What is the Difference Between Cold Storage and Archive Tiers?

Storage tiers look cheap on a pricing page. Low prices tempt you to move every old file there. Costs show up later when you need the data back.

Cold storage and archive tiers differ in access time, minimum storage period and fees. Here is what Microsoft and Google document about them and how to choose a tier before you commit years of data. New to the basics? See what cloud storage is first.

What is Cold Storage?

Cold storage is a storage tier for data you open rarely. Storing it costs less than hot storage and opening it costs more.

Google sorts its Cloud Storage classes by access frequency. Standard fits frequently accessed data. Nearline fits data you open or modify about once a month or less. Coldline fits data you access at most once a quarter. Archive fits data you access less than once a year. See Google storage classes.

Google states that every class has low latency with no offline retrieval and 99.999999999% annual durability. Microsoft names its Azure Blob Storage tiers hot, cool, cold and archive. Hot, cool and cold are online tiers with first byte access in milliseconds. See Microsoft access tiers.

What is an Archive Tier?

Archive tiers are the coldest and cheapest place to store data you must retain but almost never open. Providers use the word in different ways.

Microsoft states that the Azure archive tier is offline. You cannot open or modify an archived blob until you rehydrate it to an online tier, which can take up to 15 hours depending on the priority you choose.

Google's Archive class works differently. Data remains online with millisecond access and has a 365 day minimum storage duration.

People also use archiving to mean a managed process with retention rules, search and legal preservation. Storage tiers store bytes only and add no retention or search controls. Ask your compliance team what your rules require before you select a tier.

How do Cold Storage and Archive Tiers Differ?

Compare the tiers on minimum storage period and access time in the table.

Delete or move data before the minimum period ends and you still pay for the remaining days. Microsoft states that early deletion fees are prorated.

What Costs do Cold and Archive Tiers Add?

• Retrieval fees. Google states that Nearline, Coldline and Archive charge retrieval fees. Microsoft bills for data access, data retrieval and the destination copy when you copy out of the archive tier. See Microsoft rehydration guidance.
• Early deletion fees. Microsoft gives an example where a blob deleted or moved to hot after 45 days in archive incurs a fee for 135 days, the rest of the 180 day minimum.
• Restore speed limits. Microsoft states that priority rehydration processes at most 10 GiB per storage account per hour. Large restores take longer than small ones.
• Data transfer fees. Moving data out of a provider network can add charges. Check the price page of your provider for your region.

Add every item to the monthly storage price before you compare tiers.

How Fast can You Restore Data From an Archive Tier?

Azure archive restores take hours. Microsoft states that standard priority can take up to 15 hours for objects under 10 GB. High priority can finish in under one hour for objects under 10 GB and can take longer depending on blob size and demand. Microsoft recommends high priority for emergency restoration only.

Microsoft lists two rehydration methods. Copy Blob creates a new online copy and leaves the archived original in place so no early deletion fee applies to the original. Set Blob Tier moves the original blob and cannot be canceled once it starts.

Restore time decides whether a tier fits disaster recovery. Compare it with your recovery time objective, which is how long your business can wait for the data. Offline archive tiers suit data you can wait hours for.

How do You Choose the Right Tier?

Work through six checks in order.

1. Map your access pattern. Check how frequently you open the data using real logs from the last 12 months. Google's thresholds of once a month, once a quarter and once a year give a starting point.
2. Check minimum storage periods. Match them to your retention plan so early deletion fees never apply.
3. Check restore time. Compare it with the time you can wait for the data.
4. Add retrieval and transfer fees to the storage price using the price page of your provider.
5. Automate with lifecycle rules. Microsoft states that lifecycle policies move blobs between tiers or delete them based on creation time, last modified time or last accessed time. Last accessed time needs access time tracking and lifecycle policies cannot rehydrate an archived blob. See Azure lifecycle management.
6. Test a restore with a sample of real data before you depend on the tier.

Is Cold Storage Durable Enough for Ten Years?

Durability is the reassuring part. Google states that every Cloud Storage class has 99.999999999% annual durability. Microsoft documents at least 99.999999999% for locally redundant storage and higher figures for zone and geo redundant options. See Azure Storage redundancy.

Durability is not availability. Microsoft lists 99 percent availability for the cool, cold and archive tiers against 99.9 percent for hot.

Surviving bytes also need a plan. In ten years you still need affordable retrieval, working account access and software that opens your file formats. Test a restore on a schedule to prove the archive works. Tiers also do not replace a recovery plan for a single missing file. See how to recover deleted data from a desktop computer.