Re: o_sync in vfat driver

[lsorense@csclub.uwaterloo.ca (Lennart Sorensen)]

On Tue, Feb 28, 2006 at 08:10:44AM -0500, linux-os (Dick Johnson) wrote:
> 
> On Mon, 27 Feb 2006 col-pepper@piments.com wrote:
> 
> > On Mon, 27 Feb 2006 22:32:07 +0100, linux-os (Dick Johnson)
> > <linux-os@analogic.com> wrote:
> >
> >> Flash does not get zeroed to be written! It gets erased, which sets all
> >> the bits to '1', i.e., all bytes to 0xff.
> >
> > Thanks for the correction, but that does not change the discussion.
> >
> >> Further, the designers of
> >> flash disks are not stupid as you assume. The direct access occurs
> >> to static RAM (read/write stuff).
> >
> > I'm not assuming anything . Some hardware has been killed by this issue.
> > http://lkml.org/lkml/2005/5/13/144
> 
> No. That hardware was not killed by that issue. The writer, or another
> who had encountered the same issue, eventually repartitioned and
> reformatted the device. The partition table had gotten corrupted by
> some experiments and the writer assumed that the device was broken.
> It wasn't.
> 
> Also, if you read other elements in this thread, you would have
> learned about something that has become somewhat of a red herring.
> 
> It takes about a second to erase a 64k physical sector. This is
> a required operation before it is written. Since the projected
> life of these new devices is about 5 to 10 million such cycles,
> (older NAND flash used in modems was only 100-200k) the writer
> would have to be running that "brand new device" for at least
> 5 million seconds. Let's see:

How come I can write to my compact flash at about 2M/s if you claim it
takes 1s to erase a 64k sector?  Somehow I think your number is much too
high.  Or it can do multiple erases at the same time.

Also the 5 to 10 million is a lot higher than the numbers the makers of
the compact flash cards I use claim.

> 60 seconds per minute
> 3600 seconds per hour
> 86400 seconds per day.
> 
> 5,000,000 / 86400 = 57 days of continuous writes to the same
> sector. The writer surely would have a strange file because
> he states that even a single large file can destroy the drive
> if it is mounted with the "sync" option.
> 
> Also, the failure mode of NAND flash is not that it becomes
> "destroyed". The failure mode is a slow loss of data. The
> devices no longer retain data for a zillion years, only a
> few hundred, eventually, only a year or so. So when somebody
> claims that the flash has gotten destroyed, they need to have
> written it for a few months, then waited for a few years before
> reporting the event.

Some flash devices can be "destroyed" by loosing power in the middle of
a write, since this causes them to corrupt their table of blocks and
only the manufacturer has the ability to reset that.  Fortunately this
doesn't seem like too common a design.

Len Sorensen
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