Re: [PATCH] let fat handle MS_SYNCHRONOUS flag

[Matt Mackall <mpm@selenic.com>]

On Wed, Nov 24, 2004 at 02:00:54PM +0900, OGAWA Hirofumi wrote:
> Matt Mackall <mpm@selenic.com> writes:
> 
> > On Wed, Nov 24, 2004 at 05:24:36AM +0900, OGAWA Hirofumi wrote:
> >> Colin Leroy <colin@colino.net> writes:
> >> 
> >> > It adds MS_SYNCHRONOUS support to FAT filesystem, so that less
> >> > filesystem breakage happen when disconnecting an USB key, for 
> >> > example. I'd like to have comments about it, because as it 
> >> > seems to work fine here, I'm not used to fs drivers and could
> >> > have made mistakes.
> >> 
> >> What cases should these patches guarantee that users can unplug the
> >> USB key?  And can we guarantee the same cases by improving autofs or
> >> the similar stuff?
> >
> > Well there can be no guarantees - there will always be a race between
> > flush and hot unplug. If we're careful with write ordering, we can
> > perhaps arrange to avoid the worst sorts of corruption, provided the
> > device does the right thing when it's in the middle of an IO.
> >
> > But generally I think this is a good idea as it shrinks the window.
> 
> Things which I want to say here - do we really need the bogus
> sync-mode?

I'm not sure why you say it's bogus. Ext2 for instance has long had a
mount option similar to this and it makes sense in volatile
environments. Having the flag in the superblock seems a sensible way
of doing it as well.
 
> Current fatfs is not keeping the consistency of data on the disk at
> all.  So, after all, the data on a disk is corrupting until all
> syscalls finish, right?

This is to protect against usage patters like mv a b, oh look, it's
done, unplug. Not lots of active readers/writers.

> If so, isn't this too slow? I doubt this is good solution for this
> problem (USB key unplugging)...

Perhaps. I think the behavior should be to honor the superblock flag
by default, overridable with -o sync|async at mount.
 
> Well, it seems good as start of sync-mode though.
> -- 
> OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

-- 
Mathematics is the supreme nostalgia of our time.