Disk Emulation and Trim Instruction

Disk Emulation and Trim Instruction

[Gordan Bobic]

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?

Gordan

Re: Disk Emulation and Trim Instruction

[Avi Kivity]

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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