linux-ext4 - Re: Fwd: block level cow operation

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Message-ID: <alpine.LFD.2.00.1304091152430.22989@localhost>
Date:	Tue, 9 Apr 2013 11:56:49 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Prashant Shah <pshah.mumbai@...il.com>
cc:	linux-ext4@...r.kernel.org
Subject: Re: Fwd: block level cow operation

On Tue, 9 Apr 2013, Prashant Shah wrote:

> Date: Tue, 9 Apr 2013 14:35:56 +0530
> From: Prashant Shah <pshah.mumbai@...il.com>
> To: linux-ext4@...r.kernel.org
> Subject: Fwd: block level cow operation
> 
> Hi,
> 
> I am trying to implement copy on write operation

Hi,

In ext4 ? Why are you trying to do that ?

> by reading the
> original disk block and writing it to some other location and then
> allowing the write to pass though (block the write operation till the
> read or original block completes) I tried using submit_bio() /
> sb_bread() to read the block and using the completion API to signal
> the end of reading the block but the performance of this is very bad.
> It takes around 12 times more time for any disk writes. Is there any
> better way to improve the performance ?

I am not sure what you're trying to achieve here, but the simplest
answer is yes, there is a way to improve the performance - use
device mapper to do this. thinp target provides you with block level
cow functionality which enables you to do snapshots efficiently for
example.

-Lukas

> 
> Not waiting for the completion of the read operation and letting the
> disk write go through gives good performance but under 10% of the
> cases the read happens after the write and ends up the the new data
> and not the original data.
> 
> Regards.
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