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From Karthik Ranganathan <kranganat...@facebook.com>
Subject RE: HBASE-2312 discussion
Date Wed, 17 Mar 2010 01:04:59 GMT
@Stack: With the overwrite=false, I think option #2 looks fine.

@Todd: By "bound number of log files", I meant that the if the latest log file is log.N, the
master would try to open log.N+1, log.N+2, log.N+3 etc until one of it succeeds and the RS
cannot open more log files after that one. So the master "bounds" the number of times the
log file is opened.

I am a little nervous about the master backing off on detecting the RS's progress - because
the RS has already lost its zk lease. Not sure that if the master backs off, this will allow
everything to proceed smoothly. But probably calling sync() on zk makes sense. Will think
about this some more.

I too like option #3 because it's a useful pattern, but it was initially much easier to reason
about #2. Of course #1 is the easiest either way. Again, let me think about this more.


-----Original Message-----
From: Todd Lipcon [mailto:todd@cloudera.com] 
Sent: Tuesday, March 16, 2010 3:18 PM
To: hbase-dev@hadoop.apache.org
Subject: Re: HBASE-2312 discussion

On Tue, Mar 16, 2010 at 1:49 PM, Stack <stack@duboce.net> wrote:

> Karthik:
> Thanks for looking into this.
> Reading over the issue, you think option #2 "not clean" before Todd
> proposes changing overwrite to false.  Do you still think it so?  If
> not, then option #2 seems straight-forward.
> While option #3 is more code, its attractive in that its a pattern we
> might take on to solve other filesystem transitions; e.g. recovering
> failed compactions.  Do you think option #3 harder to verify?  The
> 'chasing logs' would be hard to do up in tests.

I think the "chasing logs" thing is actually avoidable pretty easily. I
commented on HBASE-2312 with thoughts there.

Regarding option 1, I'm not entirely against the new HDFS API, so if others
think it's a good solution we may as well go with it (we're already
requiring patched HDFS for sync, so another simple patch isn't a huge deal).

Regarding option 2, not sure what you mean be "The number of log files the
RS can create will be bound." -- can you explain?

Stack's point that #3 is a useful pattern for lots of transitions seems very
valid to me as well.

> Thanks,
> St.Ack
> P.S. Tsuna, up https://issues.apache.org/jira/browse/HBASE-2238 there
> is some discussion of why hdfs state changes has to be managed in the
> filesystem only, of how state can't bridge filesystem and zookeeper.
> On Tue, Mar 16, 2010 at 11:13 AM, Karthik Ranganathan
> <kranganathan@facebook.com> wrote:
> > Hey guys,
> >
> > Just wanted to close on which solution we wanted to pick for this issue -
> I was thinking about working on this one. There are 3 possibilities here. I
> have briefly written up the issue and the three solutions below.
> >
> > Issue:
> > There is a very corner case when bad things could happen(ie data loss):
> > 1) RS #1 is going to roll its HLog - not yet created the new one, old one
> will get no more writes
> > 2) RS #1 enters GC Pause of Death
> > 3) Master lists HLog files of RS#1 that is has to split as RS#1 is dead,
> starts splitting
> > 4) RS #1 wakes up, created the new HLog (previous one was rolled) and
> appends an edit - which is lost
> >
> > Solution 1:
> > 1) Master detects RS#1 is dead
> > 2) The master renames the /hbase/.logs/<regionserver name> directory to
> something else (say /hbase/.logs/<regionserver name>-dead)
> > 3) Add mkdir support (as opposed to mkdirs) to HDFS - so that a file
> create fails if the directory doesn't exist. Dhruba tells me this is very
> doable.
> > 4) RS#1 comes back up and is not able create the new hlog. It restarts
> itself.
> > NOTE: Need another HDFS API to be supported, Todd wants to avoid this.
> This API exists in Hadoop 0.21, but is not back-ported to 0.20.
> >
> > Solution 2:
> > 1) RS #1 has written log.1, log.2, log.3
> > 2) RS #1 is just about to write log.4 and enters gc pause before doing so
> > 3) Master detects RS #1 dead
> > 4) Master sees log.1, log.2, log.3. It then opens log.3 for append and
> also creates log.4 as a lock
> > 5) RS #1 wakes up and isn't allowed to write to either log.3 or log.4
> since HMaster holds both.
> > NOTE:  This changes the log file names, changes the create mode of the
> log files from overwrite = true to false. Master needs to create the last
> log file and open it in append mode to prevent RS from proceeding. RS will
> fail if it cannot create the next log file. The number of log files the RS
> can create will be bound.
> >
> > Solution 3:
> > 1) Write "intend to roll HLog to new file hlog.N+1" to hlog.N
> > 2) Open hlog.N+1 for append
> > 3) Write "finished rolling" to hlog.N
> > 4) continue writing to hlog.N+1
> > NOTE: This requires new types edits to go into the log file - "intent to
> roll" and "finished roll". Master has to open the last log file for append
> first. Also, master has to "chase" log files created by the region server
> (please see the issue for details) as there is an outside chance of log
> files rolling when the GC pause happens.
> >
> > In my opinion, from the perspective of code simplicity, I would rank the
> solutions as 1 being simplest, then 2, then 3. Since 1 needs another HDFS
> API, I was thinking that 2 seemed simpler to do and easier to verify
> correctness.
> >
> > What are your thoughts?
> >
> > Thanks
> > Karthik
> >
> >
> >

Todd Lipcon
Software Engineer, Cloudera

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