Disks

Formatting the disk

Formatting is the process of rewriting every sector on the disk with a specific data pattern, one that the electronics find most difficult to reproduce: if they can read this pattern, they can read anything. Microsoft calls this a Microsoft also uses the term high-level format for what we call creating a file system. Obviously it destroys any existing data, so

If you have anything you want to keep, back it up before formatting. Most modern disks don't need formatting unless they're damaged. In particular, formatting will not help if you're having configuration problems, if you can't get PPP to work or you're running out of disk space. Well, it will solve the disk space problem, but not in the manner you probably desire.

If you do need to format a SCSI disk, use camcontrol. camcontrol is a control program for SCSI devices, and it includes a lot of useful functions that you can read about in the man page. To format a disk, use the following syntax:

# camcontrol format dal

Remember that formatting a disk destroys all data on the disk. Before using the command, make sure that you need to do so: there are relatively few cases that call for formatting a disk. About the only reasons are if you want to change the physical sector size of the disk, or if you are getting "medium format corrupted" errors from the disk in response to read and write requests.

FreeBSD can format only floppies and SCSI disks. In general it is no longer possible to reformat ATA (IDE) disks, though some manufacturers have programs that can recover from some data problems. In most cases, though, in sufficient to write zeros to the entire disk:

# dd if=/dev/zero of=/dev/ad1 bs=128k

If this doesn't work, you may find formatting programs on the manufacturer's web site. You'll probably need to run them under a Microsoft platform.

Using sysinstall

If you can, use sysinstall to partition your disk. Looking at the dmesg output for our new disk, we see:

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)

You see the standard installation screen (see Chapter 5, page 60). Select Index, then Partition, and you see the following screen:

In this case, we want to partition /dev/dai, so we position the cursor on da1 (as shown) and press Enter. We see the disk partition menu, which shows that the disk currently contains three partitions:

We want a FreeBSD partition, not a Microsoft partition. At this point, we have a number of choices:

To use the whole disk, we first delete the current partition: we press the cursor down key until it highlights the FreeBSD partition. Then we press d, and the three partitions are joined into one, marked unused.

The next step is to create a new partition using the entire disk. If we press f, we get the following message:

We don't get this message if we use the a command: it just automatically assumes Yes. In this case we've decided to use the whole disk, so we move the cursor right to No and press Enter. That gives us a boot manager selection screen:

This isn't a boot disk, so we don't need any boot record, and it doesn't make any difference what we select. It's tidier, though, to select None as indicated. Then we press q to exit the partition editor, get back to the function index, and select Label. We see:

The important information on this rather empty looking menu is the information at the top about the free space available. We want to create two partitions: first, a swap partition of 512 MB, and then a file system taking up the rest of the disk. We press C, and are shown a submenu offering us all 8386733 blocks on the disk. We erase that and enter 512m, which represents 512 MB. Then we press Enter, and another submenu appears, asking us what kind of slice it is. We move the cursor down to select A swap partition :

Next, we press c again to create a new partition. This time, we accept the offer of the rest of the space on the disk, 7338157 sectors, we select A file system , and we are presented with yet another menu asking for the name of the file system. We enter the name, in this case /S:

After pressing Enter, we see:

Finally, we press W to tell the disk label editor to perform the function. We get an additional warning screen:

We're doing this online, so that's OK. We select Yes, and sysinstall creates the file system and mounts both it and the swap partition. This can take quite a while. Don't try to do anything with the drive until it's finished.

Doing it the hard way

Unfortunately, sometimes you may not be able to use the sysinstall method. You may not have access to sysinstall, or you may want to use options that sysinstall doesn't offer. That leaves us with the old way to add disks. The only difference is that this time we need to use different tools. In the following sections, we'll look at what we have to do to install this same 4 GB Seagate drive manually. This time we'll change the partitioning to contain the following partitions:

We've called this file system /newhome to use it as an example of moving file systems to new disks.

Creating a partition table

The first step is to create a PC BIOS style partition table on the disk. As in Microsoft, the partitioning program is called fdisk. In the following discussion, you'll find a pocket calculator indispensable.

If the disk is not brand new, it will have existing data of some kind on it. Depending on the nature of that data, fdisk could get sufficiently confused to not work correctly. If you don't format the disk, it's a good idea to overwrite the beginning of the disk with dd:

# dd if=/dev/zero of=/dev/da1 count=100
100+0 records in 100+0 records out
51200 bytes transferred in 1 sees (51200 bytes/sec)

We'll as sign 1 GB for Microsoft and use the remaining approximately 3 GB for FreeBSD. Our resulting partition table should look like:

(рис 11.3) Partition table on second FreeBSD disk

The Master Boot Record and the Partition Table take up the first sector of the disk, but many of the allocations are track oriented, so the entire first track of the disk is not available for allocation. The rest, up to the end of the last entire cylinder, can be divided between the partitions. It's easy to make a mistake in specifying the parameters, and fdisk performs as good as no checking. You can easily create a partition table that has absolutely no relationship with reality, so it's a good idea to calculate them in advance. For each partition, we need to know three things:

In addition, we need to decide which partition is the active partition, the partition from which we want to boot. In this case, it doesn't make any difference, because we won't be booting from the disk, but it's always a good idea to set it anyway.

