Re: BUG #15636: PostgreSQL 11.1 pg_basebackup backup to a CIFSdestination throws fsync error at end of backup

Thanks Thomas that certainly makes sense considering the commit comment as well: "Directory operations under CIFS/SMB2/SMB3 are synchronous, so fsync()".

I think will probably not use the --no-sync option then and handle for the EINVAL messages.

On Thu, Feb 14, 2019 at 5:31 PM Thomas Munro <thomas.munro@enterprisedb.com> wrote:

Hi John,
Unfortunately that applies only to directories, not to regular files
inside them.
--no-sync should get you past the problem as a workaround for now, but
then if your server(s) crash/lose power in the following seconds the
data might not be on disk.  Fsync just means that the program doesn't
return until the data is flushed, but within some short period of time
the data will be flushed anyway, and power failure in the short window
before that is really unlikely, it's more a theoretical issue that we
database hackers like to worry about.

On Fri, Feb 15, 2019 at 11:22 AM John Klann <jk7255@gmail.com> wrote:
>
> Ah thank you this all makes sense.
>
> If CIFS is synchronous and and fsync'ing is not necessary then running with the --no-sync option should be safe and possibly more performant correct?
>
> John
>
> On Thu, Feb 14, 2019 at 5:06 PM Thomas Munro <thomas.munro@enterprisedb.com> wrote:
>>
>> On Fri, Feb 15, 2019 at 10:15 AM PG Bug reporting form
>> <noreply@postgresql.org> wrote:
>> >                                 pg_basebackup: could not fsync file
>> > "/cifs/backups/<backupDirectoryName>/basebkp/base/1": Invalid argument
>> >                                 pg_basebackup: could not fsync file
>>
>> Hmm, it looks like your system gives EINVAL when you try to fsync a
>> directory.  Perhaps we should teach fsync__fname() about that here:
>>
>>         /*
>>          * Some OSes don't allow us to fsync directories at all, so we
>> can ignore
>>          * those errors. Anything else needs to be reported.
>>          */
>>         if (returncode != 0 && !(isdir && errno == EBADF))
>>         {
>>                 fprintf(stderr, _("%s: could not fsync file \"%s\": %s\n"),
>>                                 progname, fname, strerror(errno));
>>                 (void) close(fd);
>>                 return -1;
>>         }
>>
>> EINVAL actually makes more sense to me than EBADF for a filesystem
>> that can't fsync directories.  From POSIX: EINVAL = "The fildes
>> argument does not refer to a file on which this operation is
>> possible." vs EBADF "The fildes argument is not a valid descriptor."
>> It *is* a valid descriptor, it's just not a valid operation
>> (apparently).
>>
>> Quick googling on the topic tells me that CIFS directory operations
>> are "synchronous", so fsync'ing isn't necessary.  However, they only
>> made it silently do nothing in a recent version:
>>
>> https://github.com/torvalds/linux/commit/6e70c267e68d77679534dcf4aaf84e66f2cf1425
>>
>> Presumably before that you get EINVAL because there is no handler
>> registered.  The commit message even mentions that this was breaking
>> stuff like us.
>>
>> --
>> Thomas Munro
>> http://www.enterprisedb.com



--
Thomas Munro
http://www.enterprisedb.com