Re: [patch] document flash/RAID dangers

From: david
Date: Tue Aug 25 2009 - 20:21:27 EST

On Wed, 26 Aug 2009, Pavel Machek wrote:

On Tue 2009-08-25 16:56:40, david@xxxxxxx wrote:
On Wed, 26 Aug 2009, Pavel Machek wrote:

There are storage devices that high highly undesirable properties
when they are disconnected or suffer power failures while writes are
in progress; such devices include flash devices and MD RAID 4/5/6

change this to say 'degraded MD RAID 4/5/6 arrays'

also find out if DM RAID 4/5/6 arrays suffer the same problem (I strongly
suspect that they do)

I changed it to say MD/DM.

then you need to add a note that if the array becomes degraded before a
scrub cycle happens previously hidden damage (that would have been
repaired by the scrub) can surface.

I'd prefer not to talk about scrubing and such details here. Better
leave warning here and point to MD documentation.

I disagree with that, the way you are wording this makes it sound as if raid isn't worth it. if you are going to say that raid is risky you need to properly specify when it is risky

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.

you are incorrect on this.

ext3 (like every other filesystem) just accepts the risk (zfs makes some attempt to detect such corruption)

David Lang
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