Re: [PATCH 2/2] fat: mark fs as dirty on mount and clean on umount

From: Andrew Morton
Date: Wed Jan 02 2013 - 18:44:13 EST

On Fri, 28 Dec 2012 02:20:20 +0900
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> wrote:

> There is no documented methods to mark FAT as dirty. Unofficially MS
> started to use reserved Byte in boot sector for this purpose,
> at least since Win 2000. With Win 7 user is warned if fs is dirty
> and asked to clean it.
> Different versions of Win, handle it in different ways,
> but always have same meaning:
> - Win 2000 and XP, set it on write operations and
> remove it after operation was finnished
> - Win 7, set dirty flag on first write and remove it on umount.
> We will do it as fallow:
> - set dirty flag on mount. If fs was initially dirty, warn user,
> remember it and do not do any changes to boot sector.
> - clean it on umount. If fs was initially dirty, leave it dirty.
> - do not do any thing if fs mounted read-only.
> - TODO: leave fs dirty if we found some error after mount.

The changelog doesn't describe why we're making this change. Nor does
it describe the user-visible effects of this change.

AFAICT the effect is to issue a warning at mount-time to tell the
user that the fs wasn't cleanly unmounted and that the user should fsck
the volume, correct?

If so, why is this considered a desirable feature? (I can guess, but
would prefer to hear it spelled out by the experts, please).

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