Re: Direct I/O

On 2023-04-12 We 10:23, Dagfinn Ilmari Mannsåker wrote:
> Andrew Dunstan<andrew(at)dunslane(dot)net> writes:
>
>> On 2023-04-12 We 01:48, Thomas Munro wrote:
>>> On Wed, Apr 12, 2023 at 3:04 PM Thomas Munro<thomas(dot)munro(at)gmail(dot)com> wrote:
>>>> On Wed, Apr 12, 2023 at 2:56 PM Christoph Berg<myon(at)debian(dot)org> wrote:
>>>>> I'm hitting a panic in t_004_io_direct. The build is running on
>>>>> overlayfs on tmpfs/ext4 (upper/lower) which is probably a weird
>>>>> combination but has worked well for building everything over the last
>>>>> decade. On Debian unstable:
>>>>>
>>>>> PANIC: could not open file "pg_wal/000000010000000000000001": Invalid argument
>>>> ... I have a new idea: perhaps it is possible to try
>>>> to open a file with O_DIRECT from perl, and if it fails like that,
>>>> skip the test. Looking into that now.
>>> I think I have that working OK. Any Perl hackers want to comment on
>>> my use of IO::File (copied from examples on the internet that showed
>>> how to use O_DIRECT)? I am not much of a perl hacker but according to
>>> my package manager, IO/File.pm came with perl itself. And the Fcntl
>>> eval trick that I copied from File::stat, and the perl-critic
>>> suppression that requires?
>>
>> I think you can probably replace a lot of the magic here by simply saying
>>
>>
>> if (Fcntl->can("O_DIRECT")) ...
> Fcntl->can() is true for all constants that Fcntl knows about, whether
> or not they are defined for your OS. `defined &O_DIRECT` is the simplest
> check, see my other reply to Thomas.
>
>

My understanding was that Fcntl only exported constants known to the OS.
That's certainly what its docco suggests, e.g.:

    By default your system's F_* and O_* constants (eg, F_DUPFD and
O_CREAT)
    and the FD_CLOEXEC constant are exported into your namespace.

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com