I have used old snapshot files exactly once when I deleted a bunch of server
state trying to unwind a tangled

I keep a few around just for backup purposes.

On Wed, Sep 8, 2010 at 12:01 PM, Vishal K <vishalm...@gmail.com> wrote:

> Hi All,
> Can you please share your experience regarding ZK snapshot retention and
> recovery policies?
> We have an application where we never need to rollback (i.e., revert back
> to
> a previous state by using old snapshots). Given this, I am trying to
> understand under what circumstances would we ever need to use old ZK
> snapshots. I understand a lot of these decisions depend on the application
> and amount of redundancy used at every level (e.g,. RAID level where the
> snapshots are stored etc) in the product. To simplify the discussion, I
> would like to rule out any application characteristics and focus mainly on
> data consistency.
> - Assuming that we have a 3 node cluster I am trying to figure out when
> would I really need to use old snapshot files. With 3 nodes we already have
> at least 2 servers with consistent database. If I loose files on one of the
> servers, I can use files from the other. In fact, ZK server join will take
> care of this. I can remove files from a faulty node and reboot that node.
> The faulty node will sync with the leader.
> - The old files will be useful if the current snapshot and/or log files are
> lost or corrupted on all 3 servers. If  the loss is due to a disaster (case
> where we loose all 3 servers), one would have to keep the snapshots on some
> external storage to recover. However, if the current snapshot file is
> corrupted on all 3 servers, then the most likely cause would be a bug in
> ZK.
> In which case, how can I trust the consistency of the old snapshots?
> - Given a set of snapshots and log files, how can I verify the correctness
> of these files? Example, if one of the intermediate snapshot file is
> corrupt.
> - The Admin's guide says "Using older log and snapshot files, you can look
> at the previous state of ZooKeeper servers and even restore that state. The
> LogFormatter class allows an administrator to look at the transactions in a
> log." * *Is there a tool that does this for the admin?  The LogFormatter
> only displays the transactions in the log file.
> - Has anyone ever had to play with the snapshot files in production?
> Thanks in advance.
> Regards,
> -Vishal

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