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RE: [Xen-API] How Pygrub work on VHD

To: 'Anthony Xu' <anthony@xxxxxxxxx>
Subject: RE: [Xen-API] How Pygrub work on VHD
From: Dave Scott <Dave.Scott@xxxxxxxxxxxxx>
Date: Mon, 25 Jan 2010 21:13:27 +0000
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Thread-topic: [Xen-API] How Pygrub work on VHD
Hi Anthony,

> Thanks for your quick response,
> my understanding is driver (maybe in user space) behind /dev/xvda
> translates raw disk image format to VHD image format on demand, when
> pygrub works on /dev/xvda

That's correct. When a vhd-based VDI is attached to a domain, 
blktap(kernel-space) + tapdisk(user-space) do the translation from raw disk 
block accesses to vhd read/writes.

> What I'm doing is,
> 1. Create a lvm vdi on iscsi SR,
> 2. dd a vhd file to this vdi,
> 3. attach this vdi to a (empty)PV vm as device 0(vbd),
> 4. mark this vbd bootable,
> 5. then start this vm

Unfortunately this isn't going to work. The choice of whether to use blktap 
(vhd-capable) or blkback (raw device only) is a function of the SR's 
content_type. The 'iscsi' SR uses blkback :(

To see what I mean, try something like this instead:

1. Create an 'lvmoiscsi' SR
2. create a VDI in the new SR
3. look inside the new LV -- it should have vhd metadata

Are you trying to import disks in .vhd format? The most future-proof way of 
doing this is to:
* create a VDI using the API
* hotplug the VDI into a VM (eg dom0 or a domU)
* decode the .vhd data, write() it to the raw block device and use seek() to 
preserve sparseness

Simply dd'ing an existing .vhd is risky because XCP is expecting the .vhd to 
have a particular, optimized layout. In particular:
* extra space is left at the beginning of the file for later resizing
* parent locators have a particular naming convention
* blocks are carefully aligned for performance


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