I have SQL servers running on onprem and Azure VM hardware.

The backups write to a file share located on a network VM.

I am wanting to change the backup destination to Azure blob storage.

For this I have created an Azure storage account and tested the Ola backup script with credentials (account key/identity and also with shared access signature).

Is want some guidance on what is recommended values to use for the following:

  1. Block size
  2. Buffer count
  3. Max transfer size

I understand that the block blob has a 200GB file limit and this is OK for me as my compressed backups come to under 110GB.

I understand that Azure blob has a limitation of 50000 blocks * 4MB maxtransfersize.

What is the idea values to use so that I don't hit the above limit.

1 Answer 1


Conclusion: If using Backup to URL to create striped backups of large databases (over 48 GB per stripe), specify MAXTRANSFERSIZE = 4194304 and BLOCKSIZE = 65536 in the BACKUP statement.

You can stripe your backup over multiple files to support larger databases if needed.

  • I'm not using stripped backups. My compressed backups size is 110GB.
    – variable
    Commented Mar 8 at 11:24
  • What exactly is block and Max transfer size. So if block size is X and transfer size is Y, then does it mean that SQL will create multiple X blocks until the total size reaches Y and then upload it to the Azure blob storage?
    – variable
    Commented Mar 8 at 11:26
  • The BLOCKSIZE and MAXTRANSFERSIZE have been around before BACKUP to URL and are documented here: learn.microsoft.com/en-us/sql/t-sql/statements/… They are not used that often but are needed now to support larger backups on Azure DB. Commented Mar 8 at 11:44

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