Re: Uneccesary flushes waking up suspended disks

[Phillip Susi <phill@thesusis.net>]

Dave Chinner <david@fromorbit.com> writes:

> That's what I expected - I would have been surprised if you found
> problems across multiple filesystems...

How do the other filesystems know they don't need to issue a flush?
While this particular method of reproducing the problem ( sync without
touching the filesystem ) only shows on ext4, I'm not sure this isn't
still a broader problem.

Say that a program writes some data to a file.  Due to cache pressure,
the dirty pages get written to the disk.  Some time later, the disk is
runtime suspended ( which flushes its write cache ).  After that,
someone does some kind of sync ( whole fs or individual file ).  Doesn't
the FS *have* to issue a flush at that point?  Even though there is
nothing in the disk's cache, the FS doesn't know that.