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From David Alves <davidral...@gmail.com>
Subject Re: Data inconsistency after restart
Date Thu, 07 Dec 2017 20:39:36 GMT
Hi Petter

   I'd like to clarify what exactly happened and exactly what are you
referring to as "inconsistency".
   From what I understand of the first error you observed, the Kudu was
underprovisioned, memory wise, and the ingest jobs/queries failed. Is that
right? Since Kudu doesn't have atomic multi-row writes, it's currently
expected in this case that you'll end up with partially written data.
   If you tried the same job again, and it succeeded, for certain types of
operation (UPSERT, INSERT IGNORE) then the remaining rows would be written
and all the data would be there as expected.
   I'd like to distinguish this lack of atomicity on multi-row transactions
from "inconsistency", which is what you might observe if an operation
didn't fail, but you couldn't see all the data. For this latter case there
are options you can choose to avoid any inconsistency.

Best
David



On Wed, Dec 6, 2017 at 4:26 AM, Petter von Dolwitz (Hem) <
petter.von.dolwitz@gmail.com> wrote:

> Thanks for your reply Andrew!
>
> >How did you verify that all the data was inserted and how did you find
> some data missing?
> This was done using Impala. We counted the rows for groups representing
> the chunks we inserted.
>
> >Following up on what I posted, take a look at https://kudu.apache.org/doc
> s/transaction_semantics.html#_read_operations_scans. It seems definitely
> possible that not all of the rows had finished inserting when counting, or
> that the scans were sent to a stale replica.
> Before we shut down we could only see the following in the logs. I.e., no
> sign that ingestion was still ongoing.
>
> kudu-tserver.ip-xx-yyy-z-nnn.root.log.INFO.20171201-065232.90314:I1201
> 07:27:35.010694 90793 maintenance_manager.cc:383] P
> a38902afefca4a85a5469d149df9b4cb: we have exceeded our soft memory limit
> (current capacity is 67.52%).  However, there are no ops currently runnable
> which would free memory.
>
> Also the (cloudera) metric total_kudu_rows_inserted_rate_across_kudu_replicas
> showed zero.
>
> Still it seems like some data became inconsistent after restart. But if
> the maintenance_manager performs important jobs that are required to ensure
> that all data is inserted then I can understand why we ended up with
> inconsistent data. But, if I understand you correct,  you are saying that
> these jobs are not critical for ingestion. In the link you provided I read
> "Impala scans are currently performed as READ_LATEST and have no
> consistency guarantees.". I would assume this means that it does not
> guarantee consistency if new data is inserted but should give valid (and
> same) results if no new data is inserted?
>
> I have not tried the ksck tool yet. Thank you for reminding. I will have a
> look.
>
> Br,
> Petter
>
>
> 2017-12-06 1:31 GMT+01:00 Andrew Wong <awong@cloudera.com>:
>
>> How did you verify that all the data was inserted and how did you find
>>> some data missing? I'm wondering if it's possible that the initial
>>> "missing" data was data that Kudu was still in the process of inserting
>>> (albeit slowly, due to memory backpressure or somesuch).
>>>
>>
>> Following up on what I posted, take a look at https://kudu.apache.org/doc
>> s/transaction_semantics.html#_read_operations_scans. It seems definitely
>> possible that not all of the rows had finished inserting when counting, or
>> that the scans were sent to a stale replica.
>>
>> On Tue, Dec 5, 2017 at 4:18 PM, Andrew Wong <awong@cloudera.com> wrote:
>>
>>> Hi Petter,
>>>
>>> When we verified that all data was inserted we found that some data was
>>>> missing. We added this missing data and on some chunks we got the
>>>> information that all rows were already present, i.e impala says something
>>>> like Modified: 0 rows, nnnnnnn errors. Doing the verification again now
>>>> shows that the Kudu table is complete. So, even though we did not insert
>>>> any data on some chunks, a count(*) operation over these chunks now returns
>>>> a different value.
>>>
>>>
>>> How did you verify that all the data was inserted and how did you find
>>> some data missing? I'm wondering if it's possible that the initial
>>> "missing" data was data that Kudu was still in the process of inserting
>>> (albeit slowly, due to memory backpressure or somesuch).
>>>
>>> Now to my question. Will data be inconsistent if we recycle Kudu after
>>>> seeing soft memory limit warnings?
>>>
>>>
>>> Your data should be consistently written, even with those warnings.
>>> AFAIK they would cause a bit of slowness, not incorrect results.
>>>
>>> Is there a way to tell when it is safe to restart Kudu to avoid these
>>>> issues? Should we use any special procedure when restarting (e.g. only
>>>> restart the tablet servers, only restart one tablet server at a time or
>>>> something like that)?
>>>
>>>
>>> In general, you can use the `ksck` tool to check the health of your
>>> cluster. See https://kudu.apache.org/docs/command_line_tools_referenc
>>> e.html#cluster-ksck for more details. For restarting a cluster, I would
>>> recommend taking down all tablet servers at once, otherwise tablet replicas
>>> may try to replicate data from the server that was taken down.
>>>
>>> Hope this helped,
>>> Andrew
>>>
>>> On Tue, Dec 5, 2017 at 10:42 AM, Petter von Dolwitz (Hem) <
>>> petter.von.dolwitz@gmail.com> wrote:
>>>
>>>> Hi Kudu users,
>>>>
>>>> We just started to use Kudu (1.4.0+cdh5.12.1). To make a baseline for
>>>> evaluation we ingested 3 month worth of data. During ingestion we were
>>>> facing messages from the maintenance threads that a soft memory limit were
>>>> reached. It seems like the background maintenance threads stopped
>>>> performing their tasks at this point in time. It also so seems like the
>>>> memory was never recovered even after stopping ingestion so I guess there
>>>> was a large backlog being built up. I guess the root cause here is that we
>>>> were a bit too conservative when giving Kudu memory. After a reststart a
>>>> lot of maintenance tasks were started (i.e. compaction).
>>>>
>>>> When we verified that all data was inserted we found that some data was
>>>> missing. We added this missing data and on some chunks we got the
>>>> information that all rows were already present, i.e impala says something
>>>> like Modified: 0 rows, nnnnnnn errors. Doing the verification again now
>>>> shows that the Kudu table is complete. So, even though we did not insert
>>>> any data on some chunks, a count(*) operation over these chunks now returns
>>>> a different value.
>>>>
>>>> Now to my question. Will data be inconsistent if we recycle Kudu after
>>>> seeing soft memory limit warnings?
>>>>
>>>> Is there a way to tell when it is safe to restart Kudu to avoid these
>>>> issues? Should we use any special procedure when restarting (e.g. only
>>>> restart the tablet servers, only restart one tablet server at a time or
>>>> something like that)?
>>>>
>>>> The table design uses 50 tablets per day (times 90 days). It is 8 TB of
>>>> data after 3xreplication over 5 tablet servers.
>>>>
>>>> Thanks,
>>>> Petter
>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> Andrew Wong
>>>
>>
>>
>>
>> --
>> Andrew Wong
>>
>
>

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