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Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format


From: Christoph Hellwig
Subject: Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format
Date: Fri, 10 Sep 2010 15:48:59 +0200
User-agent: Mutt/1.3.28i

On Fri, Sep 10, 2010 at 08:22:14AM -0500, Anthony Liguori wrote:
> fsck will always be fast on qed because the metadata is small.  For a 
> 1PB image, there's 128MB worth of L2s if it's fully allocated (keeping 
> in mind, that once you're fully allocated, you'll never fsck again).  If 
> you've got 1PB worth of storage, I'm fairly sure you're going to be able 
> to do 128MB of reads in a short period of time.  Even if it's a few 
> seconds, it only occurs on power failure so it's pretty reasonable.

I don't think it is.  Even if the metadata is small it can still be
spread all over the disks and seek latencies might kill you.  I think
if we want to make qed future proof it needs to provide transactional
integrity for metadata updates, just like a journaling filesystem.
Given that small amount of metadata and less different kinds it will
still be a lot simpler than a full filesystem of course.




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