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Zookeeper, mail # user - Recovery time (was: Maximum size of a snapshot)

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Recovery time (was: Maximum size of a snapshot)
Flavio Junqueira 2013-07-17, 10:30
Moving the discussion to dev but keeping user on CC.

Let's step back. The reason why we started the latest discussion in this thread was because Kishore is concerned about recovery time. There are a number of improvements we have been looking at for the next release, let me go over my current understanding of the main points that add to the recovery time:

1- Before we even start leader election, each server loads state from disk to determine its last zxid. The last zxid is used in the election;
2- Once the leader is elected, it loads state from disk and take a snapshot. Loading the database again is unecessary (ZOOKEEPER-1642) and the snapshot adds latency. In fact, it is not even correct to have it there (ZOOKEEPER-1558).
3- A follower takes a snapshot before acknowledging the NEWLEADER message, so the leader has to wait until a quorum of followers finishes their snapshot.

The proposal I've heard here is to touch (1). For now, I'd rather keep (1) as is and focus on fixing (2). We might be able to do something about (3) and I'm actually not sure if there has been a discussion about it or not.


On Jul 17, 2013, at 5:40 AM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:

> Client will get session expire event only when a server explicitly tells
> the client. So any established sessions will remain in a disconnected
> state during the period
> So my comment about the need for longer session timeout might be
> incorrect. While the quorum is down during leader election, session won't
> expire during this period. When the quorum comes back, the client have to
> reconnect within session timeout in order to resume the session.  However,
> client won't be able to issue any read/write request or create a new
> session while the quorum is down.
> However, some application may need a stronger consistency guarantee. They
> will have a special logic to abort the client if it was disconnected for
> an extended period. This is because the client won't be able to tell if
> the quorum is down or there is a network partition between the client and
> the quorum.
> --
> Thawan Kooburat
> On 7/16/13 6:46 PM, "kishore g" <[EMAIL PROTECTED]> wrote:
>> Thanks Thawan. Another question to follow up, so lets say client c1 is
>> connected to leader and leader fails. Now c1 is trying to connect to
>> another zk server but all servers are busy loading snapshot and can take a
>> minute or two. According to Flavio zk servers dont accept any request
>> while
>> synchronization, but most clients dont keep that high connection timeout.
>> So does this mean clients will timeout on connection?. Is my understanding
>> correct or zk servers will accept connection requests but reject
>> read/write
>> requests.
>> thanks,
>> Kishore G
>> On Tue, Jul 16, 2013 at 3:45 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:
>>> There is a plan to work on this optimization ZOOKEEPER-1674.
>>> --
>>> Thawan Kooburat
>>> On 7/16/13 1:37 PM, "kishore g" <[EMAIL PROTECTED]> wrote:
>>>> All servers in the quorum reading the snapshot from disk as part of the
>>>> synchronization phase. From Thawan's email it looks like when ever
>>> there
>>>> is
>>>> a leader election, all zk servers read the snapshot from disk. I am not
>>>> sure why all servers should reload the snapshot from disk as this
>>>> increases
>>>> unavailability time.
>>>> On Tue, Jul 16, 2013 at 12:35 PM, Flavio Junqueira
>>>> <[EMAIL PROTECTED]>wrote:
>>>>> The synchronization phase is part of the protocol and we use it to
>>>>> guarantee that we expose a consistent view of the state. During the
>>>>> synchronization phase, servers do not accept requests.
>>>>> Which behavior are you proposing we change, Kishore?
>>>>> -Flavio
>>>>> On Jul 16, 2013, at 7:04 PM, kishore g <[EMAIL PROTECTED]> wrote:
>>>>>> Thanks for clarification Flavio. Does this mean during the leader
>>>>> election,