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Message-ID: <Pine.LNX.4.64.0812031922230.3530@artax.karlin.mff.cuni.cz>
Date: Wed, 3 Dec 2008 19:33:56 +0100 (CET)
From: Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To: Pavel Machek <pavel@...e.cz>
cc: Alan Cox <alan@...rguk.ukuu.org.uk>, Theodore Tso <tytso@....edu>,
Chris Friesen <cfriesen@...tel.com>,
kernel list <linux-kernel@...r.kernel.org>, aviro@...hat.com
Subject: Re: writing file to disk: not as easy as it looks
On Wed, 3 Dec 2008, Pavel Machek wrote:
> > > CRC errors, lost IRQs and the like are retried by the midlayer and
> > > drivers and the error handling strategies will also try things like
> > > reducing link speeds on repeated CRC errors.
> >
> > I meant for example loose cable or so --- does it make sense to retry
> > indefinitely (until the admin plugs the cable or unmounts the filesystem)
> > or return error to the filesystem after few retries?
>
> It is quite non-trivial to detect if it is "disk plugged back in"
> vs. "faulty disk unplugged, new one plugged in"... so I suppose
> automatic retry after failure of connection to disk is quite hard to
> get right.
Unless the SATA controller has the plug interrupt (very few have), there
is no way for the kernel to detect that an old SATA disk was unplugged and
a new one was plugged in.
So the answer is that the admin must not hot-swap disk unless unmounting
the filesystem or notifying the RAID layer about it. If you hot-swap
softraid1/4/5 disk, you definitely damage data, because the softraid layer
has no way to find out about the hotswap.
Mikulas
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