ddp+@andrew.cmu.edu (Drew Daniel Perkins) (07/16/90)
I have a few questions regarding SCSI disk operation and performance that I have been unable to find the answers to in the SCSI specifications that I have read. 1. When you write to a SCSI disk, when do most disks transfer the data? Immediately, or perhaps after doing any necessary seek? 2. When do most disks acknowledge the write? After the data has been transferred, or after the data has been written to the disk (i.e. after it is really on the ferrous oxide)? 3. Can SCSI disks achieve 1:1 interleaving? This depends on the answer to 2. If the disk doesn't acknowledge the write until it is on disk, then it seems unlikely that the host will be able to start the next write in time to hit the next sector without another revolution. If the disk acknowledges the write immediately after the data is transferred and before it is on disk, then it may not be giving the consistency guarantees that some file system and database software expects. Will the disk allow a second write to be started (i.e. allow the data to be transferred) before the first write completes? Thanks, Drew