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From Elliott Clark <ecl...@apache.org>
Subject Re: [DISCUSS] No regions on Master node in 2.0
Date Fri, 08 Apr 2016 08:42:41 GMT
# 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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