SIRIS, ALTO, and NAS: Properly Sizing a Datto Appliance



This article discusses how to choose the appropriate size when buying or upgrading a Datto device.


  • Datto SIRIS
  • Datto ALTO
  • Datto NAS


This article will help you size a Datto Business Continuity and Disaster Recovery (BCDR) appliance to meet your data backup needs. We recommend that you review the information presented here before you upgrade or purchase a Datto appliance.


Considerations for appliance sizing

Storage space and file size requirements

When qualifying clients for a Datto appliance, it is essential to consider both current and future storage requirements.

For Datto NAS, calculate the average amount of storage space your production shares require, and use a 2x multiplier to determine your optimal storage scenario.
  • How many machines do you need to protect?
  • Are these machines workstations or servers?
  • What is each machine's total protected size? To determine this amount, calculate the currently used space on the appliance, then add a cushion to account for future growth. You will need about 30% free space on the backed-up hard drive to allow for optimal backups. The bare minimum is 20% (anything under 30% has the potential to throw errors during the backup process).
  • How much block-level change will the system produce? Take into account databases, separate database backups, log files, application, and files change daily.
  • Are you using encrypted agents? Encrypted agents should be treated as uncompressed and will take up more space.
  • What type of files are being backed up? Certain file types cause substantial changes when accessed for utilization or updating. Large databases, media files, or production workspaces can have large sizes and thus larger backup images.
We recommend that you underestimate this number since, while block-level changes are primarily caused by file change, other factors such as disk sector changes are also involved.

Backup frequency and timespan considerations

Different server roles require different backup frequencies for proper protection. Typical requirements are as follows:

  • Exchange servers: hourly backups
  • Terminal servers: daily backups
  • Auxiliary domain controllers: several backups per week.

Other variables to consider:

  • How far back might a client need to go to restore files, folders, or entire operating systems?
  • If virtualized, how long would you need to run this virtual machine on the Datto appliance?
  • Does the client have expansion plans in place to possibly include additional servers or workstations?
For Datto Windows Agent volume space requirements, Refer to the System Requirements section of this article.

Datto best-practices

Appliance size: Datto's recommended appliance size is twice the total of the used disk space on the protected machine. This best-practice is called the 2-3x multiplier rule, and it ensures the following conditions are always met:

  • The Datto appliance has ample storage available for the first backup of the protected system (the base image, which will be a full disk snapshot)
  • The device can continuously roll this base image to the front of the chain with each incremental backup
  • The Datto appliance has storage space for backups based on the Datto recommended retention policy, as shown in Figure 1.

mceclip0.pngFigure 1: Retention policy on a Datto Device (click to enlarge)

Encrypted Agents: Backups on encrypted agents are stored uncompressed. If you are planning to use encryption to protect your agents, It may be advisable to use a 3X multiplier when determining device size.

Virtualization: The Datto appliance should have adequate storage space for local virtualization of agents.

  • When an agent is virtualized on the appliance, the Datto solution can back up the virtualized system.
  • If the local virtualization has the Rescue Agent option enabled, subsequent backups will be incremental. If not, then the next backup will be a new full base image. The immediate backup after a completed disaster recovery scenario (i.e., a completed bare metal restore, image export, etc.) may result in a new full base image as well.

Cloud synchronization: There must be enough space for the appliance's cloud synchronization. If there is not enough available space to create the off-site transfer file, the appliance's synchronization to the cloud could potentially stall.

Unexpected data growth: There is a slight safety buffer in storage size for cases in which mild unexpected data growth could potentially occur over time. When a Datto appliance's primary storage space reaches maximum capacity the optional resolution paths may be more difficult.

Datto hardware resource limitations

Datto appliances must have adequate resources to perform both daily tasks (such as taking backups) and ad hoc, resource-intensive operations (such as a local virtualization). Low system resources can severely affect local virtualizations, as the virtualized machines will seek resources that are not physically available. This can lead to virtual machines running extremely slowly, or high-resource programs crashing.

For example, a client has a single server environment (Small Business Server) running all the organization's daily operations. The server has the following hardware specifications and conditions:

  • Quad-core CPU and 32 GB of RAM
  • Storing an average of 525 GB of data on its HDD

If sizing the appliance on storage alone, the SIRIS S3B1000 appliance seems to accommodate the space of the server, as it:

  • Fits within the 2-3x multiplier
  • Provides ample space for data growth

However, if the server goes down and clients need to run off of a virtualization, the resources available may not be able to power the machine in a sufficient state to allow for operations to continue.

An S3B1000 has 32 GB of RAM and 4 CPU cores total. In this case, the best possible scenario for business continuity would use 24GB of RAM, and tax all four cores. Over-allocating RAM will hinder basic operations on the Datto appliance, while an under-provisioned virtual machine will not function as intended during the virtualization.

To ensure all of the appliance's features function correctly, the client in this example should consider a larger device, such as an S3B2000, to have enough resources for proper virtualization.

Appliance Sizing FAQs

What happens if my appliance is too small?

The following problems result from not having enough space on your Datto appliance:

  • It fills quickly after deployment, blocking future backups.
  • It cannot maintain local and off-site backup chains.
While Datto Technical Support may be able to provide a temporary remedy, issues stemming from a lack of free space will return until space is freed on the appliance, or it is upgraded.

How can I find the size of my appliance?

1. Access the Device Overview page.

Figure 2: Device overview (click for larger image)

2. Use the Local Storage Usage graphic and list to assess the size of all protected machines on your Datto appliance and the remaining local free space.

When is it time to upgrade?

If you experience failed local backups and are constantly modifying local backup retention to keep your system protected, it may be time to upgrade your appliance.

As a rule of thumb, if your appliance is utilizing 85%  or more of its local storage from backup storage, or if you do not have enough space to virtualize your largest agent, it may be time to consider upgrading options.

A "real world" sizing scenario

A small client network has four machines that need to be protected:

  • Exchange Server: 250 GB
  • Application Server: 45 GB
  • Terminal Server: 60 GB
  • Workstation: 100 GB 

The total protected space totals 455 GB. This is on the border of an S3B1000 appliance with a terabyte of storage. For local backups, this should be a suitable purchase. There are considerations to take into account:

How long will they maintain local data?

  • Do they have enough upstream bandwidth to get the images synced with the cloud consistently?
  • The recommendation is to have a minimum of 100 Kb/s upstream on a consistent, uninterrupted basis per terabyte of local storage. More is always advisable.

What if they had to restore the Exchange Server? How would this impact space?

  • If the Exchange server goes down and needs to be brought up locally on the Datto appliance via an instant virtualization, the next backup will be a base image as the hardware platform has changed.
  • Factor in the next Exchange backup being 250 GB. That will cause the appliance to quickly fill up since an additional base image is present and subsequent incremental changes will take place.
  • This may not be as pressing of a factor for smaller agents, as there is less data to back up should the machine need to be virtualized.

What kind of performance are they expecting to achieve?

  • Depending on the resources of the protected machines, what amount of processing power and RAM will be necessary for virtualized instances of the machines to operate?
  • What is the daily activity level of each machine? How many people are accessing the machines regularly?
  • Could the client survive with a virtual machine in an underpowered state that is being used in production?

If your specific sizing scenario is not covered here, contact your Datto Sales Executive for assistance.

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