Re: How does btrfs handle bad blocks in raid1?

[Roman Mamedov <rm@romanrm.net>]

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On Tue, 14 Jan 2014 14:37:46 -0700
Chris Murphy <lists@colorremedies.com> wrote:

> Reserve sectors are fundamental to ECC. If there are no more reserves, the
> status should be a failed drive, it can no longer do its own relocation of
> data experiencing transient read errors in this case.

With the Reallocated sector count being low in that case, I assume the drive
*had* a lot of reserve space, but due to a buggy firmware didn't "see the need
to use it" just yet for this particular sector.

It's not the question of what is "fundamental" or right, I am describing
observed behavior in the real world - and yes, of course, that behavior is
incorrect and probably an indication of a buggy peculiar firmware.

> I'm considering persistent write failure as a result of no more reserve
> sectors being available.

Write doesn't fail, it succeeds, but you still can't read back the bad block.
And the "reallocated sector count" does not increase.

> Well, not totally useless, if it flags the user with an hour's notice in Gnome,

If we're again talking the user GUI-level indicators, then an increase in
"Reallocated sector count", "Current pending sectors" or "Reported
uncorrectable" should also be a reason for such GUI notice to appear. And
AFAIK that's how smartd howto/manual recommends configuring it (i.e. an E-Mail
on any change in those critical attributes).

> >> a way to send a command to the firmware to persistently increase the reserve
> >> sectors at the expensive of available space - in effect it reduces the LBA
> >> count by e.g. 10MB, thereby increasing the reserve pool by 10MB.
> > 
> > Yes please that, and also a pony. :)
> 
> That seems a lot easier to implement than anything else being discussed. 

Oh really? Pushing that feature into multiple competing vendor's HDD firmwares
across diverse models, product lines, interfaces and revisions is *easier* than
mainlining a patch into the Linux kernel?... My point was that this would have
been a good feature, but no chance we're going to see it realized.

-- 
With respect,
Roman

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