The_Saver

Because there is no documentation on how to support VSS on the file system level.  
There is documentation on how to access VSS, and plenty of it.  But that's not the issue.  The problem is how the file system is supposed to handle the VSS calls.  There is NO documentation on this, in the wild.   Any documentation that may exist is internal Microsoft documentation. 
If by Samba, you mean Samba/SMB/CIFS/Windows Shares, then you're just connecting to the API.  You're relying on the underlying drive that the SMB share resides on supporting VSS.   This is the top level VSS stuff, we need the bottom/low level info, how you'd implement it on a different file system.
So, right now, we'd have to reverse engineer exactly how VSS interacts with NTFS, at the file system level.  That is not a simple thing, at all. And it would be incredibly time consuming.
If you mean a specific software, could you link it? 
Back up the underlying disks in the pool, not the pool drive. 
As for restoring .... basically the same. 
That or used something file based, or a sync utility (such as AllWays sync, good sync, free file sync, synctoy, etc).

I changed the pool service to delayed start some time ago (wanted to separate the starts of CloudDrive and DrivePool). Works fine, no problems so far.

Yeah, having the Disk Space Equalizer balancer enabled will cause issues here.  
It should be fine to use initially, but once it's done, disable it. 
Specifically, you shouldn't use these two balancers together, since they will interfere with each other.  One or the other, always. 

With the "Disk Space Equalizer" plugin turned -off-, Drivepool will still auto-balance all new files added to the Pool, even if it has to go through the SSD Cache disks first.  They merely act as a temporary front-end pool that is emptied out over time.  The fact that the SSD cache filled up may be why you're seeing balancing/performance oddness, coupled with the fact you had real-time re-balancing going on.  Try not to let those SSDs fill up. 
I would recommend disabling the Disk Space Equalizer, and just leaving the SSD cache plugin on for daily use.  If you need to manually re-balance the pool do a re-measure first, then temporarily turn the Disk Space Equalizer back on (it should kick off a re-balance immediately when toggled on).  When the re-balance is complete, toggle the Disk Space Equalizer back off.

After some testing, Pool B follows Pool A's SSD Optimiser
You can close this thread, thanks for the help!

1x128 SSD for OS, 1x8TB, 2x4TB, 2x2TB, 1x900GB. The 8TB and 1x4+1x2TB are in a hierarchical duplicated Pool, all with 2TB partitions so that WHS2011 Server Backup works. The other 4TB+2TB are in case some HDD fails. The 900GB is for trash of an further unnamed downloading client.So actually, a pretty small Server given what many users here have.

Yup. 
How they work, and how Windows handles them, ESPECIALLY over a network.  
That looks right.
That depends on how you move the data, but yeah, this is likely what will happen.  If you move data from volume to volume, this is what happens.  You only get the "smart move" if the data is on the same volume. 
If the SSD Optimizer is enabled for that pool, yes.

That should work, then.  Just wanted to make sure.
And yes, that would definitely need an SSD (and maybe even an NVMe based SSD) for that. 
If you want to use file placement rules AND the SSD Optimizer, there are some settings that you MUST change. 
 
Junctions.  If you have the option, always junctions.  Symlinks can be handled oddly in some cases.
And yeah, that software is nice
You can create multiple pools, actually.  That may be the best for what you want. 
But keep in mind that each disk can only be part of a single pool. 
 

That is quite a few disks! Glad it helped and you got to work it out.

The Disk Space Equalizer plug-in comes to mind.
https://stablebit.com/DrivePool/Plugins

Yes, that's called Hierarchical Pooling, which DrivePool supports.  The problem there, is that when you add the DB child pool to the master pool, all it's files become visible there to anyone with access to that master pool.  I was under the impression from your first post you wanted to keep the DB files completely out of any pool at all.
 
Perhaps re-defining your goals and giving a little architecture detail would help us to help you:  You have a DB with a lot of files ranging in sizes, that you want the 4 SSDs to support in a pool-style fashion (aka software RAID), but which you don't want people to see.  You also have a regular Pool of disks that hold your main non-DB style data.  And, you're using the 4 SSDs as a front-end cache - are they setup to utilize the DrivePool SSD Optimizer plugin?
Are you against having completely separate pools - one for the DB, and one for your main data store?  If not you can accomplish what you need rather easily.  Partition each of the four SSDs into two logical volumes:  the first part holds 1/4 of the DB, the second is used to cache the main data pool.  You'd make a Pool for the DB by combining all the 1/4 volumes together.  You could utilize the second volume slices on all four as your SSD Optimizer front-end.
 

