From: Michal Novotny <minovotn@redhat.com>
Subject: Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT
 option
Date: Tue, 12 Jan 2010 14:01:11 +0100
Message-ID: <4B4C7297.5030905@redhat.com>
References: <4B46FCB2.1090308@redhat.com> <4B4B84E2.1050508@redhat.com> <4B4C54DC.4040006@redhat.com> <4B4C6429.6090803@redhat.com> <4B4C67F5.1020009@redhat.com> <20100112122319.GA20596@infradead.org> <4B4C6B70.1050205@redhat.com> <20100112124600.GA7151@infradead.org>
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Cc: Ric Wheeler <rwheeler@redhat.com>, linux-ext4@vger.kernel.org
To: Christoph Hellwig <hch@infradead.org>
In-Reply-To: <20100112124600.GA7151@infradead.org>
Sender: linux-ext4-owner@vger.kernel.org

On 01/12/2010 01:46 PM, Christoph Hellwig wrote:
> On Tue, Jan 12, 2010 at 01:30:40PM +0100, Michal Novotny wrote:
>    
>> Not really, pygrub doesn't do any manipulation with file system and
>> also, it's not working on a life file system. It's called before the
>> guest boots up to read information about grub.conf/initrd and kernel for
>> PV guest and after this is read and selected in pygrub then the guest is
>> booted using the kernel and initrd extracted from the image (after which
>> the file is closed). Once again, nothing uses write support and it was
>> added just to make it use O_DIRECT for both read and write operations
>> but only pygrub uses only read support and O_DIRECT passed here is the
>> only way to make it use non-cached data.
>>      
> So what caches get in the way?  From the above it seems the situation
> is the following:
>
>   - filesystem N is a guest filesystem.  It's not usually mounted on the
>     host, except for initial setup long time ago
>    

Yes, it is really a guest file system. This is not mounted in the host 
and the reason is to get actual version of grub.conf, initrd and kernel 
to be booted...

>   - before booting a guest your "pygrub" tools needs to read files on
>     it, and it's doing so using e2fsprogs
>    

Correct.

>   - once the guest is life it uses the extN kernel driver to access the
>     filesystem
>    

That's right. So this is no longer pygrub responsibility...

> nowhere in this cycle you should have any stale cached data.  The kernel
> always makes sure to write back data on umount/reboot, as does e2fsprogs
> if actually used to write data (which you said is not the case anyway).
>    

In fact I was unable to run into those problems myself but 
reporter/customer did.

> The only data that may be in the cache are unmodified data from reads
> on the block device from either e2fsprogs or a suboptimal virtual block
> device implementation, but these can't cause any problems.
>    
Michal