Date:	Tue, 12 Jan 2010 14:33:35 +0100
From:	Michal Novotny <minovotn@...hat.com>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	Christoph Hellwig <hch@...radead.org>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT
 option

On 01/12/2010 02:29 PM, Ric Wheeler wrote:
> On 01/12/2010 08:23 AM, Michal Novotny wrote:
>> On 01/12/2010 02:12 PM, Michal Novotny wrote:
>>> On 01/12/2010 02:04 PM, Ric Wheeler wrote:
>>>> 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
>>>>
>>> That's right. I am going write an e-mail regarding this information to
>>> the reproducer if this bug and tell him that I need more information
>>> about what's happening at the customer side.
>>>
>> One more thing to point out, let's have a look at:
>> https://bugzilla.redhat.com/show_bug.cgi?id=466681#c15 .This is about
>> workaround to drop caches to be added to pygrub in the host machine
>> using this command:
>>
>> echo 1> /proc/sys/vm/drop_caches
>>
>> So this really looks like the caching issue if it's working fine after
>> dropping the caches. That may be the reason why this could be fine with
>> this patch present in e2fsprogs.
>>
>> Michal
>
> That BZ has a pretty long and twisted history, but after a quick read, 
> I still don't see why a cleanly shutdown guest would have issues with 
> caching that using O_DIRECT on read would help.
>
> We will need to dig into a bit more...
>
> ric
>
I am not saying we don't  need to dig a little bit more, we surely do 
but unfortunately I am waiting for information from reporter. But I am 
also thinking that this O_DIRECT functionality support to bypass caches 
could be useful...

Thanks,
Michal
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