[erlang-questions] Mnesia doesn't report insertion errors when underlying DETS file is full?

Ulf Wiger <>
Sat May 23 18:39:12 CEST 2015


May I suggest trying out Jobs [1]?

There, you can create a sampler which e.g. checks the size of critical tables, and generates overload events, which can automatically throttle incoming traffic, as well as notify subscribers [2].

[1] https://github.com/uwiger/jobs
[2] https://github.com/uwiger/jobs/blob/master/doc/jobs_sampler.md#subscribe-0

BR,
Ulf W


> On 23 May 2015, at 14:25, Éric Pailleau <> wrote:
> 
> Hi, 
> Thanks Ulf.
> It would be probably more smart to warn before to loose data...
> A system event 'almost_full' should be more convenient. It is sometime better to stop the application instead loose data.
> An event on each last 5 percent change could allow to handle different actions. Mail, SMS, or ultimately stop the application on almost_full_99. 
> There is already an event on database inconsistency, it would be a nice enhancement.
> Regards 
> 
> Le 23 mai 2015 13:02, Ulf Wiger <> a écrit :
>> 
>> 
>>> On 23 May 2015, at 10:38, Éric Pailleau <> wrote: 
>>> 
>>> Just wanted to know if mnesia subscribe on system events can raise a 
>>> 
>>> {mnesia_error, Format, Args} 
>>> 
>>> In such case ? 
>> 
>> It *almost* does, if you set mnesia’s debug level to ‘verbose’. ;-) 
>> 
>> The following call is made from mnesia_tm if do_update_op/3 crashes: 
>> 
>>             verbose("do_update in ~w failed: ~p -> {'EXIT', ~p}~n", 
>>                     [Tid, Op, Reason]), 
>> 
>> …which, in its turn ends up calling the following in mnesia_lib: 
>> 
>> important(Format, Args) -> 
>>     save({Format, Args}), 
>>     report_system_event({mnesia_info, Format, Args}). 
>> 
>> The only problem is that the dets operation in question doesn’t crash, but instead returns {error, Reason}, so that particular issue won’t be reported. The value *is* captured by mnesia_tm:do_update/4, and it will return the last non-ok value from the commit. However, this return value is discarded, except in a few cases where it’s captured as debug info. 
>> 
>> Of course, even if it were reported, it would be of the category “oops - your database was just corrupted!”, which is good to know, but not so easy to handle. 
>> 
>> BR, 
>> Ulf W 
>> 
>> Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc. 
>> http://feuerlabs.com 
>> 
>> 
>> 

Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.
http://feuerlabs.com





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