NEXTSTEP In Focus, Spring 1994 (Volume 4, Issue 2). 
Copyright ã1994 by NeXT Computer, Inc.  All Rights Reserved.

Adding On without Flipping Out

Mark Tacchi

NEXTSTEP provides features to make it as easy as possible to install a new disk on a computer. This article describes how to format, initialize, and partition SCSI and IDE disk drives, as well as how to mount them at boot time.

Using SCSI and IDE Drives

Installing a new disk on some computers can be complicated, particularly if you want to use sophisticated features like partitioning and automounting. NEXTSTEP addresses these difficulties with tools for formatting, initializing, and partitioning SCSI and IDE disk drives. It also lets you set up disks to mount automatically at boot time. If you want to install disks that conform to either the SCSI or IDE specifications, you should have little trouble adding them to your computer 
in any combination. 

Formatting and Initializing

Before you can use a disk with any computer, the disk must be formatted and initialized. Formatting is a low-level operation that places timing marks on the disk so that the hardware can distinguish where physical block sectors start and end. Floppy disks and other removable media with a total storage capacity of 6 megabytes or less generally aren't formatted when you buy them. In contrast, hard disks and storage media that have more than 6 megabytes of storage capacity are usually formatted at the factory where they're made. 

Initialization places an internal label and other items on the disk, such as a superblock, inodes, and other file system elements for storing, retrieving, and managing data. Generally, disks aren't initialized when you buy them.

When you connect an uninitialized or unrecognized hard disk to a computer, NEXTSTEP asks whether you want to initialize the disk or ignore it. If you choose to initialize the disk, NEXTSTEP asks you to select the disk name and file system type. It then initializes the disk with the chosen file system and mounts it. If you insert an unformatted floppy disk in the computer, NEXTSTEP follows 
the same steps but automatically formats the disk before initializing and mounting it.

In the infrequent case in which you have an unformatted disk that's larger than 6 megabytes, NEXTSTEP can't automatically format it. Instead, you must format it yourself before initializing it. To do this for a SCSI disk, use the sdform utility. There's currently no comparable utility for IDE drives in NEXTSTEP, so you must resort to DOS utilitiesÐor if your machine comes with hard disk maintenance utilities, you can use them.

For information on the sdform SCSI disk formatting utility, please see the sdform UNIX manual page.

Partitioning

When NEXTSTEP automatically initializes a hard disk, it creates a single partition that occupies the entire disk. A single partition arrangement is very simple to use and requires minimal system administration. However, sometimes you might prefer to create a disk with several partitions to manage data a special way, contain disk damage, facilitate backup operations, or export multiple file systems from the same disk with different access privileges. There are several ways to partition a disk, depending on whether the disk is a SCSI or IDE disk.

Partitioning with fdisk

If you need to access operating systems in addition to NEXTSTEP at boot time, you may wish to partition your disk with the utility fdisk. You can use NEXTSTEP's fdisk on an Intel-based computer to create up to four partitions on your drive. The command modifies the DOS partition table that resides on the boot sector of bootable disks.

To run fdisk, you must be root. You can run fdisk in interactive mode by specifying the raw device with no inquiry or action. Or, to run fdisk on the live partitionÐthe entire secondary IDE disk driveÐspecify the device hd1h:

# fdisk /dev/rhd1h

Note that the NEXTSTEP booter and fdisk command can identify only one physical NEXTSTEP partition.

To find out more about fdisk, see the fdisk UNIX manual page and Divvying It Up: 
How to Use fdiskº (NEXTSTEP In Focus 3 [Spring 1993]), NeXTanswers #1131.

BSD 4.3 UNIX partitioning

To build a disk with one or two BSD 4.3 UNIX partitions, you can use the BuildDisk application in /NextAdmin. You can also use this application to place the NEXTSTEP operating system on the disk. 

(BSD 4.3 has a fixed partition limit of 2 gigabytes (231bytes) and drives that exceed this boundary must be partitioned.)

If you want more than two partitions or some special characteristics for your disk, you must create a disktab entry and use it to initialize the disk from the command line. The /etc/disktab file contains data that describes each disk's native geometry and how the computer should partition each disk when it initializes it. You don't need to place an entry in the disktab file for every disk you want to add to the systemÐthe initialization software can read the necessary data from any disk drive that supports the SCSI MODESENSE command. Rather, you provide a disktab entry if you want to format your disk in some special way.

