You can use Windows PowerShell to turn external sharing on or off. Using the SharePoint Online Management Shell, administrators can now enforce new controls over how external users accept invitations. When enabled, the RequireAcceptingAccountMatchInvitedAccount parameter requires external users to accept invitations with the email account with which they originally received the invitation.

Disk input/output (I/O) performance is the single biggest constraint on database performance in the SharePoint world.

Running SharePoint on an underpowered or poorly designed SQL Server environment will slow performance throughout the farm.
 
Because it's equally important to make sure SQL Server databases are kept highly available, some level of disk redundancy is usually factored into physical designs for database servers. Disk redundancy is often classified by RAID levels.