From: Eric Sandeen <sandeen@redhat.com>
Subject: Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT
 option
Date: Tue, 12 Jan 2010 09:16:20 -0600
Message-ID: <4B4C9244.2050704@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>
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Cc: Michal Novotny <minovotn@redhat.com>,
	Ric Wheeler <rwheeler@redhat.com>, linux-ext4@vger.kernel.org
To: Christoph Hellwig <hch@infradead.org>
In-Reply-To: <20100112122319.GA20596@infradead.org>
Sender: linux-ext4-owner@vger.kernel.org

Christoph Hellwig wrote:
> On Tue, Jan 12, 2010 at 01:15:49PM +0100, Michal Novotny wrote:
>> I don't really know if I see your point but the thing here is that there  
>> was no way to open a file directly (ie. using O_DIRECT). The direct  
>> write support has been added only to make it possible to use both read  
>> and write directly. The main reason to create this patch was to add  
>> direct read support and flush capability won't help me at all. I am  
>> working in Red Hat, Virtualization team on Xen so I am really not that  
>> much familiar with file systems but what I needed was an option to read  
>> the data directly (using O_DIRECT) in e2fsprogs. One bug was about  
>> pygrub (Python version of GRUB of Xen PV guests that is internally using  
>> e2fsprogs functionality to access data on ext2/3/4 partition to boot the  
>> PV guests) uses outdated/cached data so some modifications were  
>> necessary to open everything directly...
> 
> So to get things staigt:  you're using e2fsprogs to manipulate a life
> filesystem and thing using O_DIRECT saves your ass?  I think you need to
> rething your model of operation fundamentally in that case. 
> 

Christoph - 

It's my understanding that nobody is doing concurrent access.

If the host reads the block device via pygrub to boot the guest,
the guest while running updates the same device when installing
a new kernel (either through the fs, or by writing to the bdev;
probably the former...), and then the guest shuts down - is there
something which will sync the host's cache (cached from the
prior read) with the updates from the guest?

I don't know nearly enough about how the virt IO goes ...

But if access is sequential between host & guest, I'd hope that
this would all just work.

If that is all supposed to work, then maybe the description of the
data flow (as Ric asked for) is not correct.

-Eric