In my C* 1.2.4 setup, I have an ssd drive of 200Gb for the data and a rotational drive for commit logs of 500Gb.
I had the unpleasant surprise during a scrub operation to fill in my ssd drive with the snapshots. That made the cassandra box unresponsive but it kept the status as up when doing nodetool status.
I am wondering if there is a way to specify the target directory for snapshots when doing a scrub.
Otherwise if you have ideas for workarounds?
I can do a column family at a time and then copy the snapshots folder, but I am open for smarter solutions.
Thanks,
H
Snapshots in Cassandra are created as hard links to your existing data files. This means at the time the snapshot is taken, it takes up almost no extra space. However, it causes the old files to remain so if you delete or update data, the old version is still there.
This means snapshots must be taken on the drive that stores the data. If you don't need the snapshot any more, just delete it with 'nodetool clearsnapshot' (see the nodetool help output for how to decide which snapshots to delete). If you want to keep the snapshot then you can move it elsewhere. It will only start using much disk space after a while, so you could keep it until you are happy the scrub didn't delete important data then delete the snapshot.
Related
Theoretical question:
Lets say I have a cassandra cluster with some data in it.
Backups are created on a daily basis.
Now a subset of data is being lost, either by application error or manual deletion.
What is the best way to restore data from existing backup?
I can think of starting a separate node with the backup disk attached, then export data manually through selects and reimport into the prod database.
That would work but sounds complicated, is there a more straight forward solution for such problems?
If its a single partition probably best bet is to use sstabledump or something like sstable-tools to read from it and just manually reinstert. If ok with restoring everything deleted from time of snapshot: reduce gcgrace to purge any tombstones with a force compact (or else they will continue to shadow the restored data) and use the sstable loader or if the token ranges are the same copy the backed up sstables back in the data directory.
I have enabled the incremental backup in the cassandra.yaml file. As I know when we enable incremental backups, cassandra will backup the data (in backups directory) only when the memtable is flushed. But what if the memtable is yet to be flushed? I won't be able to get the incremental backup right?. I know that for the memtable to be flushed there are certain conditions to be met such as time interval or memtable space. My question is how do I modify this so that even if I enter one record after the last snapshot, I can still backup entire data along with that latest entry?
Consider this example
Take the snapshot.
Clear incremental backup (backups directory)
Enter a record to a table.
Check for the incremental backup in backups directory. It is still empty.
Now how do I backup the record which is written after the last snapshot?In general how do we backup the entire upto-date data unless we take the snapshot?
You can flush the files manually with nodetool flush just before taking the backup. That way you'll always have the latest memtable flushed.
nodetool docs
If you want to backup a cluster without taking a snapshot you can do it by simply saving everything under /data folder from every node (this includes mainly the .db files stats files etc).
In order to not override files you should store it with the token information as well.
When you want to restore from this backup, you should spin up a cluster with the same number of nodes, and simply copy the data, one-to-one from each backed-up node to a restored node. Pay attention that you'll have to modify cassandra.yaml to include the relevant token in cassandra.yaml (as well as the peers/seeds/etc) for each restored node.
After all the data is copied, you can start C* process on all the nodes.
Although I checked Datastax's document about snapshot, I am still confused about what a snapshot in cassandra is. What's the function or main purpose of a snapshot?
Under the snapshot folder, I find some subfolder named in convention of this:
1426256545571-tablename
What does the number at the very beginning mean? Anyway, I just a need a easy way to know what a snapshot is.
The number is the number of ms from epoch (timestamp). A snapshot is just a local backup. It occurs automatically for some types of operations like truncate (in case done by accident and want to undo it).
They are very fast and don't cost any extra disk space up front since its just a hard link to the immutable data files. Eventually you want to clean them up though to reclaim disk as compactions occur. You can disable the auto_snapshot option in cassandra.yaml if you don't want them anymore. It is likely you will see them while doing repairs, still.
I am looking for confirmation that my Cassandra backup and restore procedures are sound and I am not missing anything. Can you please confirm, or tell me if something is incorrect/missing?
Backups:
I run daily full backups of the keyspaces I care about, via "nodetool snapshot keyspace_name -t current_timestamp". After the snapshot has been taken, I copy the data to a mounted disk, dedicated to backups, then do a "nodetool clearsnapshot $keyspace_name -t $current_timestamp"
I also run hourly incremental backups - executing a "nodetool flush keyspace_name" and then moving files from the backup directory of each keyspace, into the backup mountpoint
Restore:
So far, the only valid way I have found to do a restore (and tested/confirmed) is to do this, on ALL Cassandra nodes in the cluster:
Stop Cassandra
Clear the commitlog *.log files
Clear the *.db files from the table I want to restore
Copy the snapshot/full backup files into that directory
Copy any incremental files I need to (I have not tested with multiple incrementals, but I am assuming I will have to overlay the files, in sequence from oldest to newest)
Start Cassandra
On one of the nodes, run a "nodetool repair keyspace_name"
So my questions are:
Does the above backup and restore strategy seem valid? Are any steps inaccurate or anything missing?
Is there a way to do this without stopping Cassandra on EVERY node? For example, is there a way to restore the data on ONE node, then somehow make it "authoritative"? I tried this, and, as expected, since the restored data is older, the data on the other nodes (which is newer) overwrites in when they sync up during repair.
Thank you!
There's two ways to restore Cassandra backups without restarting C*:
Copy the files into place, then run "nodetool refresh". This has the caveat that the rows will still be older than tombstones. So if you're trying to restore deleted data, it won't do what you want. It also only applies to the local server (you'll want to repair after)
Use "sstableloader". This will load data to all nodes. You'll need to make sure you have the sstables from a complete replica, which may mean loading the sstables from multiple nodes. Added bonus, this works even if the cluster size has changed. I'm not sure if ordering matters here (that is, I don't know if row timestamps are preserved through the load or if they're redefined during load)
I have Cassandra 1.2.6 cluster running on datacenter A, each node has a solid state drive with somewhat limited space (aprox 50% of disk space is free).
Now I need to implement somehow a way of having automatic backups of each node. Ideally I want to have a way of moving all of the cluster's datafiles to a different disk (standard cheaper disks), or even to a different server in the same datacenter A and possibly moving all the data once in a while to a datacenter B in a different location.
From what I've read I can use snapshots on each node to get the files to copy using whatever tool I want and in this case I have the option to move the data to a different disk/server/datacenter.
My question is, since each of my nodes is about 50% full, taking a snapshot will consume all that space? or the hard links will consume way less space than I anticipate?, if so, is there a better way of doing this, maybe with an already made tool, or everything should be custom made when it comes to this type of backups in Cassandra?
Thanks in advance!
A hard link just creates a new directory entry for the same file (http://en.wikipedia.org/wiki/Hard_link). So a snapshot takes up effectively zero space, but you'll want to clean it up after you're done copying it off to whatever your archive is, because when the "original" sstable is deleted (typically post-compaction), space won't be reclaimed as long as the snapshot reference is still there.
My impression is that tablesnap is the most popular tool for automating backups to s3. It also supports Cassandra incremental backups. If you want more control over where you're backing up to, DataStax OpsCenter supports running a custom script when it takes snapshots.