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From Michael Theroux <mthero...@yahoo.com>
Subject Re: cassandra backup
Date Fri, 06 Dec 2013 13:39:46 GMT
Hi Marcelo,

Cassandra provides and eventually consistent model for backups.  You can do staggered backups
of data, with the idea that if you restore a node, and then do a repair, your data will be
once again consistent.  Cassandra will not automatically copy the data to other nodes (other
than via hinted handoff).  You should manually run repair after restoring a node.
  
You should take snapshots when doing a backup, as it keeps the data you are backing up relevant
to a single point in time, otherwise compaction could add/delete files one you mid-backup,
or worse, I imagine attempt to access a SSTable mid-write.  Snapshots work by using links,
and don't take additional storage to perform.  In our process we create the snapshot, perform
the backup, and then clear the snapshot.

One thing to keep in mind in your S3 cost analysis is that, even though storage is cheap,
reads/writes to S3 are not (especially writes).  If you are using LeveledCompaction, or otherwise
have a ton of SSTables, some people have encountered increased costs moving the data to S3.

Ourselves, we maintain backup EBS volumes that we regularly snaphot/rsync data too.  Thus
far this has worked very well for us.

-Mike



On Friday, December 6, 2013 8:14 AM, Marcelo Elias Del Valle <marcelo@s1mbi0se.com.br>
wrote:
 
Hello everyone,

    I am trying to create backups of my data on AWS. My goal is to store the backups on
S3 or glacier, as it's cheap to store this kind of data. So, if I have a cluster with N nodes,
I would like to copy data from all N nodes to S3 and be able to restore later. I know Priam
does that (we were using it), but I am using the latest cassandra version and we plan to use
DSE some time, I am not sure Priam fits this case.
    I took a look at the docs: http://www.datastax.com/documentation/cassandra/2.0/webhelp/index.html#cassandra/operations/../../cassandra/operations/ops_backup_takes_snapshot_t.html 
    And I am trying to understand if it's really needed to take a snapshot to create my
backup. Suppose I do a flush and copy the sstables from each node, 1 by one, to s3. Not all
at the same time, but one by one. 
    When I try to restore my backup, data from node 1 will be older than data from node
2. Will this cause problems? AFAIK, if I am using a replication factor of 2, for instance,
and Cassandra sees data from node X only, it will automatically copy it to other nodes, right?
Is there any chance of cassandra nodes become corrupt somehow if I do my backups this way?

Best regards,
Marcelo Valle.
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