I guess if you wanted to keep it simple, you could just dedicate one of the four SSDs to the database, leaving the other three for Pool use.  You won't get multi-disk-IO speeds, but you'll still get the raw IOPS speed of a single SSD.
Normally where data stores are considered I wouldn't even think of suggesting RAID due to failure rates on rebuilds and so on, but you're not dealing with RAID 5/6/etc or large storage drives scenario with the SSDs, so it would work well here if you wanted to use it.  Plus with RAID 0+1 or 1 you get the benefit of double-read speeds and redundancy.  But I can respect it if you want to go the simpler route.  Just be prepared to keep multiple daily snapshots of the data as backup points.
 
Wait... I thought you didn't want the DB files on the Pool at all.  
 

You'll need to disable the SSD Optimizer balancer.  
Do that, and see if it works (it should) 

It may have to do to with other balancers. If you have Volume Equalization and Duplication Space Optimizers active, they may need to be de-activated _or_ you need to increase the priority of the Disk Space Equalizer plug-in such that it ranks higher than the other two (but if you have StableBit Scanner, that one should always be #1 IMHO).
I have not actually used that plug-in myself though.
Edit: Did you activate e-measure though?

First, is the database a single file?  
If so, then you can't split it.
Second, you could not use hard links, since these cannot span volumes.  You'd need to use junctions (ideally) or symblinks. 
 
 

You're sorta painting yourself into a corner by wanting to use pool drives, but not the pool, AND split the database across all four of the drives.
What I'd recommend is making a RAID 0 (RAID 0+1 if you can afford the 50% space drop) stripe out of your 4 SSDs, then deciding if you want a separate volume on them just for the DB, or if it can share space with the files you move on/off and simply be in "it's own directory".  I'd think sharing a single logical volume would be okay.
You can have files/folders outside the hidden Pool directory that sit on the drive and behave normally, and which aren't seen by Pool users.  But you can't break up that DB onto separate drives without some type of underlying span mechanism, which in this case would be RAID.
You could then mount that RAID drive to both a letter, and a folder under the "C:\Users\Admin\AppData\Local" path.  Drivepool could use the letter, and you'd still be compliant using the system path for the DB.
No matter what happens, you'll want good timely backups running, since you'll be exposed to either a 4x failure rate (RAID 0) or 2x failure rate (RAID 0+1).

You could do it with a combination of a VPN, Drivepool pool(s), and Clouddrive using file share(s).  Here's how I think it could work:
The VPN connects all computers on the same local net. Each computer has a Pool to hold data, and the Pool drive shared so the local net can access it. Clouddrive has multiple file shares setup, one to each computer connected via VPN and sharing a Pool. Each local Pool can have duplication enabled, ensuring each local Clouddrive folder is duplicated locally X times. The file shares in Clouddrive are added to a new Drivepool Pool, essentially combining all of the remote computer storage you provisioned into one large volume. Note:  this is just me brainstorming, though if I were attempting it I'd start with this type of scheme.  You only need two machines with Drivepool installed and a single copy of Clouddrive to pull it off.  Essentially wide-area-storage.

Hey Guys - 
I've been using DrivePool & Scanner for a few years now and overall it's been great.  My home pool currently consists of 12 disks (11 SATA + 1 SSD for caching) totalling over 43.7tb which is assigned to my D: drive.  Being a big fan of monitoring resources, I'd love to be able to monitor the overall disk performance in some sort of desktop gadget or widget.  This is easy to do for the pool's individual disks if drive letters are assigned or within Scanner, but not the pool as a whole.  Since the pool isn't a standard disk, most applications that do this simply show the D:\ as having no activity ever unfortunately. One of the many examples of what I'd like is an older Windows Gadget "Drive Activity."
Does anyone know of an application or workaround where I could get the pool's activity to be shown for typical monitoring applications?  All I really would want is something simple which would show (or trick applications into showing) either the combined read / write totals or the highest value of the disks comprising the pool.
Thanks!