Making existing VDI disk shareable

[scottgus1]

Just in case, Mjf, you come back wondering what someone else has posted, I have an idea that could get you to your desired destination without the risky "shared" disk usage you are being warned against. (I might have missed this idea above, sorry if I did)

You want more than one guest to be able to access a single repository of data. You don't want that data on your host as a shared folder. Fair enough. Use a shared folder in another guest, and use Internal Networking to connect the guests.

Get a low-impact Linux, like a NAS OS maybe, attach your repository VDI to the NAS guest, and share a folder on the repository VDI. Attach the guests together on one Virtualbox Internal network and give each guest a static IP address, or if you want the host in the network too, use the same Host Only network on each guest (the NAS guest will probably need a static IP either way, just use one of the any applicable ranges if you use internal, or use the range picked by the Host-only).

[mjf]

Thanks for your suggestion, scottgus1. I guess sharing folders among guests is much faster than e.g. using NFS?

[scottgus1]

I'm not certain, myself, I'm not much of a Linux guy. PerryG might be able to weigh in if he sees this.
If NFS (had to google it, apparently "Network File System") is one of those things Linux can set up, just try it and see what you get.
But letting one guest act as a server for other guests is probably going to be the way you need to solve your problem.

[socratis]

Also, be aware that shared folders in VirtualBox present a very simplified file system implementation, just enough to read/write files quick and dirty. Many applications can error when using shared folders, because they expect advanced features, like file locking or access controls, which don't exist for shared folders. You should definitely take that into account.

[scottgus1]

Socratis is referring to Guest Additions Shared Folders, which can be made without having any network in the guest. Guest Additions Shared Folders are really only for file copying, not for running programs, databases, etc. You will want to set up a complete Virtualbox network with virtual NICs and an internal or host-only network, and whatever real shared folders Linux OS's can make when there's a network in place, for full file system activity.

[Perryg]

First off shared disks are not the way to go for most. The image is read only and can not be written to when the guest reboots and is only doable as a fixed disk which is not the default format. I would really like to know the use case here before I give any advise.

[scottgus1]

Hey there, Perry! Mjf was asking if a "shared" drive could be attached to multiple guests as a central depository for data all the guests would use. He was warned off the idea. I suggested using another NAS-like low-impact guest as a server with shared folders that other guests would access over an internal or host-only network. I called up your name regarding the question mjf had about using NFS as opposed to regular shared folders.

[Perryg]

It would all depend on what the share is. Is it data or is it the actual guest? Shared data would work in NSF and might work with shared folder, but I have never tried it myself. The real issue would be what happens if two or more entities were writing to the share at the same time. My preference would be data and NFS as that is not only doable but the preferred way with Linux.

[mjf]

The initial suggestion was to use shared folders on the host. Now it sounds a bit as if there might be a potential problem with simultaneous writes to files in shared folders as well. Does VirtualBox handle concurrent access to shared folders by multiple VMs?

[socratis]

Does VirtualBox handle concurrent access to shared folders by multiple VMs?

Does your operating system handle concurrent access to files/folders by multiple processes?

Hint: Not, unless there is a special setup, a referee or a "traffic cop". The networking route of NFS that was suggested has a built-in traffic cop. Specialized programs (databases come in mind) have a built-in traffic cop. VirtualBox does not.

So, don't try to fit a square peg in a round hole. Use existing tools that have been known to work well. Take an example from the physical world and apply it to the virtual. How would you setup the whole thing if we were talking about multiple PCs and not VMs? That's what you should be focusing on.

[mjf]

What makes you think that I want to use shared folders on the host?

Why don't you answer my question with a simple "no"? The fact that a given OS does not support concurrent file access does not mean that VirtualBox can't implement it for shared folders.

[socratis]

A fellow user recently reminded me:

• As people sometimes say, if you define a problem no one else has, you also have to figure out the solution no one else cares for.

The fact that a given OS does not support concurrent file access does not mean that VirtualBox can't implement it for shared folders.

You were looking for simple answers: No.

[scottgus1]

Does VirtualBox handle concurrent access to shared folders by multiple VMs

Virtualbox makes the network. The guest OS's decide how to handle files they can access over a real network shared folder.

If I and a co-worker both open a text file in Notepad on a shared folder, we are both allowed to edit, but whoever saves last gets the final version. A 3D modelling program we run uses databases which allow changes from several users at once. When my co-worker opens a MS Word or AutoCAD document, I can't open directly, but I can open read-only.

It's all up to how the programs running on the client PCs are designed to multi-access files on the server.

Don't over-think the Virtualbox side of things. You mentioned you don't want the shared files on the host, that's fine. Just set up a NAS-like guest to handle a shared folder, put your data in it, set all the guests to connect to the same Virtualbox internal or host-only network, and access the NAS's shared folder in the other development guests. If the development programs have any glitches accessing the data simultaneously, then the programs need to be investigated with their developers. The same issue would have happened if you were able to use a shared disk as you wanted to in the beginning, or if you had all physical PCs and a real wired network & router.