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Zookeeper >> mail # user >> Backups


On 1/19/12 11:30 AM, "Flavio Junqueira" <[EMAIL PROTECTED]> wrote:

>You're not talking about data corruption, are you? It is incorrect
>data that has been introduced by a user or application by mistake. Am
>I getting it right?
>On Jan 19, 2012, at 8:07 PM, Jordan Zimmerman wrote:
>> It's that very replication that creates the need for backups. In
>> there is
>> a user error or a bad injection of data, the error will quickly
>> replicate
>> to all the instances. There's no way to recover without an external
>> backup.
>> -JZ
>> On 1/19/12 10:39 AM, "Flavio Junqueira" <[EMAIL PROTECTED]> wrote:
>>> 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 backup is known to be slightly out of date.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  In these cases, an out-of-date backup may be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