Unfortunately, this technique is not guaranteed to work. Even if you write, flush then rename, the OS and disk may decide to apply these operations in a different order, breaking your guarantee.
A few years ago, I had to fix this behavior on Firefox, because it was actually causing data loss. The only techniques I found that seem to work are journaling and rolling backups + transparent recovery (which can still lose data, just one order of magnitude less often).
Ah, thankyou for fixing session restore. For years this was a really expensive (although useful) feature that caused huge disk write pressure. I used to have to move the Firefox directory to a ramdisk in order to have a usable system.
Other developers are currently working on fixing other aspects of Session Restore, including both performance issues and plugging other possible sources of data loss, but I'm not following this closely.
> Unfortunately, this technique is not guaranteed to work. Even if you write, flush then rename, the OS and disk may decide to apply these operations in a different order, breaking your guarantee.
I've heard about lying disks, but do you know of any that are still being sold today? My impression was this isn't really a thing anymore.
I don't think the OS lies if you're sufficiently careful: write the new file, fsync it, fsync the _directory_, then rename. [edit: on OS X, use F_FULLFSYNC. grr.] Now it's guaranteed that the filename has either the old or new contents. Another fsync on the directory to guarantee it's the new. I'd be interested in any evidence to the contrary.
I don't think the OS will apply the operations in a different order. I believe the fsync acts analogously to a memory barrier with regards to reordering. It may be the case that the disk will still apply the operations in a different order though.
A few years ago, I had to fix this behavior on Firefox, because it was actually causing data loss. The only techniques I found that seem to work are journaling and rolling backups + transparent recovery (which can still lose data, just one order of magnitude less often).
If you're interested, I wrote about the latter in this blog post: https://dutherenverseauborddelatable.wordpress.com/2014/06/2...