linux-kernel - Re: writing file to disk: not as easy as it looks

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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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