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Re: [Qemu-devel] Moving beyond image files

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Moving beyond image files
Date: Mon, 21 Mar 2011 21:35:38 +0000

On Mon, Mar 21, 2011 at 3:05 PM, Anthony Liguori <address@hidden> wrote:
> 2) The daemon maintains metadata for each image that includes an extent
> mapping and then a clustered allocated bitmap within each extent (similar to
> FVD).

s/clustered allocated bitmap/cluster allocation bitmap/ ?

> 3) All writes result in a sha1 being calculated before the write is
> completed.  The daemon maintains a mapping of sha1's -> clusters.  A single
> sha1 may map to many clusters.  The sha1 mapping can be made eventually
> consistent using a journal or even dirty bitmap.  It can be partially
> rebuilt easily.

Can you explain this in more detail?  A write to a single sector of a
cluster causes what to happen?  Why is the hash calculated before
acking the write and not queued in the background if the hash mapping
is only eventually consistent?

For v3:

1. Snapshots.

2. You can connect remote daemons for read-only master images.  If an
image is backed off a remote image, reads to unallocated clusters are
sent to the remote.  This also allows for a master image daemon to
keep refcounts of how many instances are currently based of an image,
and if they copy/stream that data they can drop instances and storage
administrators know the master image is safe for deletion.


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