Re: Power Down

Re: Power Down

[Bob Canup]

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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Re: Power Down

[David Woodhouse]

rcanup@go2fax.com said:
>  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.

At the block device level, that definitely shouldn't occur. The ext2 may of 
course get confused, but that's why you should be using ext3 on it if you ever 
expect it to lose power.

> 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. 

With NFTL, that's definitely the way it's designed. You write the data, write
the ECC checksum, and then mark it valid. The old version of that block remains
on the media until it's later reclaimed.

If you are interrupted during a write, it's obvious and can be fixed.





--
dwmw2




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RE: Power Down

[Oron Ogdan]

Sorry, In this email on the second line from the end it should read
I am afraid to guess NTFS and not NTFL.

Oron


-----Original Message-----
From: Oron Ogdan [mailto:Orono@m-sys.com]
Sent: Wednesday, December 08, 1999 12:59 AM
To: MTD
Subject: RE: Power Down


Since DiskOnChip is meant to be used in embedded system, we in M-systems 
do is provide a hard disk emulation on flash which is 
resistant to power failures. We power cycle the media for several 
months to check our algorithms are indeed power fail resistant.

That means that the NFTL structures on the media
are resistant to any power loss during any stage of the algorithm.
The only thing that can happen is that you will have what's called orphan 
units that need to be scanned for and released on mount.

But this only protects the logical / physical mapping, It does not 
guarantee any damage to the file system on those logical sectors was 
not caused. 

The only way to be resistant to power failures in the file system level 
is to use a log-structured file system. I heard ext3 is log-structured but
I am sure one of the Linux guys here knows more about this, Any other file
system 
that takes power failures into account, (I am afraid to guess NFTL ????). 
Some of our customers use their own home brewed LFSs and do it successfully.

Oron


-----Original Message-----
From: Bob Canup [mailto:rcanup@go2fax.com]
Sent: Tuesday, December 07, 1999 9:19 PM
To: MTD
Subject: Re:Power Down


Watch dogs are generally there to catch the problem of a run-away
machine - this ought to be a very rare occurrence.

According to Vipin's statistics about 1 in 250 random power failures
during writes to a DOC2000 results in a bad sector on the device. Since
you are required to run the chip in RW mode the only way I see to avoid
the problem is a UPS on the front end - with signaling to indicate power
failure so that an ordered shutdown could occur.

As far as the problem of a bad sector which he discussed I have not seen
any solutions other than the erase and start over one he originally came
up with - which for the reasons he discussed - is unacceptable.

The first step toward solving a problem is understanding exactly what
the problem is. My theory is that if you interrupt a sector write while
it is in progress the data and the error checking code don't match -
thus you get a bad sector. Any other theories?





To unsubscribe, send "unsubscribe mtd" to majordomo@infradead.org
To unsubscribe, send "unsubscribe mtd" to majordomo@infradead.org

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RE: Power Down

[Oron Ogdan]

Since DiskOnChip is meant to be used in embedded system, we in M-systems 
do is provide a hard disk emulation on flash which is 
resistant to power failures. We power cycle the media for several 
months to check our algorithms are indeed power fail resistant.

That means that the NFTL structures on the media
are resistant to any power loss during any stage of the algorithm.
The only thing that can happen is that you will have what's called orphan 
units that need to be scanned for and released on mount.

But this only protects the logical / physical mapping, It does not 
guarantee any damage to the file system on those logical sectors was 
not caused. 

The only way to be resistant to power failures in the file system level 
is to use a log-structured file system. I heard ext3 is log-structured but
I am sure one of the Linux guys here knows more about this, Any other file
system 
that takes power failures into account, (I am afraid to guess NFTL ????). 
Some of our customers use their own home brewed LFSs and do it successfully.

Oron


-----Original Message-----
From: Bob Canup [mailto:rcanup@go2fax.com]
Sent: Tuesday, December 07, 1999 9:19 PM
To: MTD
Subject: Re:Power Down


Watch dogs are generally there to catch the problem of a run-away
machine - this ought to be a very rare occurrence.

According to Vipin's statistics about 1 in 250 random power failures
during writes to a DOC2000 results in a bad sector on the device. Since
you are required to run the chip in RW mode the only way I see to avoid
the problem is a UPS on the front end - with signaling to indicate power
failure so that an ordered shutdown could occur.

