Another Question on node density, in this scenario:

1. we should keep time series data of some years for a heavy write system in 
Cassandra (> 10K Ops in seconds)

2. the system is insert only and inserted data would never be updated

3. in partition key, we used number of months since 1970, so data for every 
month would be on separate partitions

4. because of rule 2, after the end of month previous partitions would never be 
accessed for write requests

5. more than 90% of read requests would concern current month partitions, so we 
merely access Old data, we should just keep them for that 10% of reports!

6. The overall read in comparison to writes are so small (like 0.0001 % of 
overall time)

So, finally the question:

Even in this scenario would the active data be the whole data (this month + all 
previous months)? or the one which would be accessed for most reads and writes 
(only the past two months)? 

Could i use more than 3TB  per node for this scenario?

Sent using Zoho Mail

---- On Tue, 20 Feb 2018 14:58:39 +0330 Rahul Singh 
<> wrote ----

Node density is active data managed in the cluster divided by the number of 
active nodes. Eg. If you you have 500TB or active data under management then 
you would need 250-500 nodes to get beast like optimum performance. It also 
depends on how much memory is on the boxes and if you are using SSD drives. SSD 
doesn’t replace memory but it doesn’t hurt.


 Rahul Singh


 Anant Corporation

On Feb 19, 2018, 5:55 PM -0500, Charulata Sharma (charshar) 
<>, wrote: 

Thanks for the response Rahul. I did not understand the “node density” point.




From: Rahul Singh <>
 Reply-To: "" <>
 Date: Monday, February 19, 2018 at 12:32 PM
 To: "" <>
 Subject: Re: Right sizing Cassandra data nodes


1. I would keep opscenter on different cluster. Why unnecessarily put traffic 
and computing for opscenter data on a real business data cluster?

 2. Don’t put more than 1-2 TB per node. Maybe 3TB. Node density as it 
increases creates more replication, read repairs , etc and memory usage for 
doing the compactions etc.

 3. Can have as much as you want for snapshots as long as you have it on 
another disk or even move it to a SAN / NAS. All you may care about us the most 
recent snapshot on the physical machine / disks on a live node.


 Rahul Singh


 Anant Corporation

On Feb 19, 2018, 3:08 PM -0500, Charulata Sharma (charshar) 
<>, wrote:


Hi All,


Looking for some insight into how application data archive and purge is carried 
out for C* database. Are there standard guidelines on calculating the amount of 
space that can be used for storing data in a specific node.


Some pointers that I got while researching are;


-          Allocate 50% space for compaction, e.g. if data size is 50GB then 
allocate 25GB for compaction.

-          Snapshot strategy. If old snapshots are present, then they occupy 
the disk space.

-          Allocate some percentage of storage ( ???? ) for system tables and 
OpsCenter tables ?


We have a scenario where certain transaction data needs to be archived based on 
business rules and some purged, so before deciding on an A&P strategy, I am 
trying to analyze

how much transactional data can be stored given the current node capacity. I 
also found out that the space available metric shown in Opscenter is not very 
reliable because it doesn’t show

the snapshot space. In our case, we have a huge snapshot size. For some 
unexplained reason, we seem to be taking snapshots of our data every hour and 
purging them only after 7 days.





Cisco Systems.




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