linux-ext4 - Re: xt4 - True Readonly mount [WAS - Re: [Bug 14354] Bad corruption with 2.6.32-rc1 and upwards]

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Date:	Fri, 30 Oct 2009 11:14:23 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Alexey Fisher <bug-track@...her-privat.net>
CC:	Greg Freemyer <greg.freemyer@...il.com>,
	Ted Augustine <taugustine@...hpathways.com>,
	linux-ext4@...r.kernel.org
Subject: Re: xt4 - True Readonly mount [WAS - Re: [Bug 14354] Bad corruption
 with 	2.6.32-rc1 and upwards]

Alexey Fisher wrote:
> Am Freitag, den 30.10.2009, 10:14 -0500 schrieb Eric Sandeen:

...

>> After a little brief digging I'm not sure when the xfs mount option went 
>> in or why...
>>
>> But for both
>>
>> xfs: mount -o ro,norecovery
>>
>> and
>>
>> ext[34]: mount -o ro,noload
>>
>> I don't think either one should touch the disk.
>>
>> Also, both should skip journal replay if you set the block device 
>> readonly prior to mount (hdparm -r can do this).
> 
> Interesting tip, thank you.
> But there is some problems:
> 1. "hdparm -r" will set complete drive to ro mode. This is bad if i
> use /dev/sda1 for root and /dev/sda5 need to be forced readonly.

So point it at the partition not the drive:

[root@...n ~]# hdparm -r 1 /dev/sda1

/dev/sda1:
  setting readonly to 1 (on)
  readonly      =  1 (on)
[root@...n ~]# hdparm -r /dev/sda2

/dev/sda2:
  readonly      =  0 (off)

It doesn't change the hardware, it sets a flag on the kernel's block 
device structure.

> 2. the fact xfs and ext[3,4] use different options for true_ro make
> things complicated.

the hazards of being an open source sysadmin I guess.

> 3. the definition of ro is broken.

depends on what you mean by ro.  A user can only read from the 
filesystem so it is accurate in that respect.  Is "ro" for the fs or the 
bdev?  Semantic differences but not necessarily broken.

> 4. many frustrated admins who mounted part of raid1 only with "-o ro"

Dunno what you mean by that ...

-Eric

> Regards,
> Alexey
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