To create a disktab entry, you must determine the geometry of your disk, choose a location and size for each partition, and then add lines describing the partitions you want into the /etc/disktab file. 

Note: NeXT Computer, Inc., doesn't officially support modifications to the disktab file. You should not create disktab entries unless you have had extensive experience with disks in a UNIX environment. This information is provided to point experienced administrators to the right tools.

Analyzing the disk

To determine your drive's name, geometry, and size, contact the disk vendor or use the command scsimodes to get this information for an existing disk from the same vendor. This utility displays information about a SCSI disk. scsimodes takes a single command line argument, the name of the raw disk device. You must run this command as root:

# scsimodes /dev/rsd0a
SCSI information for /dev/rsd0a
Drive type: SEAGATE ST43400N
512 bytes per sector
99 sectors per track
21 tracks per cylinder
2737 cylinder per volume (including spare cylinders)
9 spare sectors per cylinder
21 alternate tracks per volume
5688446 usable sectors on volume

If your system already has a labeled disk of the type you want to partition, you can get a more complete set of disktab information by writing a program that invokes the getdiskbyname() system call. Please see the getdiskbyname UNIX manual page for details.

The scsimodes command is not officially documented nor supported by NeXT.

IDE drives don't respond to the scsimodes commandÐyou'll have to get data about any IDE drive from the disk drive vendor.

Editing the disktab file

Before you edit the disktab file, please be certain that you understand its format. Review the UNIX manual page on disktab and the system's existing disktab file. 

The disktab file contains a list of entries, one for each disk configuration. Each entry consists of a single UNIX ªline.º It typically takes several lines within a text editor to write an entryÐintermediate lines have the UNIX escape backslash character (\) before the carriage return, so that the entry is recognized as a single line.

Entries in disktab consist of a number of colon-separated fields. The first entry for each disk gives the names for the disk, separated by vertical bar characters (|). The last name listed is a long name that fully identifies the disk.

Figure 1 shows a typical disktab entry for a SCSI disk. This is the entry for the 2.5 gigabyte Seagate drive described by the scsimodes command above. Note that because this drive is larger than 2 gigabytesÐ231 bytes, or 2,147,483,648 bytesÐit must be partitioned into multiple BSD 4.3 UNIX partitions.

Figure 1: A disktab entry for a Seagate SCSI disk drive

ST43400N|ST43400N-512|SEAGATE ST43400N-512:\
        :ty=fixed_rw_scsi:nc#2737:nt#21:ns#99:ss#512:rm#3600:\
        :fp#160:bp#0:ng#0:gs#0:ga#0:ao#0:\
        :os=sdmach:z0#32:z1#96:hn=localhost:ro=a:\
        :pa#0:sa#4194304:ba#8192:fa#1024:ca#32:da#4096:ra#10:\
                :oa=time:ia:ta=4.3BSD:\
        :pb#4194304:sb#1494142:bb#8192:fb#1024:cb#32:db#4096:\
                :rb#10:ob=time:ib:tb=4.3BSD:

The name of the drive is the same as the name shown by the scsimodes command, with the physical sector size of the disk appended at the end. If you name a disktab entry according to this convention, the BuildDisk application can use the entry to format and initialize the disk. If you name it another way, you must use the disk command with the -i and -t options to initialize the disk according to the disktab entry.

Figure 2 shows what the other fields in the disktab entry specify.

Figure 2: Fields in a disktab file

Field	Description

ty	Type of disk. Required for all entries. Valid types are removable_rw_scsi, fixed_rw_scsi, fixed_rw_ide, removable_rw_optical, removable_rw_floppy.

ns	Number of sectors per track. This is a critical field, used for performance tuning at initialization.

nt	Number of tracks per cylinder. Helps decide where to put superblock backups. Not a critical field.

nc	Total number of cylinders on the disk. Not used by NEXTSTEP.

p[a-h]	Base sector numbers of partitions a through h. 

s[a-h]	Sizes of partitions a through h. 

b[a-h]	Block sizes for partitions a through h, in bytes. This is the size of a logical block in the file system. The NEXTSTEP file system block size is 8192 bytes.

f[a-h]	Fragment sizes for partitions a through h, in bytes. This is the Berkeley file system frag. Set it to 1024 bytes; there's no advantage to making it larger. 

