Power Down

Bob Canup rcanup at go2fax.com
Wed Dec 8 09:55:12 EST 1999

It is obvious that a physical medium such as a disk is vulnerable to
having a bad sector created by the process that I described. The proof
is simple: pop out a diskette while you are writing to it and you stand
a good chance of creating a sector in which the CRC and data are out of
sync. When you attempt to read the sector you will get a bad CRC.

This occurs in a diskette because the writing process is a serial event;
it is spread over time. So there is a window in which an interruption
can create a bad sector.

Let us assume the the DOC writes all of the bytes in a page including
the ECC code in parallel, let us also assume that you have an internal
bit which marks a sector as good when that process has completed. There
nevertheless is a time during the 'burn' of the bits where we are in an
analog state of changing the bits. If power is lost at that time - some
of the bits will not have changed to their proper state. Even if the
page is not marked as good an attempt to read the page will result in an
ECC and data which do not match and the result is a bad sector. The
sector may be easily recovered by erasing it and starting over - but as
long as there is an analog aspect to changing the states - the bits will
not all change at the same instant and a window for corruption exists.

Vipin's original post said that he saw bad sectors about once in every
250 power down cycles. Oran is telling us that can't occur.

Of course if my analysis is correct then you are safe to erase the bad
sector - it was the last one being written; the file system would then
be left in a state in which e2fsck could hopefully repair it.

Or am I off in left field with this?

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