Re: Read support for fat_fallocate()? (was [v2] fat: editions to support fat_fallocate())

From: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
To: Namjae Jeon <linkinjeon@gmail.com>
Cc: Andrew Bartlett <abartlet@samba.org>,
	linux-kernel@vger.kernel.org, Ravishankar N <cyberax82@gmail.com>,
	Amit Sahrawat <amit.sahrawat83@gmail.com>,
	Nam-Jae Jeon <namjae.jeon@samsung.com>,
	Ravishankar N <ravi.n1@samsung.com>,
	Amit Sahrawat <a.sahrawat@samsung.com>
Subject: Re: Read support for fat_fallocate()? (was [v2] fat: editions to support fat_fallocate())
Date: Mon, 18 Feb 2013 23:59:43 +0900	[thread overview]
Message-ID: <878v6lzmzk.fsf@devron.myhome.or.jp> (raw)
In-Reply-To: <CAKYAXd-DSo1R7zgu81vc9ckke+5doeEd2QPwqHt54OF-VZo34Q@mail.gmail.com> (Namjae Jeon's message of "Mon, 18 Feb 2013 23:25:11 +0900")

Namjae Jeon <linkinjeon@gmail.com> writes:

>> Hm. My concerns are compatibility and reliability. Although We can
>> change on-disk format if need, but I don't think it can be compatible
>> and reliable. If so, who wants to use it? I feel there is no reason to
>> use FAT if there is no compatible.
>>
>> Well, anyway, possible solution would be, we can pre-allocate physical
>> blocks via fallocate(2) or something, but discard pre-allocated blocks
>> at ->release() (or before unmount at least). This way would have
>> compatibility (no on-disk change over unmount) and possible breakage
>> would be same with normal extend write patterns on kernel crash
>> (i.e. Windows or fsck will truncate after i_size).
> Hi OGAWA.
> We don't need to consider device unplugging case ?
> If yes, I can rework fat fallocate patch as your suggestion.

In my suggestion, I think, kernel crash or something like unplugging
cases handles has no change from current way.

Any pre-allocated blocks are truncated by fsck as inconsistency state,
like crash before updating i_size for normal extend write.  I.e. across
unmount, nobody care whether pre-allocated or not. IOW, if there is
inconsistent between i_size and cluster chain (includes via
fallocate(2)) across unmount, it should be handled as broken state.

In short, the lifetime of pre-allocated blocks are from fallocate(2) to
->release() only.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>