linux-ext4 - RE: Can one file system be R/W mounted multiple times in Linux?

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Message-ID: <F792CF86EFE20D4AB8064279AFBA51C61EA7288F@SIXPRD3002MB028.064d.mgd.msft.net>
Date:	Fri, 9 Jan 2015 06:48:54 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Eric Sandeen <esandeen@...hat.com>
CC:	Al Viro <viro@...IV.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	"stefanha@...hat.com" <stefanha@...hat.com>
Subject: RE: Can one file system be R/W mounted multiple times in Linux?

> -----Original Message-----
> From: Eric Sandeen [mailto:esandeen@...hat.com]
> Sent: Friday, January 9, 2015 14:05 PM
> To: Dexuan Cui
> Cc: Al Viro; linux-fsdevel@...r.kernel.org; linux-ext4; stefanha@...hat.com
> Subject: Re: Can one file system be R/W mounted multiple times in Linux?
> 
> On Jan 8, 2015, at 10:59 PM, Dexuan Cui <decui@...rosoft.com> wrote:
> 
> >> -----Original Message-----
> >> From: Al Viro [mailto:viro@....linux.org.uk] On Behalf Of Al Viro
> >> Sent: Friday, January 9, 2015 11:51 AM
> >> To: Dexuan Cui
> >> Cc: linux-fsdevel@...r.kernel.org; 'linux-ext4'; stefanha@...hat.com
> >> Subject: Re: Can one file system be R/W mounted multiple times in Linux?
> >>
> >>> On Fri, Jan 09, 2015 at 03:44:16AM +0000, Dexuan Cui wrote:
> >>> The 'mount' utility allows me to mount 1 file systems multiple times at
> >> different
> >>> mount points, like "mount /dev/sdb1 /a; mount /dev/sdb1 /b".
> >>>
> >>> I tried to write from /a and /b at the same time and it seems everything is
> OK
> >>> and no data corruption happens.
> >>> I tried only  ext4 only.
> >>>
> >>> Can somebody please tell me if this usage is safe?
> >>
> >> Yes.
> >
> > Thanks for the confirmation!
> >
> > -- Dexuan
> 
> But to be clear, just in case - this is completely different from, say, mounting the
> same ext4 fs on two different nodes on a SAN, which is quite certainly not safe...
> 
> -Eric

Thank Eric for pointing this out!
Can you please explain this?  I didn't use SAN... 

Thanks,
-- Dexuan
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