From: Pavel Machek <pavel@ucw.cz>
Subject: Re: [patch] ext2/3: document conditions when reliable operation is
	possible
Date: Mon, 24 Aug 2009 20:44:21 +0200
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References: <20090312092114.GC6949@elf.ucw.cz> <200903121413.04434.rob@landley.net> <20090316122847.GI2405@elf.ucw.cz> <200903161426.24904.rob@landley.net> <20090323104525.GA17969@elf.ucw.cz> <87ljqn82zc.fsf@frosties.localdomain> <20090824093143.GD25591@elf.ucw.cz> <87f94c370908240621n32ea310sd24196084c42107a@mail.gmail.com>
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Cc: Goswin von Brederlow <goswin-v-b@web.de>,
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To: Greg Freemyer <greg.freemyer@gmail.com>
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> > +Either whole sector is correctly written or nothing is written dur=
ing
> > +powerfail.
> > +
> > + =A0 =A0 =A0 Because RAM tends to fail faster than rest of system =
during
> > + =A0 =A0 =A0 powerfail, special hw killing DMA transfers may be ne=
cessary;
> > + =A0 =A0 =A0 otherwise, disks may write garbage during powerfail.
> > + =A0 =A0 =A0 This may be quite common on generic PC machines.
> > +
> > + =A0 =A0 =A0 Note that atomic write is very hard to guarantee for =
RAID-4/5/6,
> > + =A0 =A0 =A0 because it needs to write both changed data, and pari=
ty, to
> > + =A0 =A0 =A0 different disks. (But it will only really show up in =
degraded mode).
> > + =A0 =A0 =A0 UPS for RAID array should help.
>=20
> Can someone clarify if this is true in raid-6 with just a single disk
> failure?  I don't see why it would be.
>=20
> 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.

--=20
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