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Date: Wed, 19 Jan 2011 01:12:12 +0000
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image
	format that outperforms QCOW2 by 249%
Message-ID: <20110119011212.GK30268@shareable.org>
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To: Chunqiang Tang <ctang@us.ibm.com>
Cc: qemu-devel@nongnu.org

Chunqiang Tang wrote:
> > Based on my limited understanding, I think FVD shares a 
> > lot in common with the COW format (block/cow.c).
> > 
> > But I think most of the advantages you mention could be considered as 
> > additions to either qcow2 or qed.  At any rate, the right way to have 
> > that discussion is in the form of patches on the ML.
> 
> FVD is much more advanced than block/cow.c. I would be happy to discuss 
> possible leverage, but setting aside the details of QCOW2, QED, and FVD, 
> let’s start with a discussion of what is needed for the next generation 
> image format. 

Thank you for the detailed description.

FVD looks quite good to me; it seems very simple yet performant at the
same time, due to its smart yet simple design.

> Moreover, using a host file system not only adds overhead, but 
> also introduces data integrity issues. Specifically, if I/Os uses O_DSYNC, 
> it may be too slow. If I/Os use O_DIRECT, it cannot guarantee data 
> integrity in the event of a host crash. See 
> http://lwn.net/Articles/348739/ . 

You have the same issue with O_DIRECT when using a raw disk device
too.  That is, O_DIRECT on a raw device does not guarantee integrity
in the event of a host crash either, for mostly the same reasons.

-- Jamie