Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT option

[Michal Novotny <minovotn@redhat.com>]

On 01/12/2010 04:16 PM, Eric Sandeen wrote:
> 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 PV guests) uses outdated/cached data so some modifications were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.
>
>    

Eric,
indeed, no concurrent accesses are happening that time. The guests file 
system seems to be cached since after dropping caches in host (dom0) it 
is reported to work correctly.

> 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?
>    

It appears that the data are cached in host (dom0) and another access 
(after guest update/shutdown sequence) makes dom0 still provide old 
(cached) data and not the current (newest) data.

Michal