyuki.watanabe@barclayscapital.com wrote:

Thank you.  You have been very helpful.
 
We stored only one day worth of data for now. However, we want to store 5 days worth of data eventually.
That is 5 times more disk space.
That is our main reason for us to look at older SSTables that appears to be holding only tombstones.
 
- yuki
 


From: aaron morton [mailto:aaron@thelastpickle.com]
Sent: Tuesday, September 27, 2011 8:22 PM
To: user@cassandra.apache.org
Subject: Re: Removal of old data files

Short Answer: Cassandra will actively delete files when it needs to make space. Otherwise they will be deleted "some time later". Unless you are getting out of disk space errors it's not normally something to worry about. 

Longer: 
The TTL guarantee is "do not return this data to get requests after this many seconds".
 
Data is "purged" from an SSTable when we run compactions (either minor/auto or major/manual). Purging means it will not be written in the new SSTable created by the compaction process. The main criteria for purging is that either gc_grace_seconds OR ttl have expired on the column. 

After compaction completes it's writes the -Compacted for the SSTables that were compacted. But there is a bunch of logic associated with which files are compacted, it's not "compact the oldest 3 files".  Basically it tries to compact files which are about the same size. 

Remember we *never* modify data on disk. If we want to remove data from an SSTable we have to write a new SSTable. It's one of the reasons things writes are fast http://thelastpickle.com/2011/04/28/Forces-of-Write-and-Read/

Hope that helps. 

-----------------
Aaron Morton
Freelance Cassandra Developer
@aaronmorton

On 28/09/2011, at 10:16 AM, hiroyuki.watanabe@barclayscapital.com wrote:


Now, we use TTL of 12 hours and GC grace period of 8 hours for encouraging Cassandra to remove old data/files more aggressively.  

Cassandra do remove fair amount of old data files.
Cassandra tends to removed 4 out of every 5 files.
I notice it because data file has a sequence number as a part of name.

I also noticed when Cassandra generated *-Compacted file it generated 4 file at a time.
They have consecutive numbers as file name, but skip one number from the previous group of 4.
The one missing is the file that is failed to be removed in the end and stays forever.

I looked at the Keys in an index file that failed to be removed.  If I make query of any of keys, Cassandra indicates that there is not data, which is correct because these files are older than 24 hours.  All the data must be obsolete due to TTL.     

I am wondering why Cassandra does not remove all data file whose time stamp is much older than TTL + grace period.

Does anybody have similar experience ?


Yuki Watanabe



-----Original Message-----
From: Watanabe, Hiroyuki: IT (NYK)
Sent: Friday, September 02, 2011 9:01 AM
To: user@cassandra.apache.org
Subject: RE: Removal of old data files


I see. Thank you for helpful information

Yuki



-----Original Message-----
From: Sylvain Lebresne [mailto:sylvain@datastax.com]
Sent: Friday, September 02, 2011 3:40 AM
To: user@cassandra.apache.org
Subject: Re: Removal of old data files

On Fri, Sep 2, 2011 at 12:11 AM,  <hiroyuki.watanabe@barclayscapital.com> wrote:
Yes, I see files with name like
    Orders-g-6517-Compacted

However, all of those file have a size of 0.

Starting from Monday to Thurseday we have 5642 files for -Data.db,
-Filter.db and Statistics.db and only 128 -Compacted files.
and all of -Compacted file has size of 0.

Is this normal, or we are doing something wrong?

You are not doing something wrong. The -Compacted files are just marker, to indicate that the -Data file corresponding (with the same number) are, in fact, compacted and will eventually be removed. So those files will always have a size of 0.

--
Sylvain



yuki

________________________________
From: aaron morton [mailto:aaron@thelastpickle.com]
Sent: Thursday, August 25, 2011 6:13 PM
To: user@cassandra.apache.org
Subject: Re: Removal of old data files

If cassandra does not have enough disk space to create a new file it
will provoke a JVM GC which should result in compacted SStables that
are no longer needed been deleted. Otherwise they are deleted at some
time in the future.
Compacted SSTables have a file written out with a "compacted" extension.
Do you see compacted sstables in the data directory?
Cheers.
-----------------
Aaron Morton
Freelance Cassandra Developer
@aaronmorton
http://www.thelastpickle.com
On 26/08/2011, at 2:29 AM, yuki watanabe wrote:

We are using Cassandra 0.8.0 with 8 node ring and only one CF.
Every column has TTL of 86400 (24 hours). we also set 'GC grace
second' to 43200
(12 hours).  We have to store massive amount of data for one day now
and eventually for five days if we get more disk space.
Even for one day, we do run out disk space in a busy day.

We run nodetool compact command at night or as necessary then we run
GC from jconsole. We observed that  GC did remove files but not
necessarily oldest ones.
Data files from more than 36 hours ago and quite often three days ago
are still there.

Does this behavior expected or we need adjust some other parameters?


Yuki Watanabe

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