Hi Mirantis guys, 

I have set up two Cassandra backups: 
The first backup procedure was similar to the one you want to achieve. 
The second backup used SAN features (EMC VNX snapshots) so it was very specific 
to the environment. 

Backup an entire cluster (therefore all replicas) is challenging when dealing 
with big data and not really needed. If your replicas are spread accross 
several data centers then you could backup just one data center. In that case 
you backup only one replica. 
Depending on your needs you may want to backup twice (I mean "backup the 
backup" using a tape library for example) and then store it in an external 
location for disaster recovery, requirements specification, norms, etc. 

The snapshot command issues a flush before to effectively take the snapshot. So 
the flush command is not necessary. 
https://github.com/apache/cassandra/blob/c7ebc01bbc6aa602b91e105b935d6779245c87d1/src/java/org/apache/cassandra/db/ColumnFamilyStore.java#L2213
 
(snapshotWithoutFlush() is used by the scrub command) 

Just out of curiosity, have you tried the leveled compaction strategy? It seems 
that you use STCS. 
Does your use case imply many updates? What is your read/write ratio? 

Best, 

Romain 

----- Original Message -----

From: "Denis Makogon" <dmako...@mirantis.com> 
To: "OpenStack Development Mailing List (not for usage questions)" 
<openstack-dev@lists.openstack.org> 
Sent: Friday, August 29, 2014 4:33:59 PM 
Subject: Re: [openstack-dev] [magnetodb] Backup procedure for Cassandra backend 




On Fri, Aug 29, 2014 at 4:29 PM, Dmitriy Ukhlov < dukh...@mirantis.com > wrote: 



Hello Denis, 
Thank you for very useful knowledge sharing. 

But I have one more question. As far as I understood if we have replication 
factor 3 it means that our backup may contain three copies of the same data. 
Also it may contain some not compacted sstables set. Do we have any ability to 
compact collected backup data before moving it to backup storage? 




Thanks for fast response, Dmitriy. 

With replication factor 3 - yes, this looks like a feature that allows to 
backup only one node instead of 3 of them. In other cases, we would need to 
iterate over each node, as you know. 
Correct, it is possible to have not compacted SSTables. To accomplish 
compaction we might need to use compaction mechanism provided by the nodetool, 
see 
http://www.datastax.com/documentation/cassandra/2.0/cassandra/tools/toolsCompact.html
 , we just need take into account that it's possible that sstable was already 
compacted and force compaction wouldn't give valuable benefits. 


Best regards, 
Denis Makogon 


<blockquote>



On Fri, Aug 29, 2014 at 2:01 PM, Denis Makogon < dmako...@mirantis.com > wrote: 

<blockquote>



Hello, stackers. I'd like to start thread related to backuping procedure for 
MagnetoDB, to be precise, for Cassandra backend. 

In order to accomplish backuping procedure for Cassandra we need to understand 
how does backuping work. 


To perform backuping: 

    1. 

We need to SSH into each node 
    2. 

Call ‘nodetool snapshot’ with appropriate parameters 
    3. 

Collect backup. 
    4. 

Send backup to remote storage. 
    5. 

Remove initial snapshot 



Lets take a look how does ‘ nodetool snapshot ’ works. Cassandra backs up data 
by taking a snapshot of all on-disk data files (SSTable files) stored in the 
data directory. Each time an SSTable gets flushed and snapshotted it becomes a 
hard link against initial SSTable pinned to specific timestamp. 


Snapshots are taken per keyspace or per-CF and while the system is online. 
However, nodes must be taken offline in order to restore a snapshot. 


Using a parallel ssh tool (such as pssh), you can flush and then snapshot an 
entire cluster. This provides an eventually consistent backup. Although no one 
node is guaranteed to be consistent with its replica nodes at the time a 
snapshot is taken, a restored snapshot can resume consistency using Cassandra's 
built-in consistency mechanisms. 


After a system-wide snapshot has been taken, you can enable incremental backups 
on each node (disabled by default) to backup data that has changed since the 
last snapshot was taken. Each time an SSTable is flushed, a hard link is copied 
into a /backups subdirectory of the data directory. 


Now lets see how can we deal with snapshot once its taken. Below you can see a 
list of command that needs to be executed to prepare a snapshot: 


Flushing SSTables for consistency 

'nodetool flush' 


Creating snapshots (for example of all keyspaces) 

"nodetool snapshot -t %(backup_name)s 1>/dev/null", 


where 

    * 

backup_name - is a name of snapshot 



Once it’s done we would need to collect all hard links into a common directory 
(with keeping initial file hierarchy): 


sudo tar cpzfP /tmp/all_ks.tar.gz\ 

$(sudo find %(datadir)s -type d -name %(backup_name)s)" 


where 

    * 

backup_name - is a name of snapshot, 
    * 

datadir - storage location (/var/lib/cassandra/data, by the default) 





Note that this operation can be extended: 

    * 

if cassandra was launched with more than one data directory (see cassandra.yaml 
) 
    * 

if we want to backup only: 


        * 

certain keyspaces at the same time 
        * 

one keyspace 
        * 

a list of CF’s for given keyspace 



Useful links 

http://www.datastax.com/documentation/cassandra/2.0/cassandra/tools/toolsNodetool_r.html
 

Best regards, 
Denis Makogon 

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</blockquote>




-- 
Best regards, 
Dmitriy Ukhlov 
Mirantis Inc. 

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</blockquote>



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