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Re: [Qemu-trivial] [Qemu-devel] [PATCH] Teach block/vdi about "discarded

From: Eric Sunshine
Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH] Teach block/vdi about "discarded" (no longer allocated) blocks
Date: Thu, 27 Oct 2011 12:20:15 -0400

On Oct 27, 2011, at 12:12 PM, Stefan Weil wrote:

Am 27.10.2011 10:53, schrieb Kevin Wolf:
Am 26.10.2011 21:51, schrieb Eric Sunshine:
An entry in the VDI block map will hold an offset to the actual block if the block is allocated, or one of two specially-interpreted values if not allocated. Using VirtualBox terminology, value VDI_IMAGE_BLOCK_FREE (0xffffffff) represents a never-allocated block (semantically arbitrary
content). VDI_IMAGE_BLOCK_ZERO (0xfffffffe) represents a "discarded"
block (semantically zero-filled). block/vdi knows only about

Signed-off-by: Eric Sunshine <address@hidden>

Thanks, applied to the block branch.


Kevin, I don't want to block improvements. Nevertheless
I'd like to see a small modification in this patch:
both #defines should be implemented without a type cast.
Please change them or wait until Eric sends an update.

My favorite is this:


This would also be ok:

#define VDI_UNALLOCATED 0xffffffffU
#define VDI_DISCARD     0xfffffffeU

Using the macro names and the definitions (with type cast)
from the original VirtualBox code would also be ok.

I originally implemented the change using the macro names, comments, and definitions from the VirtualBox code, but found that it made the diff so noisy that it obscured the simpler underlying change of teaching block/vdi about "discarded" blocks. Sticking with the original VDI_UNALLOCATED macro name kept the diff noise level down.

At any rate, if Kevin can amend the commit with one of your above suggestions, that would be simplest. Otherwise, I can re-roll. Let me know which is preferred.

-- ES

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