c[a-h]	Partition "cylinders-per-group" for newfs. This is a "black magic" parameter for disk block managementÐuse 32. BuildDisk or mkfs (which runs when you call newfs) lets you know if you need to change it. See d[a-h], partition density.

d[a-h]	Partition densityÐbytes-per-inodeÐfor newfs. Indicates how large files in the partition will be on the average, and thus how many inodes it should provide. A good general value is 4096. If you will have just a few large files on the disk, make this number larger; if you expect to have many very small files, specify a smaller number. If NEXTSTEP can't set up a file system with the parameters as you've set them, it will suggest that you change the cylinders-per-group parameter, c[a-h].

r[a-h]	Partition minimum left free for newfs, a value from 0 to 100. The percent of formatted disk space that should always be left free, so blocks can be distributed among files efficiently. You can save space by using a smaller minfreeÐfloppy disks have a minfree of 0% for because they're small and slow. However, a minfree less than 10% isn't usually recommended for hard disks. 

o[a-h]	Partition optimization for newfs; accepted values are space and time. Set to time if possible. Use space only when space on the disk is critically short.

i[a-h]	Whether the system should run newfs on partition during initialization with disk. Include this field in the entry for any partition that will have a UNIX file system on it; omit it otherwise.

m[a-h]	Partition mount point name. Not used for NEXTSTEPÐthe mount point is established by the automounter or by the entry in the local /etc/fstab file.

a[a-h]	Partition automount on insert. Unused by NEXTSTEP. 

t[a-h]	Partition file system type, like 4.3BSD or sound. Set to 4.3BSD. Not used now, but may be in the future.

rm	Rotational speed of the disk in rotations per minute, for performance tuning. The default value is 3600 for hard disks, 300 for floppy disks. This information isn't reported by scsimodesÐobtain the value from the disk manufacturer. If the disk is the same type as one you already have in your system, you can get this value by writing a program that calls getdiskbydev(). If you can't find the true value, the default will suffice.

ss	Sector size in bytes. Boot disks on Intel-based computers must have 512 bytes/sector. Data disks can have 1024 bytes/sector. For other architectures, both boot and data disks can have 1024 bytes/sector.

fp	Number of sectors in the "front porch" of the disk. These are 1024-byte sectors; set this value to 160 for hard drives. Floppies use 92.

bp	Number of sectors in "back porch" of the disk. 0 for all current devices.

ng	Number of alternate groups on the disk. Only for optical disks. Set to 0 or omit.

gs	Number of sectors per alternate group. Only for optical disks. Set to 0 or omit.

ga	Number of alternate sectors per alternate group. Only for optical disks. Set to 0 or omit.

ao	Sector offset of alternates in alternate group. Only for optical disks. Set to 0 or omit.

os	Name of file to boot from. Set to sdmach for NeXT computers and to mach_kernel for HP PA-RISC, Intel-based, and Sun SPARC computers.

z[0-1]	Locations of first-level boot code. Set to 32 and 96 respectively for hard disks, 32 and 32 for floppy disks.

hn	Host name to be written to label.

ro	Read-only root partition. Unused by NEXTSTEP.

rw	Read/write partition. Unused by NEXTSTEP.

For the sector fields, if you plan to have 1024-byte sectors on a 512-byte/sector disk, divide the value by 2 and round up.  You can't use 1024 bytes per sector for boot disks on Intel-based computers.

newfs only sets up the file system, whereas BuildDisk attempts to format, initialize, and build a bootable disk.  Use newfs to create an unbootable data disk.

Initializing the disk

When you've created the entry in the disktab file, you can build the disk with BuildDisk or simply initialize it with the disk command. If the disktab file contains an entry for a disk and if the entry name matches the name of the disk, BuildDisk uses the disktab entry.

# disk /dev/rsd0a

You can use disk interactively if the name of the drive matches the name of the disktab entry. The init command initializes the disk.  For help in disk's interactive mode, type help at the disk prompt. Otherwise you can enter the disktab entry name after the -t flag:

# disk -i -t ST43400N /dev/rsd0a

Unlike with other UNIX and disk systems, you don't have to start a partition on a track or cylinder boundary. However, if you don't start the partition on a boundary, you might waste a little space. Check for messages from mkfs in the Console.

