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Date:	Fri, 25 May 2007 01:25:21 +0200
From:	Karel Zak <kzak@...hat.com>
To:	Kalpak Shah <kalpak@...sterfs.com>
Cc:	Manoj Joseph <manoj@...sterfs.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	Andreas Dilger <adilger@...sterfs.com>
Subject: Re: [RFC][PATCH] Multiple mount protection

On Tue, May 22, 2007 at 01:04:42PM +0530, Kalpak Shah wrote:
> On Tue, 2007-05-22 at 12:45 +0530, Manoj Joseph wrote:
> > Kalpak Shah wrote:
> > > Hi,
> > > 
> > > There have been reported instances of a filesystem having been
> > > mounted at 2 places at the same time causing a lot of damage to the
> > > filesystem. This patch reserves superblock fields and an INCOMPAT
> > > flag for adding multiple mount protection(MMP) support within the
> > > ext4 filesystem itself. The superblock will have a block number
> > > (s_mmp_block) which will hold a MMP structure which has a sequence
> > > number which will be periodically updated every 5 seconds by a
> > > mounted filesystem. Whenever a filesystem will be mounted it will
> > > wait for s_mmp_interval seconds to make sure that the MMP sequence
> > > does not change. To further make sure, we write a random sequence
> > > number into the MMP block and wait for another s_mmp_interval secs.
> > > If the sequence no. doesn't change then the mount will succeed. In
> > > case of failure, the nodename, bdevname and the time at which the MMP
> > > block was last updated will be displayed. tune2fs can be used to set
> > > s_mmp_interval as desired.

 Frankly, I don't understand why we need this feature. The filesystem
 limitations (=not ready for clusters) should be described in docs.
 That's enough from my POV...

> >
> > What would the default value of s_mmp_interval be? 5 seconds? more?
> 
> I have set the default value to 6 seconds. Depending on specific
> conditions (hardware, etc.) it can be increased using tunefs.
> > 
> > If I am not reading this wrong a mount will take more than 
> > 's_mmp_interval' seconds to complete. Wouldn't this be too much of a 
> > penalty during boot up if the system has many 'mount at boot' filesystems?
> 
> Yes it may take a maximum of s_mmp_interval*2 seconds to mount a
> filesystem which has INCOMPAT_MMP feature set. Its up to the user to use
> this feature, if he finds the penalty is too large, he can do away with
> this feature. This feature will mostly be used for filesystems used in
> failover scenarios.

 I hope the feature will be disabled by default. It sounds strange
 that I have to way 6 secs to mount a FS if I (and 99% of Linux users)
 needn't to share same FS between two mountpoint. 
 
 I have 5 filesystems on my workstation = 30 secs penality during boot?!

> > Also, I am curious about this. Is there a test case for mounting the 
> > same filesystem multiple times? Does this use different paths to reach 
> > the device? Or is there a race? Or does it happen on a device shared by 
> > multiple hosts?
> 
> If you are using some HA software, there is the possibility of a race.
> Yes it can happen on a device shared by multiple hosts.

 That's reason why people use OCFS or GFS.

> A simple test case for this will be:
> $ dd if=/dev/zero of=img0 bs=1M count=256
> $ mke2fs -F -j img0
> $ ln img0 img1
> $ losetup /dev/loop0 img0
> $ losetup /dev/loop1 img1
> $ mount /dev/loop0 /mnt/loop0
> $ mount /dev/loop1 /mnt/loop1
> 
> This succeeds currently causing a multiple mount.

 And what? That's wrong FS usage.
 
    Karel

-- 
 Karel Zak  <kzak@...hat.com>
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