linux-ext4 - Re: How can I flush all writes before yanking the power cable?

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Date:	Thu, 07 Feb 2013 12:12:31 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Autif Khan <autif.mlist@...il.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: How can I flush all writes before yanking the power cable?

On 2/7/13 10:43 AM, Autif Khan wrote:
> The standard operating procedure to power down my machine is to switch
> it off. To work around this, we use mSATA SSDs (actually we recently
> switched from SATA SSDs) with linux on a read only partition.

Not sure the SSD part makes any significant difference, but the RO
mount should.

> This works just fine, however, we want to be able to upgrade some
> parts of the application. To do this, we have put the application on
> /app partition. We mount it read only at start up. When we want to
> upgrade the app, we remount read-write sync (mount -o remount,rw,sync
> /app) perform the write operations and remount read only.
> 
> If we yank the power cable after this, we get file system errors on
> the next reboot.

What kind of errors? (and on what kernel?  Are you mounted with
barriers enabled?)

If you use barriers, remount RO, that completes, you yank the power,
and you see corruption, I would guess one of a few things is happening:

1) You're not mounting w/ barriers, and you lose data in the SSD's cache
2) You *are* mounting w/ barriers, and the SSD is lying to you
3) There's a bug in our remount,ro path which doesn't quiesce things properly

mount -o remount,ro should be >this< close to an unmount; things should
be stable on disk when it's done.

-Eric

> We can display a message to the user telling them that it is safe to
> power down the machine.
> 
> My question is
> 
> 1) Is this the right place to discuss this or should I have posted
> this in the file systems mailing list?
> 
> 2) how can we determine that all the writes are flushed? (and this it
> is safe to yank the power cable)
> 
> 3) is there a better way to do this? - for example we may not have to
> remount read write sync - and we can force a sync before remounting
> read only or something
> 
> I have already tried "sudo sync" before remounting the filesystem as
> read only. It does not help.
> 
> Please advise.
> 
> Thanks
> 
> Autif
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