I have a tape drive connected directly to a server, which have a lot of static data. I.e. the data won't change during the backup window.
Does Amanda still need a holding disk?
I would say yes. One important reason for a cache like this is to allow the backup program to consolidate all data it needs to backup and write it to the tape in one continuous operation. This prevents the tape drive from having to start and stop all the time while the backup program is searching for data to write, which in turn is important because it will have to go back a little on the tape to find it's correct position when starting again, a process called shoe shining for obvious reasons, which puts a lot of wear on both the drive and the tape.
Modern drives are told to be much better at this because they slow down before their write buffer runs empty, reducing the shoe shining effect, but I would still try to prevent it wherever possible.
The answer to your question depends on your network environment. If you're using a hardware tape unit, Amanda will use the holding disk to stage backup data from clients until there is enough there to stream a whole tape at once. Older tape drives get very inefficient for both space and time if they have keep stopping and re-positioning the tape because the server isn't sending enough data fast enough to keep the drive streaming. If your modern drive doesn't suffer from this, you might be able to skip the holding disk.
Amanda can stream directly from the client out to the tape device, bypassing the holding disk. But, I find that most clients can't send backup data fast enough to keep the tape drive streaming. However, this can be required if the client has file systems that create backups larger than your total available holding disk space even when compressed.
If you're using virtual tapes, ie your tapes are actually cloud storage, network storage, or some other dedicated disk, then you don't need the holding disk, as there's not much benefit to having to write the same data twice, once to holding disk, and again to vtape disk.
Then again, if your vtape storage is on an unreliable network SAN (like mine has been) then the holding disk becomes very useful, as nightly backups will proceed as normal, and Amanda will simply fill up the holding disk and ignore the recalcitrant vtape device. Once the SAN is back in operation, you can flush the holding disk out to tape or vtape at any time.
The various Amanda restore commands (amrecover, amrestore, etc) generally are aware when there are dumps waiting to be flushed to tape, and they know how to restore content from those files when they are present.
To summarize, yes, Amanda will operate without a holding disk, but it will probably work much more efficiently if you have one or more holding areas defined such that their total capacity at least as large as the capacity of one of your physical tapes.