As far as the problem of a bad sector which he discussed I have not seen
any solutions other than the erase and start over one he originally came
up with - which for the reasons he discussed - is unacceptable.

The first step toward solving a problem is understanding exactly what
the problem is. My theory is that if you interrupt a sector write while
it is in progress the data and the error checking code don't match -
thus you get a bad sector. Any other theories?





To unsubscribe, send "unsubscribe mtd" to majordomo@infradead.org

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Re: Power Down

[David Woodhouse]

Orono@m-sys.com said:
> The only way to be resistant to power failures in the file system
> level  is to use a log-structured file system.

> I heard ext3 is log-structured but I am sure one of the Linux guys here
> knows more about this, 

ext3 is a journalling filesystem. I believe that it's not log-structured. 

Journalling is sufficient for protection from power failures. Stephen, 
would you care to elaborate on the difference?

> Any other file system  that takes power failures into account, (I am afraid 
> to guess NFTL ????).

I believe that NTFS is also journalling but not log-structured.

> Some of our customers use their own home brewed LFSs and do it successfully. 

Personally, I'm inclined to believe that we should run a filesystem directly 
on the flash device - rather than faking a block device and running a 'normal' 
filesystem on top of that. 

I've been rushed off my feet here with other things for a while, but as soon as
I get back to it, after I've fixed the NFTL and DiskOnChip Millennium support,
that's what I'm intending to look at.

If you're running ext2 on a DiskOnChip you should mount it with the noatime 
option if you can - this will eliminate a lot of write activity.


--
dwmw2




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Re: Power Down

[Stephen C. Tweedie]

Hi,

On Wed, 08 Dec 1999 09:54:12 +0000, David Woodhouse
<dwmw2@infradead.org> said:

> Orono@m-sys.com said:
>> The only way to be resistant to power failures in the file system
>> level  is to use a log-structured file system.

>> I heard ext3 is log-structured but I am sure one of the Linux guys here
>> knows more about this, 

> ext3 is a journalling filesystem. I believe that it's not log-structured. 

Correct.

> Journalling is sufficient for protection from power failures. Stephen, 
> would you care to elaborate on the difference?

Log-structured disks are, oddly enough, structured as a log!  The
entire data layout is in the form of a conceptually infinite log.  All
disk writes are performed at the head of the log, so writes are
necessarily sequential.  The write performance, as a result, is
unmatched.  The read performance can sometimes suffer, but these
filesystems are intended for use with huge amounts of cache, so that
the write performance is more important: most reads are from cache.

A journaled filesystem has a log *in addition to* the normal disk
structure.  The log is used for recovery after a crash but does not
store any long-term persistent data for the filesystem.

> I believe that NTFS is also journalling but not log-structured.

Indeed.

Cheers,
 Stephen


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power down

[Bob Canup]

The reason that I said that expecting anything to work during power down
is wishful thinking is this: once the voltage to a digital chip goes
below the minimum specification of the chip, the behavior of the chip
becomes indeterminate.

For example: the old Western Digital 1791 double density disk controller
chip would sometimes glitch in such a way during power down that it
would write to the floppy - you could see the floppy light blink when
this happened.

Unless chips are specifically designed to handle power down conditions
this sort of thing happens.  For example - any competently designed
Flash memory has to refuse to write if the voltage is below spec.

As to flushing the buffers and doing a shutdown when a power fail
condition occurs - I believe that Linux already has code to handle a
power down such as I described. What I have described is very similar to
a UPS signaling the kernel that power is going down. Linux can do an
ordered shutdown when it receives the signal.

Qualifying digital circuitry with a POWER GOOD signal is very similar to
protecting the circuitry with a typical 'SCR over voltage crowbar
circuit': it makes the engineer feel good - but it doesn't actually do
much of anything.

Why doesn't the crowbar work? After all, it is a text book circuit. The
answer is that the SCR is a power device which takes on the order of 10
microseconds to turn on while the delicate chips are destroyed by a few
nanoseconds of over voltage. The result is that the SCR never turns on -
the fuse blows because the weakest digital chip  shorts the power supply
to ground. One could "protect" SCR's with digital chips, but not the
other way around.

Another example of "feel good engineering" is the power on self test
which most computers have. One can only test non critical sections of
the machine: if anything critical is broken the POST won't run - and a
tech will have to figure out what is wrong. It's a bit like asking
yourself "Am I alive?" If you can ask the question the answer is always
"Yes".




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