Re: POC and rebased patch for CSN based snapshots

>> I have researched your patch which is so great, in the patch only data

>> out of 'global_snapshot_defer_time' can be vacuum, and it keep dead

>> tuple even if no snapshot import at all,right?

>>

>> I am thanking about a way if we can start remain dead tuple just before

>> we import a csn snapshot.

>>

>> Base on Clock-SI paper, we should get local CSN then send to shard nodes,

>> because we do not known if the shard nodes' csn bigger or smaller then

>> master node, so we should keep some dead tuple all the time to support

>> snapshot import anytime.

>>

>> Then if we can do a small change to CLock-SI model, we do not use the

>> local csn when transaction start, instead we touch every shard node for

>> require their csn, and shard nodes start keep dead tuple, and master node

>> choose the biggest csn to send to shard nodes.

>>

>> By the new way, we do not need to keep dead tuple all the time and do

>> not need to manage a ring buf, we can give to ball to 'snapshot too old'

>> feature. But for trade off, almost all shard node need wait.

>> I will send more detail explain in few days.

>I think, in the case of distributed system and many servers it can be

>bottleneck.

>Main idea of "deferred time" is to reduce interference between DML

>queries in the case of intensive OLTP workload. This time can be reduced

>if the bloationg of a database prevails over the frequency of

>transaction aborts.

OK there maybe a performance issue, and I have another question about Clock-SI.

For example we have three  nodes, shard1(as master), shard2, shard3, which

(time of node2) > (time of node2) > (time of node3), and you can see a picture:

http://movead.gitee.io/picture/blog_img_bad/csn/clock_si_question.png 

As far as I know about Clock-SI, left part of the blue line will setup as a snapshot

if master require a snapshot at time t1. But in fact data A should in snapshot but

not and data B should out of snapshot but not.

If this scene may appear in your origin patch? Or something my understand about

Clock-SI is wrong?