Mounting Drives and Partitions

If there is no entry in /etc/fstab for a disk or partition, the disk or partition won't be mounted until a user logs in.  Once someone logs in and Workspace Manager starts up, NEXTSTEP examines all unmounted devices to see whether they've been initialized for a file system supported by NEXTSTEP. Supported file systems include Berkeley UNIX 4.3, CD-ROM, DOS, Macintosh, and audio compact disc.

If a drive conforms to one of these standard filesystems, Workspace Manager sends a mount request to the drivers. NEXTSTEP mounts the drive on the root directory of the filesystem as /disklabelname. Drives and partitions mounted in this manner are owned by the current user. In contrast, drives and partitions without entries in /etc/fstab are owned by the current user when mounted.

Drives and partitions with /etc/fstab entries

NEXTSTEP mounts any connected drive or partition that has an entry in /etc/fstab when the computer boots. By default, such disks and partitions are owned by root.  The system disk, which holds the root file system, always has an entry in /etc/fstab, placed there when NEXTSTEP was installed.

Additional partitions on the system disk or any other disks must be mounted manually or be given an /etc/fstab entry. The automounter sees only the first partition on a disk and partitions that have entries in the /etc/fstab file. 

Conversely, NEXTSTEP will try to mount every disk it sees, either at boot time or when the Workspace starts up, unless there is an /etc/fstab entry for it that includes the -noauto option. Please see the UNIX manual page on fstab for more information. 

Creating /etc/fstab entries

Figure 3 shows three examples of /etc/fstab files. The first has entries for two SCSI drives, the second for two IDE drives, and the third for one SCSI drive and one IDE drive.

Figure 3: Entries from /etc/fstab files for different disk drive configurations

SCSI only:
/dev/sd0a / 4.3 rw,noquota,noauto 0 1
/dev/sd1a /home 4.3 rw,noquota 0 2
/dev/sd1b /data 4.3 rw,noquota 0 2

IDE only:
/dev/hd0a / 4.3 rw,noquota,noauto 0 1
/dev/hd1a /home 4.3 rw,noquota 0 2

SCSI and IDE:
/dev/hd0a / 4.3 rw,noquota,noauto 0 1
/dev/sd0a /SCSI_Disk 4.3 rw,noquota 1 2
/dev/sd1a /SCSI_CDROM 4.3 ro,noquota,removable 1 2
/dev/hd1a /IDE_Disk 4.3 rw,noquota 1 2

You can display these entries by executing mount -p once all devices are mounted; devices that have entries in the mount table, /etc/mtab, are displayed. These are formatted to match that required for /etc/fstab and can be directly appended. 

Figure 4: Output from mount -p

# mount -p | grep dev
/dev/hd0a   /   4.3    rw,noquota,noauto   0 1
/dev/sd0a   /NEXTSTEP_3.2   4.3   ro,noquota,removable   1 2
/dev/sd1a   /Seagate SCSI disk   4.3   rw,noquota   1 2
/dev/hd1a   /Secondary_IDE   4.3   rw,noquota  1 2

Although DOS partitions may appear in this output, you can't mount DOS partitions at boot time in this manner. DOS partitions require special treatmentÐsee the next section.

DOS partitions

DOS partitions are automatically mounted when a user logs into the Workspace. However, you can force the DOS partition to be mounted at boot time, so that remote users and anyone who logs in without the Workspace can access the partition.

To automount the DOS partition at boot time, follow these steps:

1	Append these lines to /etc/rc.local. Substitute the name of the device you want to mount for /dev/rhd0h.

	kl_util -l dosfs
	mount -t dos /dev/rhd0h /DOS

	In the mount command, -t dos indicates you're mounting a DOS file system.  To mount another device such as the secondary IDE drive, use /dev/rhd1h as the raw device.

2	Append this line to /etc/kern_loader.conf:

	/usr/filesystems/DOS.fs/dosfs_reloc

3	Reboot.

Note that root doesn't own this mount; the mount and its subdirectories and files are owned by the current user. This is a security hazard, but it's a DOS-ism that has existed since DOS' inception. 

Conclusion

You should now have enough information on formatting, initializing, partitioning, and mounting disks to play with them on your own.  If you run into any obstacles, refer to the UNIX manual pages for specifics.

I'll leave you with this final cautionary note: Back up valuable data before experimenting, because an honest mistake as root can be destructive to dataÐand to your day!  

Mark Tacchi is a member of the Technical Support Team. You can reach him by e-mail at tacchi@next.com. Flip Dibner contributed material on SCSI devices for this article.

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