ayac071@ut-emx.UUCP (William T. Douglass) (03/03/88)
Is there a limit in the OS or hardware that restricts the size or number of hard disks that you can attach to a mac II? Thanks, Bill
zrm@eddie.MIT.EDU (Zigurd R. Mednieks) (03/03/88)
In article <1054@ut-emx.UUCP> ayac071@ut-emx.UUCP (William T. Douglass) writes: >Is there a limit in the OS or hardware that restricts the size or number of >hard disks that you can attach to a mac II? Thanks, > Volume size: This CAN get very big. The maximum allocation block size is 2^32, and the maximum number of blocks per volume is also 2^32. Now this isn't practical since who wants a 4GB disk buffer! At least we are sure that we could put four billion and change reasonably sized disk blocks on a volume. That's more than any disk, including any optical disk, will hold. SCSI will accomodate only 7 disks using first level SCSI addressing. Using NuBus interfaces you could have up to the theoretical maximum of 2^16 drives (volumes). If you had a volume with a million billion bytes on it, and your transfer rate was 10MB/sec. it would take more than two years just to read all the information into memory. It could keep you busy. -Zigurd -- ------------------------------------------------------------------------ Zigurd Mednieks MURSU Corporation (617)424-0146 25 Exeter Street Boston, MA 02116
ephraim@think.COM (ephraim vishniac) (03/03/88)
In article <1054@ut-emx.UUCP> ayac071@ut-emx.UUCP (William T. Douglass) writes: >Is there a limit in the OS or hardware that restricts the size or number of >hard disks that you can attach to a mac II? Thanks, Disk size: the number of sectors in the drive is held in a 32-bit field in the drive queue element. Each sector is 512 bytes, so there's an obvious limit of 2^9 * 2^32 = 2^41 bytes. However, the dCtlPosition field used in low-level calls to the driver is only 32 bits and indicates a byte position, not a sector position, so there's a smaller obvious limit of 2^32 bytes (4GB). I'd be pleasantly surprised if the file system doesn't accidentally use signed arithmetic somewhere, which would cause an effective limit of 2^31 bytes (2GB). I've actually heard of people attaching >1GB disk systems to a Mac, so the real limit is somewhere between that and 4GB. You can avoid the drive size limit by partitioning the disk at a low level. The Driver Partition Map (DPM) uses 32-bit sector numbers, so this moves you back toward the 2^41 byte limit. Number of drives: You can only attach seven SCSI devices, but a single SCSI device could be several logical drives. Also, there are other attachment methods than the Apple-provided SCSI bus. For example, you could build a Nubus card with your own SCSI (or other) interface. Each mounted volume uses space in the system heap, so your heap will eventually get crowded. This is a soft limit: you'd have to experiment to find out how much of a problem it is. Ephraim Vishniac ephraim@think.com Thinking Machines Corporation / 245 First Street / Cambridge, MA 02142-1214 On two occasions I have been asked, "Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?"
dwb@Apple.COM (David W. Berry) (03/04/88)
In article <8359@eddie.MIT.EDU> zrm@eddie.MIT.EDU (Zigurd R. Mednieks) writes: >In article <1054@ut-emx.UUCP> ayac071@ut-emx.UUCP (William T. Douglass) writes: >>Is there a limit in the OS or hardware that restricts the size or number of >>hard disks that you can attach to a mac II? Thanks, >> >SCSI will accomodate only 7 disks using first level SCSI addressing. >Using NuBus interfaces you could have up to the theoretical maximum of >2^16 drives (volumes). Actually, each SCSI address/controller can have a theoretical maximum of 8 drives attached to it, common controller's support 4. Thus without using any slot's you could have 56 drives attached, the controller would have to be smart enough and the driver would take a lot of work. You could even boot off of seven of them...