A. There are essentially three units of space used for Active Directory (AD): the AD database itself, the log files generated for the database, and the SYSVOL content.
For environments where you have more than 1,000 users accessing a domain controller (DC), you should separate the AD database/SYSVOL from the AD logs—so you’ll need two separate volumes. (It’s recommended to leave the AD database and SYSVOL on the same volume.)
For the AD database, the volume size will be based on the number of objects, roughly 400MB for every 1,000 users, allowing an additional amount to accommodate expected growth. SYSVOL should usually have at least 500MB of space allocated.
The log volume size should be the total size of the log files plus 20 percent, or 500MB, whichever is larger. There shouldn’t be many log files in a normal environment because AD logging is circular—that is, after the log file is committed to AD, it’s removed. This means that unless you have a huge number of changes to AD, a couple gigabytes for the log volume will probably be plenty.