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

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Message-ID: <1256921545.3145.51.camel@mini>
Date:	Fri, 30 Oct 2009 17:52:25 +0100
From:	Alexey Fisher <bug-track@...her-privat.net>
To:	Eric Sandeen <sandeen@...hat.com>
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]

Am Freitag, den 30.10.2009, 11:14 -0500 schrieb Eric Sandeen:
> 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.

ok, got it. Every day learning something new.
It was not clear for me, after i read man hdparm: "Get/set read-only
flag for the device.  When set,  Linux  disallows write operations on
the device."

> > 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.

:( are there any plans to unify mount options?

> > 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.

Hmm... bdev. any chance to do temporary recovery and load it as external
journal if ro used? Anyway, you already pointed me to hdparm, so i can
use it too.

> > 4. many frustrated admins who mounted part of raid1 only with "-o ro"
> 
> Dunno what you mean by that ...

raid1 is down, so you need for some reasons to mount ro only one disk of
the array. Needed to do it for short time (i used -o ro), now i know
this probably was a bad idea (bad me, should read documentation). Need
to check my raid now.  Suddenly i'm not alone who doing this :(

> -Eric
> 
> > Regards,
> > Alexey
Eric, Greg,
Thank you

Regards,
Alexey.

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