linux-ext4 - Re: [RFC][PATCH] Multiple mount protection

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Message-ID: <4652A192.9010506@clusterfs.com>
Date:	Tue, 22 May 2007 13:23:54 +0530
From:	Manoj Joseph <manoj@...sterfs.com>
To:	Kalpak Shah <kalpak@...sterfs.com>
CC:	linux-ext4 <linux-ext4@...r.kernel.org>,
	Andreas Dilger <adilger@...sterfs.com>
Subject: Re: [RFC][PATCH] Multiple mount protection

Kalpak Shah wrote:

>> 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.

Ah, if the HA-software doesn't deal with multiple mounts for filesystems 
it is managing, then I would claim that the software is flawed. :)

But yes, turning on MMP would help.

It might also help to make the frequency at which sequence number gets 
updated (currently 5 sec) tunable. Would making that also a field in the 
super block be a bad idea (set only by mkfs/tunefs)?

It might also be worthwhile to write the dev_t, the path of the device 
and the hostname to the s_mmp_block, along with the random sequence. (I 
assume there is enough space.) If the mount is being failed because of a 
multiple mount  scenario, these fields could be used to provide useful 
diagnostics.

My $ 0.02. :)

Regards,
Manoj
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