What’s New In Windows Home Server Vail Drive Extender?

Mark Vayman, Program Manager for the Windows Home and Small Business Server Team, has posted a summary of the major new features in Drive Extender for Windows Home Server (WHS) Vail.

Of course not everything is new and some Drive Extender (DE) functionality is carried over from the current version:

  • Duplication can be turned on/off per folder.
  • Duplicated folders can survive a single hard drive failure.
  • Storage pool can be easily expanded using different drive types and various sizes.

The new and improved features are listed below (I have added my own emphasis):

  • For duplicated folders, data is duplicated in real time to two separate drives – there is no hourly migration pass.
  • File system level encryption (EFS) and compression are now supported for Drive Extender folders.
  • File conflicts are gone, duplication works as intended for files in use as it is performed at the block level now.
  • The remaining amount of data to synchronize/duplicate is reported per storage pool.
  • All storage operations are executed in the background without blocking other server operations. Specifically, drive removal can be issued without impacting the online state of shares.
  • Drives in a storage pool can be named with a custom description to enable physical identification of the drive in the server.
  • Drive serial number and exact connection type is reported for each drive.
  • Drives which are bigger than 2TB can be added to a storage pool.
  • iSCSI storage devices can be added to the a storage pool.
  • The system drive can be excluded from the storage pool.
  • A new low-level storage check and repair diagnostic operation was added.
  • All storage operations are performed with very low I/O priority to ensure they don’t interfere with media streaming.A new “folder repair” operation is available which runs chkdsk on the folder’s volume.
  • To protect against silent storage errors (bit flips, misdirected writes, torn writes), additional information is appended to each 512-byte sector stored on drive. In particular, each sector is protected by a CRC checksum, which enables Drive Extender to detect data read errors, perform realtime error correction and self-healing (up to 2 bit errors per sector if duplication is disabled, and any number of bit errors if duplication is enabled) and report the errors back to the user and application. The overhead for this additional data is roughly 12% of drive space.
  • Data drives in storage pools can be migrated between servers, and appear as a non-default pool. A non-default pool can be promoted to a default pool if no default pool exists.

Some features though have been lost, the most significant of which (in my opinion) is that a “data drive from a storage pool cannot be read on machine not running the ‘Vail’ server software“. This is explained elsewhere in another thread by Bulat Shelepov, Test Lead (Drive Extender), Windows Home and Small Business Server Team:

Drive Extender v2 is a volume driver that sits under the file system … [and] presents its data as regular NTFS volumes (each share, e.g. Music, Videos, etc. is a separate volume with its own drive letter) …  Internally, these NTFS volumes are sliced […] into 1 GB chunks, which are distributed (in multiple copies if duplication is enabled) across multiple physical disks […] DEVolume.sys [currently a Vail only driver] is the only driver […] that can parse this layout and present the aforementioned NTFS volumes to applications. Consequently, any system not running DEVolume.sys is currently unable to retrieve the data from directly connected DEv2 disks – that includes all client Windows OS.

I certainly hope that DEVolume.sys can be packaged for other Windows operating systems (at least for Windows 7 which is a close enough relative to Windows Server 2008 R2).

Beyond this it is also interesting to note the following point:

  • Internally, the “Vail” software has been tested with up to 16 hard drives and with up to 16 TB of total storage capacity. We’re aware of a number of bugs that occur beyond these limits, so please keep your beta installations under 16 drives and 16 TB total drive space.

This goes someway towards explaining the ten hard drive limit for the Vail Beta, and again this is something that Microsoft are working on, though it appears that the number of disks is less important than the total size of the storage pool. Again, hopefully this issue will be addressed prior to the release of Vail.


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