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From Jimmy Xiang <jxc...@gmail.com>
Subject Re: [DISCUSS] No regions on Master node in 2.0
Date Fri, 08 Apr 2016 15:27:27 GMT
One thing I'd like to say is that it makes the master startup much
more simpler and realiable to put system tables on master.

Even if proc-v2 can solve the problem, it makes things complicated,
right? I prefer to be sure that meta is always available, in a
consistent state.

If we really need to split meta, we should have an option for most
users to have just one meta region, and keep it on master.


On Fri, Apr 8, 2016 at 8:03 AM, Matteo Bertozzi <theo.bertozzi@gmail.com> wrote:
> # Without meta on master, we double assign and lose data.
>
> I doubt meta on master solve this problem.
> This has more to do on the fact that balancer, assignment, split, merge
> are disjoint operations that are not aware of each other.
> also those operation in general consist of multiple steps and if the master
> crashes you may end up in an inconsistent state.
>
> this is what proc-v2 should solve. since we are aware of each operation
> there is no chance of double assignment and similar by design.
>
> The master doesn't need the full meta to operate properly
> it just need the "state" (at which point of the operation am I).
> which is the wal of proc-v2. given that we can split meta or meta
> remote without any problem. since we only have 1 update to meta to
> update the location when the assignment is completed.
>
> also at the moment the master has a copy of the information in meta.
> a map with the RegionInfo, state and locations. but we are still doing
> a query on meta instead of using that local map directly.
> if we move meta on master we can remove that extra copy, but that
> will tight together meta and master making impossible to offload meta, if
> we need to.
>
>
> In my opinion with the new assignment you have all the main problem solved.
> we can keep regions on master as we have now,
> so you can configure it to get more performance (avoid the remote rpc).
> but our design should allow meta to be split and to be hosted somewhere
> else.
>
> Matteo
>
>
> On Fri, Apr 8, 2016 at 2:08 AM, 张铎 <palomino219@gmail.com> wrote:
>
>> Agree on the performance concerns. IMO we should not hurt the performance
>> of small(maybe normal?) clusters when scaling for huge clusters.
>> And I also agree that the current implementation which allows Master to
>> carry system regions is not good(sorry for the chinglish...). At least, it
>> makes the master startup really complicated.
>>
>> So IMO, we should let the master process or master machine to also carry
>> system regions, but in another way. Start another RS instance on the same
>> machine or in the same JVM? Or build a new storage based on the procedure
>> store and convert it to a normal table when it is too large?
>>
>> Thanks.
>>
>> 2016-04-08 16:42 GMT+08:00 Elliott Clark <eclark@apache.org>:
>>
>> > # Without meta on master, we double assign and lose data.
>> >
>> > That is currently a fact that I have seen over and over on multiple
>> loaded
>> > clusters. Some abstract clean up of deployment vs losing data is a
>> > no-brainer for me. Master assignment, region split, region merge are all
>> > risky, and all places that HBase can lose data. Meta being hosted on the
>> > master makes communication easier and less flakey. Running ITBLL on a
>> loop
>> > that creates a new table every time, and without meta on master
>> everything
>> > will fail pretty reliably in ~2 days. With meta on master things pass
>> MUCH
>> > more.
>> >
>> > # Master hosting the system tables locates the system tables as close as
>> > possible to the machine that will be mutating the data.
>> >
>> > Data locality is something that we all work for. Short circuit local
>> reads,
>> > Caching blocks in jvm, etc. Bringing data closer to the interested party
>> > has a long history of making things faster and better. Master is in
>> charge
>> > of just about all mutations of all systems tables. It's in charge of
>> > changing meta, changing acls, creating new namespaces, etc. So put the
>> > memstore as close as possible to the system that's going to mutate meta.
>> >
