→ Applies to: Hyperion 9.x and above

Overview

On Hyperion, not all system RAM is available for virtual machines. Two reservations are made by default before any RAM is allocated to VMs:

• 12 GB reserved for the operating system
• 20% of total RAM reserved for ARC cache (ZFS's storage cache)

The remaining RAM is what's actually usable for running VMs.

IMPORTANT
This differs from Syneto appliances running ESXi, where the RAM reserved for storage cache is set explicitly by the RAM assigned to the SynetoOS VM (commonly 32 GB), rather than calculated as a percentage of total host RAM. Customers moving from an ESXi-based deployment to Hyperion, or sizing a new Hyperion system, should account for this difference — usable VM RAM on Hyperion is not simply "total RAM minus 32 GB."

EXAMPLE

On a system with 250 GB of total RAM:

Allocation	Amount
Reserved for OS	12 GB
Reserved for ARC cache (20% of 250 GB)	50 GB
Usable for VMs	188 GB

NOTE
The ARC cache reservation scales with total RAM. A larger system reserves proportionally more RAM for ARC cache by default, which can result in less usable VM RAM than expected if this isn't accounted for during sizing.

Why this matters

If memory sizing for a Hyperion deployment doesn't account for these two reservations, the number of VMs the system can actually run may be lower than originally planned.

RAM Sizing Examples by Configuration

How to check current configuration

Step 1. Connect to Hyperion appliance via SSH as admin

ssh admin@<your_ip_address_or_hostname>

Step 2. Become Super User

sudo su -

Step 3. Check the system's memory allocation configuration

syneto-memory-info

EXAMPLE OUTPUT

============================================================
CURRENT SYSTEM MEMORY ALLOCATION DECISIONS
============================================================

📊 SYSTEM RESOURCES:
  Total RAM:                 250 GB
  Available Disk:            40 GB (raw)
  Effective Available Disk:  160 GB (including existing swap)

🧮 MEMORY BREAKDOWN:
  OS Reserved:                12 GB
  ARC Cache:                  50 GB
  Usable RAM for VMs:         188 GB

💾 RECOMMENDED SWAP CONFIGURATION:
  zRAM Size:                  15 GB
  Disk Swap Chunks:           15 × 8 GB
  Total Disk Swap:           120 GB
  Total Swap:                135 GB

📋 ACTIVE SWAP CONFIGURATION:
  zRAM Size:                  4 GB
  Disk Swap Chunks:           15 × 8 GB
  Total Disk Swap:           120 GB
  Total Swap:                124 GB

🖥️  VIRTUAL MACHINE CAPACITY:
  Max VM Memory:             323 GB
  Memory Efficiency:   1.3x RAM

📈 SWAP UTILIZATION:
  Disk Utilization:       100.0% of available
  Swap/RAM Ratio:      0.5:1

⚙️  CURRENT ARC SETTING:
  Current ARC Max:           50 GB
  Recommended ARC:            50 GB

Resizing the ARC Cache

The ARC cache reservation is configurable. It is expressed as a percentage of total RAM and can be lowered to increase usable VM RAM.

For step-by-step instructions, see: ("How to Resize the ARC Cache in Hyperion")

IMPORTANT
Do not lower the ARC cache reservation below 20 GB. ARC cache directly affects storage read performance — reducing it too far can negatively impact the performance of all VMs running on the system, not just free up capacity.