linux-kernel - Re: How to flush the disk write cache from userspace

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Message-Id: <200701180115.54772.rcorreia@wizy.org>
Date:	Thu, 18 Jan 2007 01:15:54 +0000
From:	Ricardo Correia <rcorreia@...y.org>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: How to flush the disk write cache from userspace

On Tuesday 16 January 2007 00:38, you wrote:
> As always with these things, the devil is in the details. It requires
> the device to support a ->prepare_flush() queue hook, and not all
> devices do that. It will work for IDE/SATA/SCSI, though. In some devices
> you don't want/need to do a real disk flush, it depends on the write
> cache settings, battery backing, etc.

Is there any chance that someone could implement this (I don't have the 
skills, unfortunately)? Maybe add a new ioctl() to block devices, so that it 
doesn't break any existing code?

I believe it's a very useful (and relatively simple) feature that increases 
data integrity and reliability for applications that need this functionality.

I think it must be considered that most people have disk write caches enabled 
and are using IDE, SATA or SCSI disks.

I also think there's no point in disabling disks' write caches, since it slows 
writes and decreases disks' lifetime, and because there's a better solution.

Personally, I'm not really interested in specific filesystem behaviour, since 
my application uses block devices directly (it's a filesystem itself). 
Although I think all filesystems should guarantee data integrity in the face 
of fsync() or metadata modifications, even if it costs a little performance.

Thank you.
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