Grokbase Groups HBase dev April 2010
FAQ

On Tue, Apr 6, 2010 at 9:46 AM, Jean-Daniel Cryans wrote:

Yes it is, you will be missing a RS ;)
How do you detect this, though?

It might be useful to add a counter in ZK for region server crashes. If the
master ever notices that a RS goes down, it increments it. Then we can check
the before/after for a job and know when we might have lost some data.

-Todd

General rule when uploading without WAL is if there's a failure, the
job is screwed and that's the tradeoff for speed.

J-D
On Tue, Apr 6, 2010 at 9:36 AM, Todd Lipcon wrote:
On Tue, Apr 6, 2010 at 9:31 AM, Jean-Daniel Cryans <jdcryans@apache.org
wrote:
The issue isn't with the write buffer here, it's the WAL. Your edits
are in the MemStore so as far as your clients can tell, the data is
all persisted. In this case you would need to know when all the
memstores that contain your data are flushed... Best practice when
turning off WAL is force flushing the tables after the job is done,
else you can't guarantee durability for the last edits.
You still can't guarantee durability for any of the edits, since a failure
in the middle of your job is undetectable :)

-Todd

J-D
On Tue, Apr 6, 2010 at 4:02 AM, Lars George wrote:
Hi,

I have an issue where I do bulk import and since WAL is off and a
default write buffer used (TableOutputFormat) I am running into
situations where the MR job completes successfully but not all data is
actually restored. The issue seems to be a failure on the RS side as
it cannot flush the write buffers because the MR overloads the cluster
(usually the .META: hosting RS is the breaking point) or causes the
underlying DFS to go slow and that repercussions all the way up to the
RS's.

My question is, would it make sense as with any other asynchronous IO
to return a Future from the put() that will help checking the status
of the actual server side async flush operation? Or am I misguided
here? Please advise.

Lars


--
Todd Lipcon
Software Engineer, Cloudera


--
Todd Lipcon
Software Engineer, Cloudera

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