Re: Disk Emulation and Trim Instruction

[Avi Kivity <avi@redhat.com>]

On 08/14/2009 01:11 AM, Gordan Bobic wrote:
> With the recent talk of the trim SATA instruction becoming supported 
> in the upcoming versions of Windows and claims from Intel that support 
> for it in their SSDs is imminent, it occurs to me that this would be 
> equally useful in virtual disk emulation.
>
> Since the disk image is a sparse file, it always only grows, and 
> eventually it will grow to it's full intended size even if the actual 
> used space is a small fraction of the container size. Since the trim 
> instruction tells the disk that a particular block is no longer used 
> (and can thus be scheduled for erasing as and when required), the same 
> thing could be used to reclaim space used by sparse files backing the 
> VM. It would allow for higher overcommit of disk usage on VM farms.
>
> Is this feature likely to be available in KVM soon?

We do want trim support for the reasons you mention as well as others.  
I don't know about anyone working to implement it though.

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