Microsoft recomends virtual disks should not be sparse?

NTDEV
1

Hello,

A diff between WDK 10 and WDK 10 TH2 showed this change in wording for
an error code for virtual disks, as you can see they have added sparse
files to what to avoid in adition to compressed or encrypted. From time to
time it has been said that virtual disks can not be cached because of a
limitation in the cache manager and that explains the uncompressed
and unencrypted requirement but I wonder why sparse is added, sparse files
are very good for virtual disk because they can start small and grow,
Vmware recomends the use of sparse files for their virtual machines.

Win 10:
//
// MessageId: STATUS_VIRTUAL_DISK_LIMITATION
//
// MessageText:
//
// The requested operation could not be completed due to a virtual disk
system limitation. On NTFS, virtual hard disk files must be uncompressed
and unencrypted. On ReFS, virtual hard disk files must not have the
integrity bit set.
//
#define STATUS_VIRTUAL_DISK_LIMITATION ((NTSTATUS)0xC03A001AL)

Win 10 TH2:
//
// MessageId: STATUS_VIRTUAL_DISK_LIMITATION
//
// MessageText:
//
// The requested operation could not be completed due to a virtual disk
system limitation. Virtual hard disk files must be uncompressed and
unencrypted and must not be sparse.
//
#define STATUS_VIRTUAL_DISK_LIMITATION ((NTSTATUS)0xC03A001AL)

Bo Branten

2

Just use Dynamic VHD(X) instead of a sparse one.

–
Maxim S. Shatskih
Microsoft MVP on File System And Storage
xxxxx@storagecraft.com
http://www.storagecraft.com

“Bo Branten” wrote in message news:xxxxx@ntdev…
>
> Hello,
>
> A diff between WDK 10 and WDK 10 TH2 showed this change in wording for
> an error code for virtual disks, as you can see they have added sparse
> files to what to avoid in adition to compressed or encrypted. From time to
> time it has been said that virtual disks can not be cached because of a
> limitation in the cache manager and that explains the uncompressed
> and unencrypted requirement but I wonder why sparse is added, sparse files
> are very good for virtual disk because they can start small and grow,
> Vmware recomends the use of sparse files for their virtual machines.
>
> Win 10:
> //
> // MessageId: STATUS_VIRTUAL_DISK_LIMITATION
> //
> // MessageText:
> //
> // The requested operation could not be completed due to a virtual disk
> system limitation. On NTFS, virtual hard disk files must be uncompressed
> and unencrypted. On ReFS, virtual hard disk files must not have the
> integrity bit set.
> //
> #define STATUS_VIRTUAL_DISK_LIMITATION ((NTSTATUS)0xC03A001AL)
>
>
> Win 10 TH2:
> //
> // MessageId: STATUS_VIRTUAL_DISK_LIMITATION
> //
> // MessageText:
> //
> // The requested operation could not be completed due to a virtual disk
> system limitation. Virtual hard disk files must be uncompressed and
> unencrypted and must not be sparse.
> //
> #define STATUS_VIRTUAL_DISK_LIMITATION ((NTSTATUS)0xC03A001AL)
>
> Bo Branten
>
>

3

A sparse file for a VHD doesn’t make sense because VHD blocks are not mapped to a file block 1:1.