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

[Andy Lutomirski <luto@amacapital.net>]

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.

--Andy