During an on-premises-to-cloud migration, there are two approaches to moving data and applications as part of a cloud migration strategy: offline or online migration. Each approach offers different pros and cons from the perspective of performance, cost savings and security.
What is offline cloud migration?
In an offline migration, an enterprise uses physical storage media -- such as portable disk drives or storage appliances -- to move workloads to a cloud environment. Under this method, admins acquire portable storage media, copy data from local systems onto it, then physically deliver the media to a cloud provider. After that, the provider uploads the data to its cloud via a local network connection -- which is faster than the public internet -- or connects the storage media directly to its cloud servers. Once data is in the cloud, the offline migration is considered complete and the portable storage media can be decommissioned or reused.
This on-premises-to-cloud migration approach usually costs more than an online transfer because enterprises need to rent or purchase large volumes of storage media, and then pay to relocate it. This method also typically requires more planning and effort than an online migration because admins must acquire portable storage media and manage the multistep process of moving it physically to a cloud provider. They must also manage the physical security risks that could arise from unauthorized parties accessing portable storage media that contains sensitive data.
What is online cloud migration?
An online cloud migration uses a network -- either a cloud provider's direct connection service or the public internet -- to transfer data and applications to a cloud data center in real time. Online migrations are simpler overall because IT teams can copy data in one step from their local infrastructure to the cloud. Online migration typically also offers cost savings compared to offline approaches, because the only expense is whatever it costs to pay for the bandwidth necessary to transfer data from on premises to a cloud. Most cloud providers don't charge ingress fees for incoming data -- they only charge for egress, meaning data that leaves their clouds -- so there's no added cost due to ingress.
The major disadvantage with online cloud migrations is that they can be slow, especially when businesses have large amounts of data to transfer, due to network bandwidth limitations. In addition, moving data over the internet can expose data to network-borne security risks.
Choose the right migration method
The biggest factors to consider when choosing an on-premises-to-cloud migration method are how much data you need to move and how quickly you have to move it. Although an online migration might be the easiest and most popular choice, it can be problematic for organizations with large amounts of data and a strict migration time frame.
For example, if an organization has only 1 TB of data to migrate and a 1 GBps network connection is available, the online migration should take under three hours using a standard internet connection. However, 1,000 TB would likely take more than 100 days to transfer over the same network connection -- in which case, an offline migration would be a better approach.
Total transfer time is important to consider not just because it determines how long the migration process will take, but also how much data syncing must be performed post-migration to ensure the cloud-based copy of your data perfectly represents the most recent on-premises version of the data. This is crucial; if workloads remain operational on premises while migration is underway, the data moved into the cloud might end up being different than the most up-to-date data that exists on premises. This happens because data will constantly change while workloads are running, so if you begin migration based on one version of on-premises data, the on-premises data is likely to change by the time the migration finishes.
One way to address this challenge is to shut down workloads prior to migration so data remains consistent between the on-premises and cloud-based copies. However, that might not be acceptable if you have mission-critical workloads that can't be suspended. Another approach is to perform the migration, then do a data sync between the on-premises and cloud-based data copies that only updates the data that has changed since migration. Tools like rsync -- which supports copying only those files that are different between two data sources -- or commercial tools based on rsync, are useful for this purpose. However, the amount of syncing to perform will depend on how much difference there is between the on-premises and cloud-based copies of the data once the migration is complete. And the faster the migration is completed, the fewer differences there are likely to be.
The reliability of the network connection must also be considered when evaluating on-premises-to-cloud migration methods. If a network connection is intermittent -- or bandwidth availability fluctuates unpredictably -- an online migration could take longer than is acceptable. Some data may also need to be recopied due to failed transmissions over a flaky connection, exacerbating the time and difficulty required to perform online migration. There's a much lower risk of data transfer issues when performing offline migration because physical storage media and local uploads are generally more reliable than using the internet.
Security is another issue. If an online network can't be trusted, or it's not possible to encrypt sensitive data before the migration, an offline migration might be necessary. That said, think about the physical security risks of offline data migration, if you choose that route.
Whether you choose online or offline migration, data compression is one of a few techniques that can accelerate the process. Data compression reduces the number of bits required to represent data, so more data can move using the same network bandwidth or less physical storage media. In addition, enterprises can transfer the most important data -- such as data that powers mission-critical workloads -- first to streamline the online migration. This technique enables users to take advantage of the cloud more quickly, even if the overall migration takes a while. Scanning data to remove redundant or unnecessary files prior to migration can also reduce the total amount of data to be migrated and speed the migration process.
Cloud migration tools
Out of these two on-premises-to-cloud migration approaches, the online method remains the most popular, because most organizations don't have massive volumes of data to move. As a result, cloud migration tools cater mostly to online strategies. To smooth the process, most of these tools have assessment and cost estimation features, as well as automation capabilities.
Some popular native and third-party online migration tools include the following:
- AWS Server Migration Service
- Azure Migrate
- Google Cloud Migrate to Virtual Machines
- Carbonite Migrate
- PlateSpin Migrate
The main advantage of using a third-party cloud migration tool over a provider's native offering is flexibility. Most third-party tools support migrations into multiple clouds, whereas major cloud providers' tools are only compatible with their respective platforms. Additionally, enterprises can use third-party tools in conjunction with cloud vendors' native tools.
Offline migrations can also be performed using cloud vendors' own services or third-party tools. Popular options include the following:
- AWS Snowball
- AWS Snowmobile
- Google Data Transfer Appliance
- Azure Data Box
- IBM Cloud Mass Data Migration
A key difference between cloud vendor offline migration tools and third-party offerings is that customers who choose cloud vendor offerings may have to purchase or rent data storage media directly from cloud vendors. For example, Azure Data Box requires the use of a special storage device -- called a Data Box -- that customers purchase through Azure. AWS Snowball likewise requires customers to use storage appliances supplied by Amazon. With third-party tools, businesses can generally use any storage media to complete their cloud migration.