I had a similar starting point with customers usually running setups similar to
- 2 or more dual-cpu servers with Raid5 systems running Hyper-V for the usual VMs:
- Domain Controller
- Print Server
- File Server
- SQL Server for ERP System
- SQL Server running VEEAM for backup
The customers wanted faster systems and were ready to replace the old servers (while keeping the ERP systems). We as sysadmins were fed up with VEEAM as it was such a pain with corrupted backup chains.
So we opted for a change and started small:
Veeam was replaced by ABB which required the 1st NAS. Then we understood how you can protect yourself against ransomware with BRTFS and snapshots. Then we went for off-site backups with snapshot replication which usually led to the 2nd and 3rd NAS.
Over the years we used ABB for backup and for me personally, ABB+Snapshot Replication is by far the best plattform for small to mid-seized companies.
In hindsight the performance bottleneck were never the servers but the network. A properly tuned Windows/Linux file server will always beat a Synology NAS. But as the performance never arrived at the desk of my clients the “low performance of Synology NAS systems” was not relevant.
So we kept the old servers that now are running VMs for PDC (AD, DNS, DHCP), Print Server and SQL. Having eliminated the storage role significantly reduced the workload on the servers. Having a sound backup and good smart monitoring (https://www.hdsentinel.com/) we inserted PCIe NVMe adapters and we got realy responsive servers a very low cost.
The money saved on licenses for backup software was then invested into 10G network equipment (check the Unifi XG24 24*RJ45 10G @ 1300USD) which had a huge impact on the user experience.
Back then a typical low-budget setup for an office with 20-50 clients were 2-3 DS1520+/DS1522+ as file & ABB servers. Default HDDs are Seagate EXOS 16TBs which for the last 2 years didn’t cause any issues. We recently bough a some DS1823xs+ and were very pleasently surprised of the significantly increased performance. I consider the DS1823xs+(same CPU, 110G. 8bays) a slightly downgraded FS2500 (210G, 12 bays).
With a given budget I would rather invest in 2-3 smaller models - mostly DS+ models as they digest almost any HDD you throw on them. Multiple NAS allow to set up (immutable) snap-shot replications which are huge life savers. Starting from DS16xx you can add PCIe 10G NICs which allows for HA solutions. You can install Central Management to have a single dashbord for monitoring and policy management (TIME SYNC between AD and all NAS)
Also, with multiple NAS you can better partition them to specific jobs
- All SSD btrfs NAS for VMs (prefereably with original Synology SSD)
- All HDD ext4 NAS for Surveillance Station
- All HDD btrfs NAS for ABB (3rd party HDDS)
I did not have good experience with Synology Directory Server (AD and AD Forest version 2008) on smaller DS units as they did not seem to properly handle GPOs. Seen the good performance of the DS1823xs+ I have no problems using it as host for a virtual PDC. Seeing that your currenty set-up only has 16GB Ram I would expect the accounting software to run nicely too. But separate your server roles on different VMs.
In your situation I would take a smaller unit (DS1622+/DS1822+) with Synology SSDs (Raid1 2x 1TB) and virtualize the ERP system. If it works buy the DS1823xs+ (Raid 5 with 3x 1TB) and migrate to this system. Use the other NAS for ABB (the more bays the better). Use the DS918+ for Off-site backup to the biggest branch office and setup shared folder sync. Max out RAM on all System. Buy 2/3 16/18/20TB drives for HDD to extend your snapshot chain and add disks when capacity > 60%
If you end with 2 identical boxes where the RAM is not soldered to the MCB you could transfer the Synology RAM from the replication box to the main box, max out the replication box with 3rd party RAM and save some more money.