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

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Message-ID: <878v6lzmzk.fsf@devron.myhome.or.jp>
Date:	Mon, 18 Feb 2013 23:59:43 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Namjae Jeon <linkinjeon@...il.com>
Cc:	Andrew Bartlett <abartlet@...ba.org>, linux-kernel@...r.kernel.org,
	Ravishankar N <cyberax82@...il.com>,
	Amit Sahrawat <amit.sahrawat83@...il.com>,
	Nam-Jae Jeon <namjae.jeon@...sung.com>,
	Ravishankar N <ravi.n1@...sung.com>,
	Amit Sahrawat <a.sahrawat@...sung.com>
Subject: Re: Read support for fat_fallocate()? (was [v2] fat: editions to support fat_fallocate())

Namjae Jeon <linkinjeon@...il.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@...l.parknet.co.jp>
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