My brother’s company are looking to replace their ailing Xserve with a Synology NAS. He asked me a question I don’t know the answer to. How does Spotlight work, if at all, with a Synology volume?
I.e. do one or more of the Macs have to “see” the volume and perform the index themselves, and if so indexed, will other Macs accessing the volume take advantage of what is already scanned?
I think Synology’s file system would be a problem. I’ve been able to index Apple file systems on network drives before, but never my Synology shares.
If you mean integration into the CMD + Space spotlight bar that won’t work.
If you mean to be able to search in Finder on a drive that will work if indexing is turned on.
Right now the overall experience of using Synology and Apple together isn’t that great.
Their iOS apps are outdated and do not fully integrate into the Files app. Meaning, they show up as file providers, but you won’t be able to navigate in the Files app’s column view. Instead a pop-up will open with a custom interface of Synology’s apps. You can however establish direct connections via SMB or SFTP/SSH.
On macOS the situation currently is even worse. Due to Apple’s questionable SMB support and the discontinuation of AFP navigating Synology network shares can be a frustrating endeavor. According to rumors Synology will react to Apple’s mistakes with DSM 7, which is supposedly released in summer this year. In the mean time I highly advise to ditch Finder and resort to using a SFTP program to access Synology network shares.
Just to add on what folks above already mentioned; indexing a large amount of files across a network is never a great or smooth experience. I found the back/forth traffic of spotlight doing it’s thing on the Synology drive slow, and on occasion leading to instabilities (yeah, vague I know, no time to troubleshoot these things if a reset gets me going again).
Needless to say, I am not a great fan of spotlight on external (or internal) drives. If I need to find something on the NAS I use devonthink’s indexing abilities - just on the folder where I know or suspect my file is.
Actually, with AFP activated, the DiskStation (with Universal Search activated) will let your Macs make use of the remote search index. I think there are some technical limitations to how many folders can be indexed, though, so keep that in mind. (Only 1000 main folders, I think, with unlimited subfolders. May or may not at all be relevant.)
As has already been mentioned by leo, AFP has been depreciated, but if your brother’s company has been using Xserves until now, that‘s likely not a problem for them (yet)?
Also, SMB support on macOS is still not without fault anyway, afaik. 
I‘m using my MacBook occasionally to access my company‘s DiskStation via AFP, and search works pretty well. The overall performance of navigating through remote directories feels a bit underwhelming sometimes, but that might be more related to our specific setup. (Not a perfect network setup, no SSD cache for the NAS, etc.)
Interestingly, Acronis offers a software package for Windows Servers to provide AFP file indexing to macOS clients. Very expensive and I have not tested it, but stumbled across it several times as a recommendation:
If for some weird reason (sounds like a primarily macOS-focused envrionment, but who knows), they end up with a Windows Server, then this might be a field to look further into.
Edit:
https://www.synology.com/en-global/knowledgebase/DSM/help/SynoFinder/universalsearch_index
This sounds like you can even connect your Macs via SMB, and they will still use the remote universal search index. I cannot test this today, unfortunately.