Re: md-raid paranoia mode?

[NeilBrown <neilb@suse.de>]

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On Thu, 12 Jun 2014 12:28:14 +0600 Roman Mamedov <rm@romanrm.net> wrote:

> On Thu, 12 Jun 2014 10:15:32 +0800
> Brad Campbell <lists2009@fnarfbargle.com> wrote:
> 
> > On 11/06/14 14:48, Bart Kus wrote:
> > > Hello,
> > >
> > > As far as I understand, md-raid relies on the underlying devices to
> > > inform it of IO errors before it'll seek redundant/parity data to
> > > fulfill the read request.  I have, however, seen certain hard drives
> > > report successful reads while returning garbage data.
> > 
> > If you have drives that return garbage as valid data then you have far 
> > greater problems than what you are suggesting will fix. So much so I 
> > suggest you document these instances and start banging a drum announcing 
> > them in a name and shame campaign. That sort of behavior from storage 
> > devices is never ok, and the manufacturer needs to know that.
> 
> If your RAM can return garbage, that's not a justification for having ECC RAM.
> ECC RAM is a gimmick invented by weak conformist people. Instead, you should go
> and loudly scream at the manufacturer who sold you that RAM! Errors from RAM
> are never OK! RAM should always work perfectly! And if it doesn't, you have
> greater problems. We shall not tolerate this behavior! So go get a drum and
> start banging it as loudly as you can! Name and shame the manufacturer who
> sold you that RAM. Fight the power, brother!!!

Your screwdriver is leaking  (*).

Hard drives contain ECC.  It should ensure undetected errors are an
*extremely* rare event (more rare than bugs in the md code).

If your ECC RAM started returning bad data without telling you, would you
build a complex virtual memory system to load every byte from two different
DIMMs into CPU registers and compare them before trusting them?

I know that hard drives can return bad data.  I've seen it happen.  I don't
think that trying to "fix" it in the md/raid layer is appropriate.

File-systems and higher level data management systems (e.g. git) are much
better placed to detect such errors than md/raid is.  Supposedly btrfs will
DTRT with your drives (though TRT is to RMA them, and I don't think btrfs
has an RMA plugin yet).

> 
> You can probably tell just how sick I am of reasoning like yours. That's why
> we can't have nice things (md-side resiliency for the cases when you need/want
> it), and sadly Neil is of the same opinion as you.
> 

In general, if you want nice things you need to pay for them.  If you are
willing to pay I suspect you can find someone who is willing to provide.

NeilBrown

(*)http://www.zazzle.com/a_bad_analogy_is_like_a_leaky_screwdriver_tshirts-235102919981826183

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