[erlang-questions] mnesia sync_transactions not fsynced?

[Dan Gudmundsson]

Disk log cashes writes as well so you can loose many writes when
halting the process.

/Dan

On Fri, Oct 28, 2011 at 1:05 AM, Jon Watte <jwatte@REDACTED> wrote:
>
>>
>> runtest(N) ->
>>    mnesia:sync_transaction(
>>        fun () -> mnesia:write(testtab, #testrec{id = N}, write) end),
>>    mnesia:sync_transaction(fun () -> mnesia:delete({testtab, N}) end),
>>    runtest(N-1).
>>
>
>
> This code runs two separate transactions.  One to add a record, and one to
> delete a record. If you bring down the node after the first one has
> committed, but before the second one has committed, then you will see one
> record when you start up again.
> Are you claiming that if you run this from 100 through 0, and crash the node
> at 10, you may see a record with the value 40 in the database? If that is
> the case, then the cause is simply that all of the transactions from 39 .. 0
> never wrote to disk before the crash. This can, roughly, be thought of as a
> SQL database achieving some particular point in its binlogs, but nothing
> beyond that point.
> The observation here is that mnesia:sync_transaction(), under the conditions
> observed, does not fulfill the "Durable" part of ACID transactions -- a
> committed transaction (writing value 39, say) was observed by the system,
> but after re-start, that point was not durable.
> Whether mnesia:sync_transaction() "should be" durable is not something I'm
> qualified to answer :-) However, reading the documentation, it certainly
> *sounds* as if it should provide durability:
>>
>> This function waits until data have been committed and logged to disk (if
>> disk is used) on every involved node before it returns
>
> Thus, assuming that you don't have a kernel/device-based write
> cache/power-fail outage (but simply an Erlang process death outage), then I
> would expect the last record seen on disk to be the last record observed in
> the test function.
>
> Sincerely,
> jw
>
>
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