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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-12233) Bring back root table
Date Sat, 11 Oct 2014 05:32:33 GMT

    [ https://issues.apache.org/jira/browse/HBASE-12233?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14168009#comment-14168009

stack commented on HBASE-12233:

bq.  We found a 5x win on meta versions. 

5x in what? i/o? memory used?  Ability to cache?  Shrinkage in HDFS size occupied by meta?

bq. We haven't tried meta region replicas.

When it lands, a while yet it seems, this will scale read i/o only.  Not write i/o.  It won't
do anything about the size of meta in the fs.  We'll waste mem caching the same stuff multiple
times, once per replica.  Also, IMO regards stale reads of meta I'd suggest we not be cavalier.
 Methinks it will make for a new class of problems.

bq.  We haven't tried allowing a shared cache infront of meta.

At first blush, adding an external system to manage our cache seems way more complicated than
adding back an extra tier.  I'd have to hear more.

bq. We haven't tried a more compact meta table representation

That'd work for the in-memory representation -- being able to represent bigger clusters in-mem
-- but it does nothing to address i/o carried by meta-carrying server nor does it address
size of meta in HDFS (write-amplification continually rewriting big files).  We'll also be
burning CPU at a higher rate going from compact representation to pb over and over again.

bq. We haven't tried picking smaller split keys.

Fair point.  We should do this for sure.  You think this would make difference when hosting
1M regions or 50M?  How much?  10/20%?

bq. We haven't tried stripe compactions on meta.

Stripe compactions works best when time-series writes.  It makes the compaction story worse
when writes are evenly distributed.  Meta could be of either type in any particular deploy.

bq. We haven't gotten block encoding working for meta.

Doesn't address i/o, caching (we can cache encoded but currently we just dumbly decompress
each access -- would have to add smarts), and block encoding would probably help some with
size-on-disk but what, cut size by 50% max?  Maybe.  3G to 1.5G region when 1M regions at
best?  It'd still be too big?

bq. While split meta helps maybe 0.1% of HBase's users.

Factor in that we want to pivot and tell folks that small regions is the way to go (compactions):
i.e. 10x or 100x what they are carrying currently... so being simplisitic your 0.1% becomes
1% or 10% .  Also this fraction are actually our most critical users, our 'fortune' 100 as
it were, the reason for hbase, the folks who use us because they need to scale.

bq. he NN is still a bottle neck on cluster size

Yeah. But that is another issue we should address separately (it so happens that the folks
proposing this patch may be able to help  in this regard).

bq. So making every user and every cluster feel the pain of increased complexity and mttr....

...lets discuss this.  Its a problem I agree.

bq. ....so that we the HBase developers can perform a thought experiment .....

This is not a thought experiment as I understand it.  The lads are up against a scaling problem.

bq. I am for sure against bringing root back in for branch-1 

For sure root will not be back for branch-1. That said, I do not preclude a patch that community
brothers and sisters might need that adds root back as a bridge across versions as long as
it behind a million safety valves and switches so it does not disturb the mainline code paths
in case they need a bridge to branch-1 and beyond. 

This is a tough one.  Should we go to the mailing list with this?  I can rehash the doc I
wrote up over on the 1M/50M JIRA or I can paraphrase bits of  the nice job [~mantonov] did
summarizing this issue in his recent meetup talk.

Good on you [~eclark]

> Bring back root table
> ---------------------
>                 Key: HBASE-12233
>                 URL: https://issues.apache.org/jira/browse/HBASE-12233
>             Project: HBase
>          Issue Type: Sub-task
>            Reporter: Virag Kothari
>            Assignee: Virag Kothari
>         Attachments: HBASE-12233.patch
> First step towards splitting meta.

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