Re: [PATCH v3] introduce sys_syncfs to sync a single file system

[Ted Ts'o <tytso@mit.edu>]

On Mon, Mar 14, 2011 at 01:10:42PM -0700, Andrew Morton wrote:
> 
> There might one day be a requirement to be able to initiate a
> resource-management-style writeback against a whole filesystem.  When
> that happens, we'll regret not having added a "mode" argument to
> sys_syncfs().

I'm a bit nervous about exposing WB_SYNC_NONE to userspace, because
its semantics are *definitely* hard to describe.  For example, at the
moment if you do a WB_SYNC_NONE writeback, the writeback code will
clamp the amount of data written back for each inode to
MAX_WRITEBACK_PAGES (1024) pages.  Do we want to document that?
Probably not!  But if we don't document it, what can userspace expect?

If you just issue a writeback_inodes_sb(), it's not the case that it
will start a process that will eventually write out everything (i.e.,
it's not the equivalent of a non-blocking data integrity sync).  It
just means, "write out some stuff".

I could imagine userspace wanting to start a non-blocking writeout of
all data blocking pages, and which doesn't cause queue flush / barrier
requests.  (i.e., a non-blocking-non-barrier-issuing-but-otherwise-a-
data-integrity writeback) But that's not something that the current
writeback machinery can do easily, at least not today.

It wouldn't hurt to have a "flags" field which we could expand later
--- but that can lead to portability headaches for userspace programs
that don't know whether a particular kernel is going to support a
particular flag or not.  So it's certainly not a panacea.

	   	   	    	 	       - Ted