From: Ric Wheeler <rwheeler@redhat.com>
Subject: Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT
 option
Date: Tue, 12 Jan 2010 08:04:43 -0500
Message-ID: <4B4C736B.6080403@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> <4B4C7297.5030905@redhat.com>
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Cc: Christoph Hellwig <hch@infradead.org>, linux-ext4@vger.kernel.org
To: Michal Novotny <minovotn@redhat.com>
In-Reply-To: <4B4C7297.5030905@redhat.com>
Sender: linux-ext4-owner@vger.kernel.org

On 01/12/2010 08:01 AM, Michal Novotny wrote:
> 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

If the guest is the only one (when running) that installs a new grub.conf file 
and kernel and it shuts down properly, you should be good. It if does not shut 
down cleanly, it could have a stale grub.conf file (or worse, a partially 
written one), but using O_DIRECT to bypass the file system cache should not help.

If we cannot reproduce this failure, sounds like we need to go back and get a 
better understanding of what the customer saw?

ric