mis@seiden.com (Mark Seiden) (02/28/90)
Original-posting-by: mis@seiden.com (Mark Seiden) Original-subject: format vs diag and Wren VII Reposted-by: emv@math.lsa.umich.edu (Edward Vielmetti) format is obviously less mature than the standalone diag, but its big advantages is that you can run it while doing other ordinary things, though not on a disk which has a mounted partition. since a lot of us are adding disks to/repairing blocks on existing systems, that's operationally more typical than formatting standalone... a few comments on large scsi disks (e.g. Wren VII): the ONLY thing important from the viewpoint of the disk's embedded scsi controller is the block address, so all we really care about is the number of blocks on the disk after formatting. cylinders, alternate cylinders, heads, sectors per track, bytes per track, are all artifacts of ESDI and SMD, and those artifacts are carefully preserved in the disk label and format.dat for no good reason i know of. anyone know to the contrary? what i usually do when getting a big scsi disk is find out the maximum addressable block. ignore the mfr supplied numbers of hds and cylinders. compute my own hdsprime * cylindersprime * sectorsprime <= maximum addressable block. /usr/games/factor finally has a known purpose! these numbers are almost completely arbitrary! the closer the product is, the less space is wasted. the smaller a "cylinder" is, the less waste in backup disk labels and the more waste in cylinder group overhead. concerning cylinder groups and zone recording techniques: The big CDC disks have variable numbers of sectors per track depending on whether you're on the inner tracks or on the outer tracks. The number is constant within a zone, a contiguous band of cylinders. This means BSD file system cylinder groups within any sizeable file system cannot be both of fixed size and also be aligned starting on a cylinder boundary. If you knew, or could find out (you can, with some difficulty) at what cylinder numbers the zones begin, you could arrange a good compromise where cylinder groups begin at cylinder boundaries, but what would you gain? that a cylinder group doesn't begin at a cylinder boundary doesn't necessarily impact performance, since it's only *relative* rotational position (and only 3 bits of it at that!) that is used for disk block allocation. for seeks between one cylinder and another that *happens* to cross a zone boundary, you may lose a rotation, but there are relatively few zones on a disk and when you seek you shouldn't be surprised in losing anyway. (while i have you here...) is format.dat documented anywhere? (what is cache? trks_zone? asect? fmt_time?) is there any convenient way to do a Read Capacity to find out the maximum addressable block on a formatted scsi disk? mark seiden, mis@seiden.com