Re: Linux 2.6.3-rc3 - IDE DMA errors on Thinkpad A30

[Eric D. Mudama]

On Sun, Feb 15 at 22:47, Jeff Garzik wrote:
> Chip Salzenberg wrote:
> > Still: I wonder if the occasional bad sector is really that bad.
> > Shirley, at the unreal densities of today's drives, the development of
> > bad sectors is inevitable?  (Especially in a laptop drive that's
> > bounced around in normal use.)
>
> Open argument :)
>
> A lot of smart people will argue that a bad sector every now and again 
> occurs, and "I've run my server's disks that way for years."
>
> Other equally smart people argue that modern IDE disks reserve space for 
> remapping bad sectors.  If you run out of sectors that the drive is 
> willing to silently remap for you, you should toss the disk and buy a 
> new one.

Yes, definitely.  There are a *lot* of spare sectors on a modern IDE
drive -- Running out of spares is an extremely rare event, and usually
indicates that the drive has been operating in severe conditions for
its lifetime.  (>55C + vibe sorts of things)

> There is of course the caveat that it is impossible to avoid the drive 
> returning "bad sector", instead of silently remapping, on reads.

Yes, because at that point, there's nothing to remap.  If the drive
couldn't read it, and knows it isn't in its 2/8MB cache, remapping is
pointless until there is known-good data to apply.  Therefore, only on
the write is there a good reason to examine and possibly remap an LBA.

> Oh, and I just thought of something else.  Current Linux filesystems 
> will, on a read error, usually mark it as a bad sector and move on. 
> Really, they should attempt to write to the bad sector before 
> considering it bad.
>
> As a result, current kernels will AFAICT assume a sector is bad even 
> when the drive politely swaps a good sector in place for you.
>
> One for the todo list, I suppose...  a useable workaround for this is 
> probably good ole 'e2fsck -c', i.e. badblocks...  That says "check again 
> to see if this sector is bad", and -hopefully- will unmark bad blocks 
> that were incorrectly marked bad.

Agreed.

In most cases, a simpe write to the supposedly-defective LBA will get
the drive to resolve whether it is a permanent media defect that needs
to be remapped, or else functional media that has gone bad for some
other reason. (of which there are several... excessive cold/heat,
vibration, poor power, etc)


--
Eric D. Mudama
edmudama@xxxxxxxxxxxxxxxxxxxxx

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