ZCB allows users to perform backups to disk only, backups to cloud only, or backups to both disk and cloud.
This article discusses the Backup to Cloud operation, which has a few limitations and drawbacks when compared to the other backup types. The limitations are technical, and are largely related to the power and speed of the machine that runs ZCB.
It is important to note that none of the limitations discussed below are hard limits. Not every system will have exactly the same limitations. Nor will every system encounter the limitations below.
Zmanda recommends that the Backup to Disk and Cloud be used whenever possible. It is generally faster and more stable than a Backup to Cloud only.
Encryption and Compression can have a large effect on upload speed during a Backup to Cloud operation.
If enabled, ZCB has to compress and/or encrypt each block of data before it is uploaded. It takes time to do the compression and/or encryption, and both encryption and compression are CPU-intensive processes. The time it takes to compress and/or encrypt will vary. The power of the CPU, the type of data, and the load on the CPU will all play a part in how long it takes to compress and/or encrypt data.
It is easily possible, especially on older, less powerful machines, for the time it takes to compress and/or encrypt a block of data to be greater than the time it takes to upload that block of data.
In other words, it's possible that ZCB can upload a block of data so fast that the next one isn't ready yet! And even though we are working on several blocks of data at a time, this delay can bring average transfer speeds down.
It still takes time to compress/encrypt the data during a Backup to Disk and Cloud. However, this is done during the backup to disk phase. Once the backup to disk is complete, ZCB will upload the resulting archive to the cloud at full speed. There is no waiting for more blocks of data to be compressed or encrypted. The backup is already done, and it just needs to be sent to the cloud.
ZCB uses Volume Shadow Copy (VSS) snapshots to perform backups. Your machine must keep the snapshot intact during the entire duration of your backup.
During a Backup to Cloud, the snapshot must be active and intact for the entire duration of the backup. This can be a very long time, especially for large backups on a slow connection.
While a snapshot is active, it must account for all data written to the disk. The more data is written, the larger the snapshot grows. Snapshots are allocated a limited amount of space by Windows. If a snapshot grows larger than this allocated space, it "breaks". The backup fails.
Backups to Cloud keep the snapshot open for a long time. Long enough that even a light load on the machine can cause the snapshot to break. A machine under a heavy load will cause the snapshot to break faster.
In general, this means that Backups to Disk and Cloud are more stable overall than a Backup to Cloud only, especially for large, long-running operations.
A full description of this behavior can be found in another Knowledgebase Article: Direct Backups to Cloud run for a while before failing.
While a Backup to Cloud can be paused and resumed, it cannot recover from complete failure. Nor can a Backup to Disk be recovered if it fails. If a snapshot fails, for any reason, the backup must be restarted.
The upload portion of a Backup to Disk and Cloud can be resumed, even if it fails. The backup has already completed and resides on the local disk. ZCB tracks what has been uploaded, and can just pick up where it left off. ZCB will even try to do it for you automatically, as seen in How does ZCB handle the loss of network connectivity to the cloud?
With a Backup to Cloud, the backup and upload processes are linked. If either fails, the backup will fail, and cannot be resumed.
Backups to Cloud are viable in most situations on most machines, but the limitations above are real and should not be discounted.
Backup to Disk and Cloud is the recommended backup type. It is faster, more stable, and recovers from errors better than a Backup to Cloud only. If you feel you're running afoul of any of the limitations above, try a Backup to Disk and Cloud.