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Zookeeper, mail # user - Backups


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Re: Backups
Flavio Junqueira 2012-01-19, 18:39
Hi Ted, Znodes for leader election, group membership, etc, can all be  
recreated, so why should I back them up instead of recreating the  
znodes? In fact, one might bring back a previous snapshot of the  
system that reflects an incorrect system state.

In the case that one stores data that can't be recovered by other  
means, I understand the need, but then we have the durability problem  
that I mentioned and you apparently agreed. Also, ZooKeeper is a  
replicated service, so why can't you simply rely upon the replication  
strategy that ZooKeeper provides to you already? Again, I'm trying to  
understand the use cases here.

Thanks,
-Flavio

On Jan 19, 2012, at 7:11 PM, Ted Dunning wrote:

> A backup can still be useful.  It is a common property that a database
> backup is known to be slightly out of date.
>
> Such a backup can still be very useful.  In many systems, the most  
> common
> cause of error is simple human intervention.  This especially  
> applies to
> file systems and databases, but can still apply to ZK if an admin
> carelessly tries to clean up part of the namespace and accidentally  
> cleans
> up all of it.  This should be much less common with ZK because manual
> adjustments are so much less a part of standard operation, but they  
> can
> still occur.  In these cases, an out-of-date backup may be enormously
> valuable.
>
> If somebody wants a precise backup from a particular moment in time,  
> the
> best option is to use the snapshot capabilities exposed by various  
> file
> systems.  Traditional NAS vendors all support this.  At a lower cost  
> and
> complexity point, you can get this from MapR clusters exposed as NFS  
> or by
> a ZFS file system.  This option also allows you to keep multiple  
> snapshots
> from points in the past.
>
> What Jordan is doing would allow backups without special storage  
> devices
> and, with good backup of the log, would allow nearly current  
> recovery in
> the event of catastrophic loss.  Yes, this loses some durability,  
> but it is
> still very desirable.
>
> On Thu, Jan 19, 2012 at 11:07 AM, Flavio Junqueira <fpj@yahoo-
> inc.com>wrote:
>
>> Since you started this thread, I've been thinking about the idea of
>> backing up, and I'm not sure I understand the motivation and if it  
>> is ok to
>> violate safety properties.
>>
>> Given that ZooKeeper is used for coordination, I would think that  
>> in many
>> cases all its state can be reconstructed in an algorithmic manner.  
>> Perhaps
>> the use case for a backup would be the one in which it is being  
>> used as a
>> database, for example, to keep the metadata of a file system.  
>> Periodic
>> backups or even keeping an observer, however, won't guarantee that  
>> if you
>> bring the system up using that backup you'll have all committed  
>> operations.
>> The state of the leader reflects all committed operations, but one  
>> needs to
>> have the latest state of the transaction log to not miss an update.
>>
>> But, it is true that I'm assuming that you can't miss updates. If  
>> you can
>> miss updates, then that's a different story. By missing updates  
>> we'll be
>> violating durability, which is  a property that ZooKeeper is  
>> supposed to
>> provide, so I'm trying to understand in which cases violating  
>> durability
>> would be acceptable. If it is not acceptable and you still want to  
>> have a
>> backup, then I don't see a way other than shutting down the clients  
>> before
>> you take a backup, which doesn't seem to be what is being proposed  
>> here.
>>
>> -Flavio
>>
>>
>> On Jan 18, 2012, at 1:38 AM, Jordan Zimmerman wrote:
>>
>> Neha - can you send me your email address. Send it to:
>>> [EMAIL PROTECTED]
>>>
>>> On 1/17/12 10:10 AM, "Neha Narkhede" <[EMAIL PROTECTED]>  
>>> wrote:
>>>
>>> Jordan,
>>>>
>>>> I'd be interested in previewing it. Let me know.
>>>>
>>>> Thanks,
>>>> Neha
>>>>
>>>> On Mon, Jan 16, 2012 at 5:42 PM, Jordan Zimmerman
>>>> <[EMAIL PROTECTED]> wrote:

flavio
junqueira

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