Sorry for the delay.
I discussed this case internally.
Unfortunately, there is no direct way to cancel cloud upload in the VTL free version. You can change a secret key in Azure,and after several retries, VTL will stop sending tapes to the cloud.
As to optimal part size, this depends a lot on your network throughput. Overall, try increasing the part size in incrementals and see how the upload goes (i.e., trial and error). What I see people do is split to 4 GB pieces, yet it is not necessarily the size you might be looking for. Specifying a larger part size might also result in tape cloud upload failure, as you have seen, as the bandwidth might be too small. Also, please try setting the same part size for S3-compatible, since part size may be read from there as well.
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