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January 23, 2014

Atlantis ILIO for Persistent VDI – how safe is my data?

Andrew Wood - Atlantis


Common concerns from those seeing Atlantis ILIO for persistent desktops for the first time is “how quickly do unique changes get saved to the optimised disk?” as well as “could my desktop data get corrupted?

To address these concerns, it is useful to understand how the Microsoft Windows OS commits data, and then we can discuss how Atlantis ILIO ensures data integrity.


How does the Windows OS Commit Data?

In a Microsoft Windows guest VM, an application writes data asynchronously into the page cache of the operating system.  This is then lazily flushed to disk. To maintain data integrity – a two phase commit is used (usually through a mechanism like a file system journal).  It is useful to note that data integrity is guaranteed, but not data loss – even with NTFS.  If we assume that the Windows data flush interval is X milliseconds: in Windows, you can lose X ms worth of data in power loss.


Atlantis ILIO Storage Optimization for Persistent VDI

Atlantis ILIO Persistent VDI is offered in two modes :-

  • Atlantis ILIO Persistent VDI (In-Memory)  enables customers to deploy virtual desktop VMs using a hypervisor’s host RAM as storage, providing a solution that is optimized for a maximum performance. Using In-Memory mode, the virtual desktop VMs are stored on a highly optimized datastore provided by an Atlantis ILIO software appliance with the unique data continuously replicated to shared SAN/NAS storage for persistence.

  • Atlantis ILIO Persistent VDI (Disk Backed) is a persistent VDI deployment model that optimises shared SAN or NAS for storage. Atlantis ILIO Persistent VDI (Disk-based) is a solution that runs in a dedicated Atlantis ILIO virtual machine that is logically placed between the virtual desktop VMs and the storage for the virtual desktops.


Atlantis ILIO Storage with Integrity

When you introduce an Atlantis ILIO to your host, once the guest VM writes to its virtual disk, that I/O is received by the Atlantis ILIO. The ILIO will de-duplicate and acknowledge it even though data is still in memory.  If your VM is hosted In–Memory this acknowledgement happens once the data is mirrored to to the Replication Host. If your VM is hosted on a disk backed ILIO appliance, this acknowledgement happens once the data is written into the ILIO page cache.

Atlantis ILIO also maintains a two phase commit, and will write data to a journal and then to disk. This flush interval is Y milliseconds. Importantly, we have designed the appliance to ensure that Y is always smaller than X.

Atlantis ILIO will never lose more data than is already in the windows page cache which would have been lost in the first place (even without Atlantis ILIO, or on a physical system).

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