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

On Thu 2015-05-07 12:53:46, Andy Lutomirski wrote:

On Thu, May 7, 2015 at 12:09 PM, Richard Weinberger
[off-list ref] wrote:

quoted

On Thu, May 7, 2015 at 7:20 PM, Zach Brown [off-list ref] wrote:

quoted

On Thu, May 07, 2015 at 10:26:17AM +1000, Dave Chinner wrote:

quoted

On Wed, May 06, 2015 at 03:00:12PM -0700, Zach Brown wrote:

quoted

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.

I'm using unison, and yes, I believe I already seen failures from
mmap(). I'd like to see that fixed, and I can test the patches.

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
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