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Re: [Qemu-discuss] What does "COW" mean?

From: shhuiw
Subject: Re: [Qemu-discuss] What does "COW" mean?
Date: Wed, 3 Sep 2014 17:24:50 +0800 (CST)

At 2014-09-03 03:29:55, "Kevin Wolf" <address@hidden> wrote:
>Am 03.09.2014 um 04:06 hat shhuiw geschrieben:
>> At 2014-09-02 04:33:50, "shhuiw" <address@hidden> wrote:
>> >
>> >
>> >Hi,
>> >
>> >I'm new to qemu community, and I'm trying the COW image format (old but 
>> >simple:-).
>> >I have read through its source code, and didn't find anything about 'copy 
>> >on write'.
>> >I wonder wthat "COW" stands for?
>> Sorry for my unclear expression.
>> I mean when copy-on-write happens if COW image format is used, and how the 
>> COW driver code handles cop-on-write?
>It is what happens in cow_write(). Initially the sector is marked as
>unallocated in the bitmap and you don't need space for it. Any reads are
>redirected to the backing file. As soon as you write to it, it gets
>marked as allocated and written into the image file. This is the COW

This kind of cow is different from what I have known about process management 
and filesystems like btrfs.
It's more like allocation-on-demand.

Thanks, Kevin.



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