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From aaron morton <aa...@thelastpickle.com>
Subject Re: Commit log periodic sync?
Date Mon, 27 Aug 2012 08:08:51 GMT
> Brutally. kill -9.
that's fine. I was thinking about reboot -f -n

> We are wondering if the fsync of the commit log was working.
I would say yes only because there other reported problems. 

I think case I would not expect to see data lose. If you are still in a test scenario can
you try to reproduce the problem ? If possible can you reproduce it with a single node ?

Cheers


-----------------
Aaron Morton
Freelance Developer
@aaronmorton
http://www.thelastpickle.com

On 25/08/2012, at 11:00 AM, rubbish me <rubbish.me@googlemail.com> wrote:

> Thanks, Aaron, for your reply - please see the inline.
> 
> 
> On 24 Aug 2012, at 11:04, aaron morton wrote:
> 
>>> - we are running on production linux VMs (not ideal but this is out of our hands)
>> Is the VM doing anything wacky with the IO ?
> 
> Could be.  But I thought we would ask here first.  This is a bit difficult to prove cos
we dont have the control over these VMs.
> 
>>  
>> 
>>> As part of a DR exercise, we killed all 6 nodes in DC1,
>> Nice disaster. Out of interest, what was the shutdown process ?
> 
> Brutally. kill -9.
> 
> 
>> 
>>> We noticed that data that was written an hour before the exercise, around the
last memtables being flushed,was not found in DC1. 
>> To confirm, data was written to DC 1 at CL LOCAL_QUORUM before the DR exercise. 
>> 
>> Was the missing data written before or after the memtable flush ? I'm trying to understand
if the data should have been in the commit log or the memtables. 
> 
> Missing data was those written after the last flush.  These data was retrievable before
the DR exercise.
> 
>> 
>> Can you provide some more info on how you are detecting it is not found in DC 1?
>> 
> 
> We tried hector, consistencylevel=local quorum.  We had missing column or the whole row.
 
> 
> We tried cassandra-cli on DC1 nodes, same.
> 
> However once we run the same query on DC2, C* must have then done a read-repair. That
particular piece of result data would appear in DC1 again.
> 
> 
>>> If we understand correctly, commit logs are being written first and then to disk
every 10s. 
>> Writes are put into a bounded queue and processed as fast as the IO can keep up.
Every 10s a sync messages is added to the queue. Not that the commit log segment may rotate
at any time which requires a sync. 
>> 
>> A loss of data across all nodes in a DC seems odd. If you can provide some more information
we may be able to help. 
> 
> 
> We are wondering if the fsync of the commit log was working.  But we saw no errors /
warning in logs.  Wondering if there is way to verify....
> 
> 
>> 
>> Cheers
>> 
>> -----------------
>> Aaron Morton
>> Freelance Developer
>> @aaronmorton
>> http://www.thelastpickle.com
>> 
>> On 24/08/2012, at 6:01 AM, rubbish me <rubbish.me@googlemail.com> wrote:
>> 
>>> Hi all
>>> 
>>> First off, let's introduce the setup. 
>>> 
>>> - 6 x C* 1.1.2 in active DC (DC1), another 6 in another (DC2)
>>> - keyspace's RF=3 in each DC
>>> - Hector as client.
>>> - client talks only to DC1 unless DC1 can't serve the request. In which case
talks only to DC2
>>> - commit log was periodically sync with the default setting of 10s. 
>>> - consistency policy = LOCAL QUORUM for both read and write. 
>>> - we are running on production linux VMs (not ideal but this is out of our hands)
>>> -----
>>> As part of a DR exercise, we killed all 6 nodes in DC1, hector starts talking
to DC2, all the data was still there, everything continued to work perfectly. 
>>> 
>>> Then we brought all nodes, one by one, in DC1 up. We saw a message saying all
the commit logs were replayed. No errors reported.  We didn't run repair at this time. 
>>> 
>>> We noticed that data that was written an hour before the exercise, around the
last memtables being flushed,was not found in DC1. 
>>> 
>>> If we understand correctly, commit logs are being written first and then to disk
every 10s. At worst we lost the last 10s of data. What could be the cause of this behaviour?

>>> 
>>> With the blessing of C* we could recovered all these data from DC2. But we would
like to understand why. 
>>> 
>>> Many thanks in advanced. 
>>> 
>>> Amy
>>> 
>>> 
>> 
> 


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