Ted,

I am not sure if this was covered yet but if so I apologize in advance
for beating a dead horse.

The current opinion is that if you want to make meta data (eg the out of
order rename case) reliable you must use sync... Ok, that would require
a huge number of changes to many programs but it is at least
theoretically doable. However in doing so aren't you rendering laptop
mode essentially useless? As far as I know and from mjg59 confirmed was
the case you can't delay syncs. So now any time in the future any
application that wants its data to be reliable if only for meta data
cases it will cause a drive to spin up causing battery life to go down
the tubes. Yes SSDs are much more power efficient and are the wave of
the future, but from where I am it looks like they will continue to be
the wave of the future, what with even the Intel SSDs being only rated
for 20GB/day data transfer.

Also, you mentioned before that attempting to track meta data ordering
for files in memory would cause entaglement forcing a write of all data
to disk. Is that just due to the way the current kernel filesystem layer
works or a problem that is not fixable? If it were fixable then sync's
in code would only be needed for critical writes since meta data being
out of order would no longer cause the serious problems it does today in
cases with no sync calls. This would allow much more efficient power
management of hard drives since a laptop could potentially go for a very
long time without needing to spin up the hard drive (or bring the SSD
out of low power mode).

You also did mention that in the case of saving to a remote filesystem
that sync is actually needed to determine if it is even possible to save
to it. With the way the desktop in the past had worked, mounting remote
filesystems using the kernel and using posix apis, it was unknown what
the user would be saving to so apps might just sync to be safe. However
I think this case may not be as big of an issue anymore, on the desktop
at least, due to moving to api's such as gio which already know if they
are saving to a remote filesystem so can sync only in those cases, at
least aiui.

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Ext4 data loss
https://bugs.launchpad.net/bugs/317781
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