If you do not need range scans (and assuming Random Partitioner), I would probably go with B. I tend to feel better when things are spread out.

I'm not sure on any overhead on asking the coordinator to send requests to a lot of nodes. But I feel that it will make better use of new nodes added to the cluster, so you would get more ops by doing them on more machines. I may be wrong, dont have an understanding of the overheads involved.

If you needed range scans you could build a secondary index of your own. Also scenario B gives you room to store more columns for a key in the future.

I did read somewhere once that even in a standard CF it's not a good idea to have millions of columns. I think it was probably related to the issues below.

Hope that helps
Aaron



On 14 Jul, 2010,at 08:00 AM, "Kochheiser,Todd W - TOK-DITT-1" <twkochheiser@bpa.gov> wrote:

So it would appear that 0.7 will have solved the requirement that a single row must be able to fit in memory.  That issue aside, how would one expect the read/write performance to be in the scenarios listed below?

 


From: Mason Hale [mailto:mason@onespot.com]
Sent: Tuesday, July 13, 2010 8:41 AM
To: user@cassandra.apache.org
Subject: Re: Consequences of having many columns

 

Currently there is a limitation that each row must fit in memory (with some not insignificant overhead), thus having lots of columns per row can trigger out-of-memory errors. This limitation should be removed in a future release. 

 

Please see:

  - https://issues.apache.org/jira/browse/CASSANDRA-16  (notice this is marked as resolved now)

 

Mason

On Tue, Jul 13, 2010 at 9:38 AM, Kochheiser,Todd W - TOK-DITT-1 <twkochheiser@bpa.gov> wrote:

I recently ran across a blog posting with a comment from a Cassandra committer that indicated a performance penalty when having a large number of columns per row/key.  Unfortunately I didnít bookmark the blog posting and now I canít find it.  Regardless, since our current plan and design is to have several thousand columns per row/key, it made me question our design and if it might cause unintended performance consequences.  As a somewhat concrete example for discussion purposes, which of the following scenarios would ďpotentialĒ perform better or worse?

 

Assume:

  • Single ColumnFamily
  • Three node cluster
  • 10 to 1 read/write ratio (10 reads to every write)

 

Scenario A:

 

  • 10k rows
  • 5k columns/row
  • Each column ~ 64kB
  • Hot spot for writes and reads would be a single column in each row (the hot column would change every hour).  We would be accessing every row constantly, but in general accessing just a few columns in each. 
  • A low volume of reads accessing ~100 columns per row (range queries would work)
  • Access is generally direct (row key / column key) 
  • Data growth would be horizontal (adding columns) as apposed to vertically (adding rows)
  • This is our current design

 

Scenario B:

 

  • 50M rows/keys
  • 1 column/key
  • Each column ~ 64kB
  • Hot spot for writes and reads would be the single column in 10k rows, but the 10k rows accessed would change every hour.
  • Access would generally be direct (row key / column key)
  • Data growth would be vertically (adding rows 10k at a time) as apposed to horizontal (adding columns)

 

Scenario C:

 

  • 5k rows/keys
  • 10k columns/row
  • Each column ~64kB
  • Hot spot for writes and reads would be every column in a single row.  Row being access would change every hour
  • Access is generally direct (row key / column key)
  • Low volume of queries accessing a single column in many rows
  • Data growth would be by adding rows, each with 10k column.

 

In all three scenarios the amount of data is the same but the access pattern in different.  From an application coding perspective any of the approaches are feasible, although the data is easier to think about in Scenario A (i.e. fewer mental gymnastics and fewer composite keys).  In all of the scenarios there are 10k columns that are constantly accessed (read and write).

 

Some thoughts: Scenario A has the advantage of evenly distributing reads/writes across all cluster nodes (I think).  Scenario B has the potential advantage of having one column per row (I think) but *not* necessarily distributing evenly reads/writes across all cluster nodes.  Iím not serious about Scenario C, but it is an option.  Scenario C would probably cause one node in the cluster to take the brunt of all reads/writes so I think this design would be a bad idea.  And, if having lots of columns is a bad idea then this is even worse than scenario A.

 

Regards,

Todd