Re: [PATCH] Add quick erase format option

From: Stefani Seibold
Date: Mon Aug 09 2010 - 09:54:41 EST


Am Montag, den 09.08.2010, 14:29 +0300 schrieb Artem Bityutskiy:
> On Mon, 2010-08-09 at 10:52 +0200, Stefani Seibold wrote:
> > Am Montag, den 09.08.2010, 09:37 +0100 schrieb David Woodhouse:
> > > On Mon, 2010-08-09 at 09:25 +0100, stefani@xxxxxxxxxxx wrote:
> > > > From: Stefani Seibold <stefani@xxxxxxxxxxx>
> > > >
> > > > This patch add a quick format option which skips erasing of already erased
> > > > flash blocks. This is useful for first time production environments where
> > > > the flash arrived erased.
> > > >
> > > > Signed-off-by: Stefani Seibold <stefani@xxxxxxxxxxx>
> > >
> > > This scares me, given the lengths we had to go to in JFFS2 to cope with
> > > blocks which *look* like they're erased, but which actually start losing
> > > data as soon as you start writing to them because the erase didn't
> > > complete.
> > >
> >
> > I know the drawback. This is why it is only an option which must be
> > enabled. And in most use cases there is a subsequent ubimkvol, which
> > will fail if the flash is not correct initialized.
> >
> > Flash are normally delivered erased. So this save in our production
> > environment (Nokia Siemens Networks) about 5 minutes per device (256 MB
> > NOR CFI Flash).
> >
> > The old JFFS2 was very fast to install the first time on a flash, it was
> > only a simple mount of the MTD partition.
>
> Not sure what you do, but both UBI and UBIFS auto-format flash if it is
> empty, and attaching empty flash should be very fast.
>

I was never able to mount UBIFS without a previous ubimkvol, despite the
flash is already erased.

> But yes, the first volume creation ioctl will block until everything is
> erased, although this is just an implementation issue and in theory,
> fixable.
>

Here are my timing results for mounting an empty flash as UBIFS:

ubiattach /dev/ubi_ctrl -m 5 -d 1 --> 2.023 sec
ubimkvol /dev/ubi1 -m -N flash --> 294.574 sec
mount -t ubifs -o sync ubi1:flash /mnt --> 0.221 sec

or

ubiformat /dev/mtd5 --> 299.111 sec
ubiattach /dev/ubi_ctrl -m 5 -d 1 --> 0.129 sec
ubimkvol /dev/ubi1 -m -N flash --> 1.784 sec
mount -t ubifs -o sync ubi1:flash /mnt --> 0.220 sec

So there is no real benefit between an empty flash and a formated flash.

And this are the timing results for formating and mounting an empty
flash with my patched ubiformat tool:

ubiformat /dev/mtd5 -E --> 5.475 sec
ubiattach /dev/ubi_ctrl -m 5 -d 1 --> 0.130 sec
ubimkvol /dev/ubi1 -m -N flash --> 1.699 sec
mount -t ubifs -o sync ubi1:flash /mnt --> 0.220 sec

As you can see this is 296,818 vs. 7,522 or 40 times faster!

But maybe i do something wrong. Could you explain this?

> > Which the quick format option i have now only a slightly first time
> > installation overhead compared to JFFS2. Without this option the
> > overhead is more than 5 minutes.
>
> Are you flashing an UBI image in production? Then what you can do if you
> want to be faster is to flash only the blocks which contain image date,
> and leave the rest intact, UBI will erase them and write EC header to
> them when you first boot the device.
>

No, we only initialize the flash, mount the UBIFS it and copy files.

> So I think it is better to add an --pristine-flash option, or something
> like this. In this case ubiformat won't erase anything, and will assume
> everything is 0xFFed without reading. This should be faster and I think
> is better to do.
>

My patch assumes nothing, it check if the EC header is 0xff and i think
this is safer than your suggestion. My patch skips the erase if all
bytes in the header are 0xff skip, otherwise erase it.

- Stefani


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/