Re: [PATCH RFC] vfs: add a O_NOMTIME flag

[Andy Lutomirski <luto@amacapital.net>]

On Thu, May 7, 2015 at 1:02 PM, Richard Weinberger <richard@nod.at> wrote:
> Am 07.05.2015 um 21:53 schrieb Andy Lutomirski:
>> On Thu, May 7, 2015 at 12:09 PM, Richard Weinberger
>> <richard.weinberger@gmail.com> wrote:
>>> On Thu, May 7, 2015 at 7:20 PM, Zach Brown <zab@redhat.com> wrote:
>>>> On Thu, May 07, 2015 at 10:26:17AM +1000, Dave Chinner wrote:
>>>>> On Wed, May 06, 2015 at 03:00:12PM -0700, Zach Brown wrote:
>>>>>> Add the O_NOMTIME flag which prevents mtime from being updated which can
>>>>>> greatly reduce the IO overhead of writes to allocated and initialized
>>>>>> regions of files.
>>>>>
>>>>> Hmmm. How do backup programs now work out if the file has changed
>>>>> and hence needs copying again? ie. applications using this will
>>>>> break other critical infrastructure in subtle ways.
>>>>
>>>> By using backup infrastructure that doesn't use cmtime.  Like btrfs
>>>> send/recv.  Or application level backups that know how to do
>>>> incrementals from metadata in giant database files, say, without
>>>> walking, comparing, and copying the entire thing.
>>>
>>> But how can Joey random user know that some of his
>>> applications are using O_NOMTIME and his KISS backup
>>> program does no longer function as expected?
>>>
>>
>> Joey random user can't have a working KISS backup anyway, though,
>> because we screw up mtime updates on mmap writes.  I have patches
>> gathering dust that fix that, though.
>
> Hmmm, I thought mtime will be updated upon msync()?
> Assuming a sane application is using msync()...
>

So would I.  Unfortunately, mtime is updated on the page fault that
makes an mmapped page writeable, thus guaranteeing that the resulting
mtime is stale if you mmap a file, write to it, unmap it, and close
it.  It's much more stale if you mmap it, write, wait for a while but
not long enough that the page is automatically written back, write
again, unmap, and close.

--Andy