Re: [patch] ext2/3: document conditions when reliable operation ispossible

From: Pavel Machek
Date: Mon Aug 24 2009 - 14:44:33 EST

> > +Either whole sector is correctly written or nothing is written during
> > +powerfail.
> > +
> > +       Because RAM tends to fail faster than rest of system during
> > +       powerfail, special hw killing DMA transfers may be necessary;
> > +       otherwise, disks may write garbage during powerfail.
> > +       This may be quite common on generic PC machines.
> > +
> > +       Note that atomic write is very hard to guarantee for RAID-4/5/6,
> > +       because it needs to write both changed data, and parity, to
> > +       different disks. (But it will only really show up in degraded mode).
> > +       UPS for RAID array should help.
> Can someone clarify if this is true in raid-6 with just a single disk
> failure? I don't see why it would be.
> And if not can the above text be changed to reflect raid 4/5 with a
> single disk failure and raid 6 with a double disk failure are the
> modes that have atomicity problems.

I don't know enough about raid-6, but... I said "degraded mode" above,
and you can read it as double failure in raid-6 case ;-). I'll prefer
to avoid too many details here.

(cesky, pictures)
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