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Re: [PATCH] fsfreeze: suspend and resume access to an filesystem

To: Masatake YAMATO <yamato@xxxxxxxxxx>
Subject: Re: [PATCH] fsfreeze: suspend and resume access to an filesystem
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Thu, 13 May 2010 09:54:12 -0500
Cc: util-linux-ng@xxxxxxxxxxxxxxx, e2fsprogs-ext4@xxxxxxxxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-cluster@xxxxxxxxxx, htaira@xxxxxxxxxx
In-reply-to: <20100513.163313.67205054690538917.yamato@xxxxxxxxxx>
References: <1194896399.685821273733580088.JavaMail.root@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> <20100513.163313.67205054690538917.yamato@xxxxxxxxxx>
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Masatake YAMATO wrote:
> Hi,
> 
> (The disscussion can be found at
>  http://thread.gmane.org/gmane.linux.utilities.util-linux-ng/3181/focus=3193)
> 
>> Hello.
>>
>> I understand reason when it use with device-mapper.
>> I think, fsfreeze command need for filesystem on physical block device 
>> without device-mapper.
>> For example, by storage controller based LUN snapshot.
>>
>> # fsfreeze -f /data
>> # ssh root@xxxxxxxxxxx "take snapshot lun0"
>> # fsfreeze -u /data
>>
>> * /data is mounted physical block device(/dev/sdb1)
> 
> As Hajime wrote, taking snapshot in physical storage level is popular
> situation. It seems that xfs_freeze can be used for the purpose but
> the name `xfs_freeze' gives the impression that the command is only 
> for xfs. 
> 
> My argument can be applicable to gfs2_tool, too.  "gfs2_tool freeze"
> also does ``ioctl(fd, FIFREEZE, 0)''.
> 
> 
> One of the solution is to add xxx_freeze for each file system implementation
> which has freeze/unfreeze methods to eash util-xxx, xxx-progs or xxx-utils. 
> e.g. Adding ext4_freeze or ext3_freeze command to e2fsprogs package.
> 
> However, I think this is not good idea. Linux provides file system neutral
> interface already. So it is better to have file system neutral 
> command(fsfreeze)
> and the command is included in file system neutral package, util-linux-ng.

I tend to agree, since there is a common interface, there is no reason to
have filesystem-specific tools which all do the same thing.

Note that xfs_freeze existed long before the common interface, and in fact
the common ioctl number was chosen based on the xfs number, so that explains
the existence of the xfs-specific tool, and why it does happen to work
now on non-xfs filesystems....

-Eric

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