Disks

One of the most important parts of running any computer system is handling data on disk. We have already looked at UNIX file handling in "File systems and devices" . In this chapter, we'll look at two ways to add another disk to your system, and what you should put on them. In addition.

Adding a hard disk

When you installed FreeBSD, you created file systems on at least one hard disk. At a later point, you may want to install additional drives. There are two ways to do this: with sysinstall and with the traditional UNIX command-line utilities.

There was a time when it was dangerous to use sysinstall after the system had been installed: there was a significant chance of shooting yourself in the foot. There's always a chance of doing something wrong when initializing disks, but sysinstall has become a lot better, and now it's the tool of choice. It's good to know the alternatives, though. In this section we'll look at sysinstall, and on page 209 we'll see how to do it manually if sysinstall won't cooperate.

We've been through all the details of disk layout and slices and partitions in "Before you install" , so I won't repeat them here. Basically, to add a new disk to the system, you need to:

• Install the disk physically. This usually involves power cycling the machine.
• Barely possibly, format the disk. Without exception, modern disks come preformatted, and you only need to format a disk if it has defects or if it's ancient. In many cases the so-called "format" program doesn't really format at all.
• If you want to share with other operating systems, create a PC style partition table on the disk.
• Define a FreeBSD slice (which the PC BIOS calls a "partition").
• Define the partitions in the FreeBSD slice.
• Tell the system about the file systems and where to mount them.
• Create the file systems.

These are the same operations that we performed in "Installing FreeBSD" .

Disk hard ware installation

Before you can do anything with the disk, you have to install it in the system. To do this, you must normally shut down the system and turn the power off, though high-end SCSI enclosures allow hot-swapping, changing disks in a running system. If the disk is IDE, and you already have an IDE disk on the controller, you need to set the second disk as "slave" drive. And you may have to set the first disk as "master" drive: if you only have one drive, you don't set any jumpers, but if you have two drives, some disks require you to set jumpers on both disks. If you don't do this, the system will appear to hang during the power-on self test, and will finally report some kind of disk error.

Adding a SCSI disk is more complicated. You can connect up to 15 SCSI devices to a host adapter, depending on the interface. Many systems restrict the number to 7 for compatibility with older SCSI interfaces. Typically, your first SCSI disk will have the SCSI ID 0, and the host adapter will have the SCSI ID 7. Traditionally, the IDs 4, 5, and 6 are reserved for tape and CD-ROM drives, and the IDs 0 to 3 are reserved for disks, though FreeBSD doesn't impose any restrictions on what goes where.

What ever kind of disk you're adding, look at the boot messages, which you can retrieve with the dmesg command. For example, if you're planning to add a SCSI device, you might see:

sym0: <875> port 0xc400-0xc4ff mem 0xec002000-0xec002fff,0xec003000-0xec0030ff irq 10
at device 9.0 on pci0
sym0: Symbios NVRAM, ID 7, Fast-20, SE, NO parity
sym0: open drain IRQ line driver, using on-chip SRAM
sym0: using LOAD/STORE-based firmware.
sym0: SCAN FOR LUNS disabled for targets 0.
sym1: <875> port 0xc800-0xc8ff mem 0xec001000-0xec001fff,0xec000000-0xec0000ff irq 9
at device 13.0 on pci0
sym1: No NVRAM, ID 7, Fast-20, SE, parity checking
further down...
Waiting 3 seconds for SCSI devices to settle sa0
at sym0 bus 0 target 3 lun 0
sa0: <EXABYTE EXB-8505SMBANSH2 0793> Removable Sequential Access SCSI-2 device
sa0: 5.000MB/s transfers (5.000MHz, offset 11) sa1 at sym0 bus 0 target 4 lun 0
sa1: <ARCHIVE Python 28849-XXX 4.CM> Removable Sequential Access SCSI-2 device
sa1: 5.000MB/s transfers (5.000MHz, offset 15) sa2 at sym0 bus 0 target 5 lun 0
sa2: <TANDBERG TDC 3800 -03:> Removable Sequential Access SCSI-CCS device
sa2: 3.300MB/s transfers
pass4 at sym0 bus 0 target 4 lun 1
pass4: <ARCHIVE Python 28849-XXX 4.CM> Removable Changer SCSI-2 device
pass4: 5.000MB/s transfers (5.000MHz, offset 15) cd0 at sym0 bus 0 target 6 lun 0
cd0: <NRC MBR-7 110> Removable CD-ROM SCSI-2 device
cd0: 3.300MB/s transfers
cd0: cd present [322265 x 2048 byte records]
da0 at sym1 bus 0 target 3 lun 0
da0: <SEAGATE ST15230W SUN4.2G 0738> Fixed Direct Access SCSI-2 device
da0: 20.000MB/s transfers (10.000MHz, offset 15, 16bit), Tagged Queueing Enabled
da0: 4095MB (8386733 512 byte sectors: 255H 63s/t 522C)

This output shows two Sym bios SCSI host adapters /dev/(sym0 and /dev/syml), three tape drives/dev/(sa0, /dev/sal and /dev/sa2), a CD-ROM drive /dev/(cd0), a tape changer /dev/(pass4), and also a disk drive /dev/da0 on ID 3, which is called a target in these messages. The disk is connected to the second host adapter, and the other devices are connected to the first host adapter.

