Re: [patch] document flash/RAID dangers

[david]

On Wed, 26 Aug 2009, Pavel Machek wrote:

> > > > > THESE devices have the property of potentially corrupting blocks being
> > > > > written at the time of the power failure,
> > > >
> > > > this is true of all devices
> > >
> > > Actually I don't think so. I believe SATA disks do not corrupt even
> > > the sector they are writing to -- they just have big enough
> > > capacitors. And yes I believe ext3 depends on that.
> >
> > Pavel, no S-ATA drive has capacitors to hold up during a power failure
> > (or even enough power to destage their write cache). I know this from
> > direct, personal knowledge having built RAID boxes at EMC for years. In
> > fact, almost all RAID boxes require that the write cache be hardwired to
> > off when used in their arrays.
>
> I never claimed they have enough power to flush entire cache -- read
> the paragraph again. I do believe the disks have enough capacitors to
> finish writing single sector, and I do believe ext3 depends on that.

keep in mind that in a powerfail situation the data being sent to the 
drive may be corrupt (the ram gets flaky while a DMA to the drive copies 
the bad data to the drive, which writes it before the power loss gets bad 
enough for the drive to decide there is a problem and shutdown)

you just plain cannot count on writes that are in flight when a powerfail 
happens to do predictable things, let alone what you consider sane or 
proper.

David Lang
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