Re: RAID5 -> RAID6 conversion, please help

[NeilBrown <neilb@suse.de>]

On Tue, 10 May 2011 21:04:29 -0400 Dylan Distasio <interzone@gmail.com> wrote:

> It's possible I did use different versions but I thought I had
> upgraded both of them right before the reshapes.  Sorry if this is an
> elementary question, but does writing the 2nd parity block always to
> the last drive instead of rotating it increase the odds of a total
> loss of the array since the one specific drive always has the 2nd
> parity block?

No.  The only possibly impact is a performance impact, and even that would be
hard to quantify.

The reason that parity is rotated is to avoid a 'hot disk'.  Every update has
to write the parity block and if they are all on one disk then every write
will generate a write to that one disk.

Because of the way md implements RAID6, every write involves either a read or
a write to every device, so there is no real saving in rotating parity.

I think (but am open to being corrected) that rotating parity is only
important for RAID6 if the code implements 'subtraction' as well as
'addition' for the Q syndrome (which md doesn't) and if you have at least 5
drives, and you probably wouldn't notice until you get to 7 or more drives.

... so it might make sense to make mdadm default to converting to the -6
layout...
You can request it with "--layout=preserve".


> 
> If so, do you think normalizing would be worth the risk of something
> going wrong with that operation?  I'm just trying to get a feel for
> how much of a difference this makes.

Not worth the risk.

NeilBrown



> 
> 
> > mdadm first converts the RAID5 to RAID6 in an instant atomic operation which
> > results in the "-6" layout.  It then starts a restriping process which
> > converts the layout.
> >
> > If you end up with a -6 layout then something when wrong starting the
> > restriping process.
> >
> > Maybe you used different version of mdadm?  There have probably been bugs in
> > some versions..
> >
> > NeilBrown
> >
> >
> >
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