storage blob copy start-batch needs "prefix" arg. Not pattern.

[worldspawn]

Is your feature request related to a problem? Please describe.
When trying to use start-batch I wanted to copy all files beginning with a prefix. Prefix in this case is how prefix is applied with this command:

az storage blob list --container-name $container --account-name testbullclip --prefix $_.name

This appears to do a performant query against blob storage. I get results in seconds. Start-batch doesn't this. It has --pattern, what --pattern does I couldn't say but it appears to be scanning every single blob name. My container has 1.34M blobs making the implementation of --pattern of no use at all.

Describe the solution you'd like
Use whatever magic the blob list --prefix arg is using with a new arg for start-batch called --prefix.

Describe alternatives you've considered
I've tried using azcopy but it has the same sort of implementation with --include-pattern.

[Juliehzl]

Hi @worldspawn, thanks for your feedback. Two things to do in next Sprint:

1. Find a way to add --prefix for az storage blob copy start-batch
2. Integrate Azcopy v10.3 and add --include/exclude related flags to az storage copy.
   We may do both of the above improvements or one of them, but anyway we will provide a way to copy with prefix.

[Juliehzl]

Since Azcopy v10.3 will bring some breaking changes, we plan to add breaking changes information in this sprint in PR #11546 and will do the change in next sprint.