You can finally "redirect" OST files with FSLogix Office Containers!

Outlook in cached mode with non-persistent VDI seemed like a pipe dream until FSLogix came along.

When I last wrote about FSLogix, it was with regards to their Profile Containers product that, for the first time, allowed you to deliver Windows Search, redirect folders (without using folder redirection), and even use Outlook OST files across the network. Every one of us has tried to use those things and failed, most likely due to the fact that things like folder redirection amount to a “mini denial-of-service attack” (Thanks, Helge Klein, for that awesome analogy).

FSLogix saw a lot of interest in Profile Containers, but ran into some headwinds when dealing with customers that wanted the Outlook OST redirection functionality, but had already invested in a profile management tool that they didn’t want to abandon. Because of this, FSLogix cooked up Office Containers.

If you were already familiar with Profile Containers, it’s easiest to think of Office Containers as the same product, just with the profile stuff stripped out. The real story is a bit more complex, because FSLogix had to do some work to make it run standalone and not interfere with other profile management products. While they were in there, though, they also added some optimizations that will filter back into the Profile Containers product over time.

If you’re not familiar with either product, here’s the gist:

The brains behind FSLogix is their filesystem filter driver, which acts as a traffic cop for filesystem activities in certain files, folders, and registry entries (FileSystemLogix–get it?!). When you use their App Container product, this driver is hiding applications from Windows entirely, toggling whether or not they’re hidden with the push of a button. The first version of App Containers required all the applications to be installed in the local image.

Later versions added the ability for the driver to mount VHD files across the network and to redirect applications to the VHD file, thereby not requiring huge disk images. It’s this technology that then morphed into Profile Containers, followed by Office Containers, because now FSLogix could effectively redirect any portion of the filesystem or registry to a network share without actually using Folder Redirection. And, they could do all of that on persistent, non-persistent, terminal servers, or even physical desktops!

The reason it works so well is that when you mount a VHD file across the network, there is a single SMB connection to that VHD file. All data accessed from that VHD file is sent as plain old data without all the craziness that SMB adds every time a single file is opened, read, and closed (along with lots of other operations). So instead of “open, read, close” on every file in your OST (which creates that “mini denial-of-service attack”), you get one “open” as the VHD is mounted, one “close” as the VHD is unmounted, and in between is just read/write operations. It’s that increase in efficiency that allows them (and you!) to “redirect” folders, Outlook OST files, and even Windows Search (but that only works on single-user systems).

Back to Office Containers

As they developed this, they realized that the OST file is a very live database, and that they don’t want to rely on a lock on that VHD file for the entire time. To combat this, one of the changes they made was to create a separate, temporary VHD file used as a transaction log. Transactions are written to this temporary VHD file, then committed to the primary VHD file (and the real OST) when the opportunity presents itself. All of this is handled by the driver, and it’s completely hands-off.

Implementing Office Containers amounts to simply installing the driver, enabling it, telling it where the VHD files live, and how big they should be. There are no reboots required, so deployment is something you can do over lunch. As long as Outlook is using default data file settings, you don’t even have to configure anything there since the filesystem filter driver intercepts and redirects all actions that target those files.

A beta of Office Containers is available, and FSLogix would appreciate your feedback as they approach the release, which should be in mid-July. When released, it will sell for as low as $10 per user, per year, which seems like a no-brainer to enable functionality that we’ve wanted for the last fifteen years.


In the near future, FSLogix will add support for the new model of Microsoft licensing, which will prevent users from having to re-authenticate so often. This especially applies to Office 365 and Skype for Business. They’ll capture the licensing components after they’re set up the first time so that a user doesn’t have to re-authenticate each time the app is delivered to them.

In addition, they’ll also begin caching OneDrive just like they do for Outlook, which Microsoft should love. This, as well as the licensing stuff, should be done by the end of the year.

Longer term, they’re looking at adding support for Windows Search in multiuser environments, which seems possible but will take some work.

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So how does this compare to VMware App Volumes?