linux-ext4 - Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG

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Date:	Tue, 12 Jan 2010 06:59:37 -0500
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Michal Novotny <minovotn@...hat.com>
CC:	linux-ext4@...r.kernel.org, Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT
 option

On 01/12/2010 05:54 AM, Michal Novotny wrote:
> On 01/11/2010 09:06 PM, Ric Wheeler wrote:
>> On 01/08/2010 04:36 AM, Michal Novotny wrote:
>>> This patch extends functionality of e{2|4}fsprogs to add
>>> EXT2_FLAG_DIRECT flag to be passed to ext2fs_open2() function. This
>>> internally calls open() function with O_DIRECT and handles the memory
>>> alignment for both read and write operations.
>>> In some cases direct access to devices is necessary and that was the
>>> main reason for this patch to be done.
>>>
>>> The main reason why this was done is that pygrub (used by xen
>>> virtualization user-space package, it's a python version of grub for
>>> paravirtualized guests) sometimes uses outdated version of grub.conf
>>> file. Modifications to xen package were *not* enough because
>>> e2fsprogs doesn't open the files directly. That's why I added
>>> EXT2_FLAG_DIRECT support to make read/write operations work directly
>>> when passed. It's been tested with pygrub like mentioned above for
>>> read operation and it's working fine.
>>>
>>> Signed-off-by: Michal Novotny <minovotn@...hat.com>
>>
>> Can you add to this some kind of data flow overview? Seems like a
>> really odd way to update this file system...
>>
>> ric
>>
> What do you mean by adding some kind of data flow overview? I am not
> working on file systems but I needed option open files directly to be
> added to e2fsprogs to make pygrub working right with not using
> cached/outdated data so this was the first patch I did for file system
> so I don't realy know what do you mean by data flow overview ...
>
> Thanks,
> Michal

Hi Michael,

What would be useful is where the data is written (guest/host) and the state of 
the file system (mounted/unmounted). Which app it is that writes the data that 
gets cached, who needs to read it and misses that cached data.

 From the above, it is really not clear that you should not be able to use 
existing interfaces to flush any cached data to disk....

ric

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