[erlang-questions] Mnesia deleting log file

[Jesper Louis Andersen]

This thread makes we wonder about the following section in the Mnesia
documentation:

"If a power failure occurs during the dump, this can cause the randomly
accessed DAT files to become corrupt. If the parameter is set to false,
Mnesia copies the DAT files and target the dump to the new temporary files.
If the dump is successful, the temporary files are renamed to their normal
DAT suffixes. The possibility for unrecoverable inconsistencies in the data
files becomes much smaller with this strategy. However, the actual dumping
of the transaction log becomes considerably slower. The system designer
must decide whether speed or safety is the higher priority."

In UNIX(Posix) a rename(2) call is atomic. The worst case, which is an OS
crash might leave you with a temporary file and the target file, hardlinked
as if ln() was called, but the move itself is atomic, even under OS
crashes. Of course, this assumes you correctly wrote your temporary file,
fsync()'ed the file and then performed the rename() as you should.

With that information in mind, what is the scenario which can make mnesia
fail if the dump_log_update_in_place is set to false? There may be a
window, but the operating system cannot be the provider of this window at
all, unless it is incorrectly implemented or configured[1]. Hence my
question.

[1] The primary concern are write caches on disks which can have a write
hole unless they have proper (working!) battery backup of their cache or
are able to write their memory to a small solid state part of the chip. Or
if you are using ZFS where copy-on-write semantics completely eliminates
the write hole.



On Wed, Nov 22, 2017 at 2:29 PM Michael Schmidt <
Michael.K.Schmidt@REDACTED> wrote:

> Hi Arun,
>
>
>
> If you happen to reboot when Mnesia is writing out its logfile, it can
> become corrupt (and lose data).  We fight this issue on embedded devices a
> lot.
>
>
>
> Look “Configuration Parameters” section here:
>
> http://erlang.org/doc/man/mnesia.html#id69584
>
>
>
> You will want to set dump_log_update_in_place to false.  This will make
> it write out the table to a new file, and rename it the very end.
>
>
>
> One other option to look at is dump_log_time_threshold to change how
> often items are flushed to disk.  For configuration data, you can likely
> set this to a few seconds.
>
>
>
> These options can be set via the sys.config file as well
>
>
>
> Mike
>
>
>
> *From:* erlang-questions-bounces@REDACTED [mailto:
> erlang-questions-bounces@REDACTED] *On Behalf Of *Arun
> *Sent:* Tuesday, November 21, 2017 10:16 PM
> *To:* erlang-questions@REDACTED
> *Subject:* [erlang-questions] Mnesia deleting log file
>
>
>
> Dear all,
>
> I've a program written in erlang which uses Mnesia application as the
> database application. I've a table created by name "configuration_table"
> which stores
>
> certain configurations that need to be persistent. Occasionally, whenever
> I restart my program the following error is thrown by mnesia and I end up
> losing
>
> all the persistent configurations.
>
> I've searched about this problem in the erlang documentation and all it
> tells is "Node not running". What could be the probable cause for this
> problem
>
> and how do I fix it?
>
> Mnesia('test@REDACTED <ommp82_plus@REDACTED>'): Data may be missing,
> Corrupt logfile deleted:
> "/home/utl/mnesia_database/configuration_table.DCL", {node_not_running,
> 'test@REDACTED <ommp82_plus@REDACTED>'}
>
>
> ______________________________________________________________________
> This email has been scanned by the Symantec Email Security.cloud service.
> ______________________________________________________________________
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20171122/f146b783/attachment.htm>