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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] [PATCH 2/3] block-nbd: fix use of protocols in backing files and nbd probing |
Date: | Sat, 11 Sep 2010 12:27:43 -0500 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.12) Gecko/20100826 Lightning/1.0b1 Thunderbird/3.0.7 |
On 09/11/2010 11:53 AM, Stefan Hajnoczi wrote:
On Sat, Sep 11, 2010 at 3:04 PM, Anthony Liguori<address@hidden> wrote:Additionally, there's a spurious read when using an nbd protocol that can be quite destructive when using copy-on-read. Potentially, this can lead to probing an image file over top of NBD but this is completely wrong as NBD devices are not growable.Can you describe the copy-on-read scenario where the 2 KB probe read is a problem? Accessing a fixed size image file over NBD is probably uncommon, but I'm not sure if there's a reason to forbid it.
I think the better solution is to explicitly specific raw with nbd. IOW, I think -drive file=nbd:localhost:1026,format=raw should work the same way. I still feel slightly weird about probing happening with nbd. It seems like it could only result in badness.
The specific scenario is migration. I'm using a copy-on-read file on the destination and I want to be sure that I don't read any blocks (since they're copied) until the source stops execution.
Regards, Anthony Liguori
Stefan
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