[openbeos] Re: sync daemon?
- From: "André Braga" <meianoite@xxxxxxxxx>
- To: openbeos@xxxxxxxxxxxxx
- Date: Thu, 18 Jan 2007 16:15:58 -0200
On 1/18/07, Waldemar Kornewald <wkornewald@xxxxxxxxxxxx> wrote:
Hi,
On 1/18/07, François Revol <revol@xxxxxxx> wrote:
> Ideally each device driver would report its liking in terms of usage.
> A simple way would be to just use the removable flag to force sync ops
> and disable periodical syncing.
Aren't specifically removable drives vulnerable to losing data because
the user forgot to unmount? I've seen *waaay* to many users just
unplug their USB stick in WinXP and Linux without prior unmounting. Of
course, this problem doesn't exist with all removable drives. The
system should take into account if the volume must be ejected with a
button (and if this signal can be intercepted).
Sometimes Windows XP decides to disable the write cache on removable
drives, forcing every operation to be done synchronously; unmounting
these devices essentially mean flushing the caches onto the media, and
since the write operations would be synchronous, there would be no
cache to flush. I guess François meant that.
However that just kills performance.
So we're left with some options. Not to cache writes and force them to
happen synchronously and kiss goodbye every hope for performance
(eek); to cache writes, sync them from time to time and give the final
syncing job to the unmount operation (more reasonable and every OS is
doing that nowadays); or the perhaps best option which I'm yet to find
implemented in ANY operating system:
To put a pseudo full journal (not only metadata) on top of every
removable media and warn the user if there were any uncommited write
jobs the moment he ejected the media, asking him to kindly reinsert it
and wait for the progress meter on the dialog to complete before
removing it again. It would have to be FS independent, like the
gjournal GEOM layer being developed for FreeBSD 7. But we don't really
need that level of sophistication to implement this idea; a relatively
easy implementation is to just log all the write operations and them
flush as appropriate, or maybe layering a sparse image on top of the
removable media FS and commit the changes as appropriate; the latter
happens to be very similar to how VMWare and QEMU 0.8.2 handle
snapshots.
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