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From "Hiller, Dean" <>
Subject Re: data clean up problem
Date Tue, 28 May 2013 19:26:37 GMT
Don't do any delete != "need to free the disk space after retention period" which you have
in both your emails.  My understanding is TTL is an expiry and just like tombstones will only
be really deleted upon a compaction(ie. You do have deletes via TTL from the sound of it).
 If you have TTL of 1 day, it does not immediately go away until the next compaction is run
and then the compaction may not run on all rows???   I am not quite sure there except the
data stays in the sstable until it is compacted into a new sstable and is thrown away then
as long as TTL has passed.


From: cem <<>>
Reply-To: "<>" <<>>
Date: Tuesday, May 28, 2013 1:17 PM
To: "<>" <<>>
Subject: Re: data clean up problem

Thanks for the answer but it is already set to 0 since I don't do any delete.


On Tue, May 28, 2013 at 9:03 PM, Edward Capriolo <<>>
You need to change the gc_grace time of the column family. It defaults to 10 days. By default
the tombstones will not go away for 10 days.

On Tue, May 28, 2013 at 2:46 PM, cem <<>>
Hi Experts,

We have general problem about cleaning up data from the disk. I need to free the disk space
after retention period and the customer wants to dimension the disk space base on that.

After running multiple performance tests with TTL of 1 day we saw that the compaction couldn't
keep up with the request rate. Disks were getting full after 3 days. There were also a lot
of sstables that are older than 1 day after 3 days.

Things that we tried:

-Change the compaction strategy to leveled. (helped a bit but not much)

-Use big sstable size (10G) with leveled compaction to have more aggressive compaction.(helped
a bit but not much)

-Upgrade Cassandra from 1.0 to 1.2 to use TTL histograms (didn't help at all since it has
key overlapping estimation algorithm that generates %100 match. Although we don't have...)

Our column family structure is like this:

Event_data_cf: (we store event data. Event_id  is randomly generated and each event has attributes
like location=london)

row                  data

event id          data blob

timeseries_cf: (key is the attribute that we want to index. It can be location=london, we
didnt use secondary indexes because the indexes are dynamic.)

row                  data

index key       time series of event id (event1_id, event2_id….)

timeseries_inv_cf: (this is used for removing event by event row key. )

row                  data

event id          set of index keys

Candidate Solution: Implementing time range partitions.

Each partition will have column family set and will be managed by client.

Suppose that you want to have 7 days retention period. Then you can configure the partition
size as 1 day and have 7 active partitions at any time. Then you can drop inactive partitions
(older that 7 days). Dropping will immediate remove the data from the disk. (With proper Cassandra.yaml

Storing an event:

Find the current partition p1

store to event_data to Event_data_cf_p1

store to indexes to timeseries_cff_p1

store to inverted indexes to timeseries_inv_cf_p1

A time range query with an index:

Find the all partitions belongs to that time range

Do read starting from the first partition until you reach to limit


Could you please provide your comments and concerns ?

Is there any other option that we can try?

What do you think about the candidate solution?

Does anyone have the same issue? How would you solve it in another way?

Thanks in advance!


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