>> > # If you want to make meta faster then moving it to other regionservers
>> > makes things worse.
>> >
>> > Meta can get pretty hot. Putting it with other regions that clients will
>> be
>> > trying to access makes everything worse. It means that meta is competing
>> > with user requests. If meta gets served and other requests don't, causing
>> > more requests to meta; or requests to user regions get served and other
>> > clients get starved.
>> > At FB we've seen read throughput to meta doubled or more by swapping it
>> to
>> > master. Writes to meta are also much faster since there's no rpc hop, no
>> > queueing, to fighting with reads. So far it has been the single biggest
>> > thing to make meta faster.
>> >
>> >
>> > On Thu, Apr 7, 2016 at 10:11 PM, Stack <stack@duboce.net> wrote:
>> >
>> > > I would like to start a discussion on whether Master should be carrying
>> > > regions or not. No hurry. I see this thread going on a while and what
>> > with
>> > > 2.0 being a ways out yet, there is no need to rush to a decision.
>> > >
>> > > First, some background.
>> > >
>> > > Currently in the master branch, HMaster hosts 'system tables': e.g.
>> > > hbase:meta. HMaster is doing more than just gardening the cluster,
>> > > bootstrapping and keeping all up and serving healthy as in branch-1; in
>> > > master branch, it is actually in the write path for the most critical
>> > > system regions.
>> > >
>> > > Master is this way because HMaster and HRegionServer servers have so
>> much
>> > > in common, they should be just one binary, w/ HMaster as any other
>> server
>> > > with the HMaster function a minor appendage runnable by any running
>> > > HRegionServer.
>> > >
>> > > I like this idea, but the unification work was just never finished.
>> What
>> > is
>> > > in master branch is a compromise. HMaster is not a RegionServer but a
>> > > sort-of RegionServer doing part serving. So we have HMaster role, a new
>> > > part-RegionServer-carrying-special-regions role and then a full-on
>> > > HRegionServer role. We need to fix this messyness. We could revert to
>> > plain
>> > > branch-1 roles or carrying the
>> > > HMaster-function-is-something-any-RegionServer-could-execute through to
>> > > completion.
>> > >
>> > > More background from a time long-past with good comments by the likes
>> of
>> > > our Francis Liu and Mighty Matteo Bertozzi are here [1], on unifying
>> > master
>> > > and meta-serving. Slightly related are old discussions on being able to
>> > > scale by splitting meta with good comments by our Elliott Clark [2].
>> > >
>> > > Also for consideration, the landscape has since changed. [1] was
>> written
>> > > before we had ProcedureV2 available to us where we could record
>> > > intermediate transition states local to the Master rather than remote
>> as
>> > > intermediate updates to an hbase:meta over rpc running on another node.
>> > >
>> > > Enough on the background.
>> > >
>> > > Let me provoke discussion by making the statement that we should undo
>> > > HMaster carrying any regions ever; that the HMaster function is work
>> > enough
>> > > for a single dedicated server and that it important enough that it
>> cannot
>> > > take a background role on a serving RegionServer (I could go back from
>> > this
>> > > position if evidence HMaster role could be backgrounded). Notions of a
>> > > Master carrying system tables only are just not on given system tables
>> > will
>> > > be too big for a single server especially when hbase:meta is split (so
>> we
>> > > can scale). This simple distinction of HMaster and RegionServer roles
>> is
>> > > also what our users know and have gotten used to so needs to be a good
>> > > reason to change it (We can still pursue the single binary that can do
>> > > HMaster or HRegionServer role determined at runtime).
>> > >
>> > > Thanks,
>> > > St.Ack
>> > >
>> > > 1.
>> > >
>> > >
>> >
>> https://docs.google.com/document/d/1xC-bCzAAKO59Xo3XN-Cl6p-5CM_4DMoR-WpnkmYZgpw/edit#heading=h.j5yqy7n04bkn
>> > > 2.
>> > >
>> > >
>> >
>> https://docs.google.com/document/d/1eCuqf7i2dkWHL0PxcE1HE1nLRQ_tCyXI4JsOB6TAk60/edit#heading=h.80vcerzbkj93
>> > >
>> >
>>

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