Installing an external SCSI device

External SCSI devices have two cable connectors: one goes towards the host adapter, and the other towards the next device. The order of the devices in the chain does not have to have anything to do with the SCSI ID. This method is called daisy chaining. At the end of the chain, the spare connector may be plugged with a terminator, a set of resistors designed to keep noise off the bus. Some devices have internal terminators, however. When installing an external device, you will have to do one of the following:

• If you are installing a first external device (one connected directly to the cable connector on the backplane of the host adapter), you will have to ensure that the device provides termination. If you already have atl east one internal device, the host adapter will no longer be at one end of the chain, so you will also have to stop it from providing termination. Modern SCSI host adapters can decide whether they need to terminate, but older host adapters have resistor packs. In the latter case, remove these resistor packs.
• If you are adding an additional external device, you have two choices: you can remove a cable in the middle of the daisy chain and plug it into your new device. You then connect a new cable from your device to the device from which you removed the original cable.

  Alternatively, you can add the device at the end of the chain. Remove the terminator or turn off the termination, and plug your cable into the spare socket. Insert the terminator in your device (or turn termination on).

You can add external SCSI devices to a running system if they're hot-pluggable. It might even work if they're not hot-pluggable, but it's not strictly the correct thing to do, and there's the risk that you might damage something, possibly irreparably. After connecting the devices, powering them up and waiting for them to come ready, run camcontrol rescan. For example, if you added a second disk drive to the second host adapter in the example above, you might see:

# camcontrol rescan 1
dal at syml bus 0 target 0 lun 0
dal: <SEAGATE ST15230W SUN4.2G 0738> Fixed Direct Access SCSI-2 device
dal: 20.000MB/s transfers (10.000MHz, offset 15, 16bit), Tagged Queueing Enabled
da1: 4095MB (8386733 512 byte sectors: 255H 63s/t 522C)
Re-scan of bus 1 was successful

There's a problem with this approach: note that /dev/dal has ID 0, and the already present /dev/da0 has ID 3. If you now reboot the system, they will come up with the device names the other way round. We'll look at this issue in more detail in the next section.

Installing an internal SCSI device

Installing an internal SCSI device is much the same as installing an external device. Instead of daisy chains, you have a tat band cable with a number of connectors. Find one that suits you, and plug it into the device. Again, you need to think about termination:

• If you are installing the device at the end of the chain, it should have termination enabled. You should also disable termination for the device that was previously at the end of the chain. Depending on the device, this may involve removing the physical terminators or setting a jumper.
• If you are installing the device in the middle of the chain, make sure it does not have termination enabled.

In this chapter, we'll look at two ways of installing a drive in an existing SCSI chain. We could be in for a surprise: the device ID we get for the new drive depends on what is currently on the chain. For example, consider our example above, where we have a chain with a single drive on it:

da0 at sym1 bus 0 target 3 lun 0
da0: <SEAGATE ST15230W SUN4.2G 0738> Fixed Direct Access SCSI-2 device
da0: 20.000MB/s transfers (10.000MHz, offset 15, 16bit), Tagged Queueing Enabled
da0: 4095MB (8386733 512 byte sectors: 255H 63S/T 522C)

This drive on target (ID) 2. If we put our new drive on target 0 and reboot, we see:

da0 at sym1 bus 0 target 0 lun 0
da0: <SEAGATE ST15230W SUN4.2G 0738> Fixed Direct Access SCSI-2 device
da0: 20.000MB/s transfers (10.000MHz, offset 15, 16bit), Tagged Queueing Enabled
da0: 4095MB (8386733 512 byte sectors: 255H 63S/T 522C)
da1 at sym1 bus 0 target 3 lun 0
da1: <SEAGATE ST15230W SUN4.2G 0738> Fixed Direct Access SCSI-2 device
da1: 20.000MB/s transfers (10.000MHz, offset 15, 16bit), Tagged Queueing Enabled
da1: 4095MB (8386733 512 byte sectors: 255H 63S/T 522C)

At first glance, this looks reasonable, but that's only because both disks are of the same type. If you look at the target numbers, you'll notice that the new disk is /dev/da0,not /dev/dal. The target ID of the new disk is lower than the target ID of the old disk, so the system recognizes the new disk as /dev/da0, and our previous /dev/da0 has become /dev/dal .

This change of disk ID can be a problem. One of the first things you do with a new disk is to create new disk labels and file systems. Both offer excellent opportunities to shoot yourself in the foot if you choose the wrong disk: the result would almost certainly be the complete loss of data on that disk. Even apart from such catastrophes, you'll have to edit /etc/fstab before you can mount any file systems that are on the disk. The alternatives are to wire down the device names, or to change the SCSI IDs. In FreeBSD 5.0, you wire down device names and busses by adding entries to the boot configuration file /boot/device.hints. We'll look at that on page 575.