linux-ext4 - Re: Question on delalloc

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Message-ID: <51ADF914.6070209@redhat.com>
Date:	Tue, 04 Jun 2013 09:26:28 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	宋柏翰 <solarispika@...il.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: Question on delalloc

On 6/3/13 2:00 PM, 宋柏翰 wrote:
> Hi everybody,
> 
> I am new to ext4 and doing research on Android with ext4 as file
> system. These days, I have a question on ext4's delayed allocation
> against ext4_sync_file.
> I have learned that delalloc won't guarantee file data's integrity on
> power failure, since those delayed allocated buffer heads won't be
> handled by jbd2.

I just want to address your first assertion regarding delalloc:

> In order to protect data, user programs need to fsync
> those files to be secured.

This is true with or without delayed allocation.

With delayed allocation, the blocks are chosen a the time of the IO.
Without delayed allocation, the blocks are chosen at the write syscall time.

But in both cases, data is only in memory after the write(), and is not
guaranteed to be on disk until an fsync or similar data integrity syscall.

http://lwn.net/Articles/457667/

-Eric
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