We specify the partitions we don't want by giving them a type, start sector and end sector of 0. Our disk has 8386733 sectors, numbered 0 to 8386732. Partitions should start and end on a cylinder boundary, and we want the Microsoft partition to be about 1 GB. 1GB is 1024 MB, and 1 MB is 2048 sectors of 512 bytes each, so theoretically we want 1024 x 2048, or 2197152 sectors. Because of the requirement that partitions begin and end on a "cylinder" boundary, we need to find the closest number of "cylinders " to this vaflue. First we need to find out how big a "cylinder "is. We can do this by running /fdisk without any options:

# fdisk dal
******* Working on device /dev/dal *******
parameters extracted from in-core disklabel are:
cylinders=13726 heads=13 sectors/track=47 (611 blks/cyl)

Figures below won't work with BIOS for partitions not in cyl 1 
parameters to be used for BIOS calculations are:
cylinders=13726 heads=13 sectors/track=47 (611 blks/cyl)

fdisk: invalid fdisk partition table found
Media sector size is 512
Warning: BIOS sector numbering starts with sector 1 
Information from DOS bootblock is:
The data for partition 1 is:
<UNUSED>
The data for partition 2 is:
<UNUSED>
The data for partition 3 is:
<UNUSED>
The data for partition 4 is:
sysid 165,(FreeBSD/NetBSD/386BSD)
   start 47, size 8386539 (4094 Meg), flag 80 (active)
       beg: cyl 0/ head 1/ sector 1;
       end: cyl 413/ head 12/ sector 47

You'll notice that /fdisk has decided that there is a FreeBSD partition in partition 4. That happens even if the disk is brand new. In fact, this is a less desirable feature of /fdisk: it "suggests" this partition, it's not really there, which can be really confusing. This printout does, however, tell us that /fdisk thinks there are 611 sectors per cylinder, sowe divide 2197152 by 611 and get 3423.327 cylinders. We round down to 3423 cylinders, which provestobe2091453 sectors. This is the length we give to the first partition.

We use the remaining space for the FreeBSD partition. How much? Well, dmesg tells us that there are 8386733 sectors, but if you look at the geometry that /fdisk outputs, there are 13726 cylinders with 13 heads (tracks) per cylinder and 47 sectors per track. 13726 x 13 x 47 is 8386586. This rounding down is the explanation for the missing data at the end of the disk that we saw on page 204. The best way to calculate the size of the FreeBSD partition is to take the number of cylinders and multiply by the number of tracks per cylinder. The FreeBSD partition starts behind the Microsoft partition, so it goes from cylinder 3423 to cylinder 13725 inclusive or 10303 cylinders. At 611 sectors per cylinder, we have a total of 6295133 sectors in the partition. Our resulting information is:

Next we run fdisk in earnest by specifying the -i option. During this time, you may see messages on the console:

da1: invalid primary partition table: no magic

The message no magic doesn't mean that fdisk is out of purple smoke. It refers to the fact that it didn't find the so-called magic number, which identifies the partition table. We don't have a partition table yet, so this message isn't surprising. It's also completely harmless.

fdisk prompts interactively when you specify the -i flag:

# fdisk -i dal
******* Working on device /dev/da1 *******
parameters extracted from in-core disklabel are:
cylinders=13726 heads=13 sectors/track=47 (611 blks/cyl)

Figures below won't work with BIOS for partitions not in cyl 1
parameters to be used for BIOS calculations are:
cylinders=13726 heads=13 sectors/track=47 (611 blks/cyl)

Do you want to change our idea of what BIOS thinks ? [n] Enter pressed Media sector size is 512
Warning: BIOS sector numbering starts with sector 1
Information from DOS bootblack is:
The data for partition 1 is:
sysid 165,(FreeBSD/NetBSD/386BSD)
   start 0, size 8386733 (4095 Meg), flag 80 (active)
       beg: cyl 0/ head 0/ sector 1; 
       end: cyl 522/ head 12/ sector 47
Do you want to change it?  [n] y
Supply a decimal value for "sysid (165=FreeBSD)"  [0] 6
Supply a decimal value for "start"  [0]    Enter pressed Supply a decimal value for "size"  [0] 2091453
Explicitly specify beg/end address ?  [n]    Enter pressed
sysid 6,(Primary 'big' DOS (> 32MB))
   start 0, size 2091453 (1021 Meg), flag 0 
      beg: cyl 0/ head 0/ sector 1;
      end: cyl 350/ head 12/ sector 47
Are we happy with this entry?  [n] y
The data for partition 2 is:
<UNUSED>
Do you want to change it?  [n] y
Supply a decimal value for "sysid (165=FreeBSD)"  [0] 165 Supply a decimal value for "start"  [0] 2091453 
Supply a decimal value for "size"  [0] 6295133 
Explicitly specify beg/end address ?  [n]    Enter pressed
sysid 165,(FreeBSD/NetBSD/386BSD)
start 2091453, size 6295133  (3073 Meg), flag 0
   beg: cyl 351/ head 0/ sector 1;
   end: cyl 413/ head 12/ sector 47
Are we happy with this entry?  [n] y
The data for partition 3 is:
<UNUSED>
Do you want to change it?  [n] Enter pressed
The data for partition 4 is:
sysid 165,(FreeBSD/NetBSD/386BSD)
    start 47, size 8386539 (4094 Meg), flag 80 (active)
      beg: cyl 0/ head 1/ sector 1;
      end: cyl 413/ head 12/ sector 47 
Do you want to change it?  [n] y
The static data for the DOS partition 4 has been reinitialized to:
sysid 165,(FreeBSD/NetBSD/386BSD)
start 47, size 8386539 (4094 Meg), flag 80 (active)
    beg: cyl 0/ head 1/ sector 1;
    end: cyl 413/ head 12/ sector 47
Supply a decimal value for "sysid (165=FreeBSD)"  [165] 0 Supply a decimal value for "start"  [47] 0 
Supply a decimal value for "size"  [8386539] 0 
Explicitly specify beg/end address ?  [n] Enter pressed 
<UNUSED>
Are we happy with this entry?  [n] y 
Do you want to change the active partition?  [n] y 
Supply a decimal value for "active partition"  [1] 2 
Are you happy with this choice [n] y

We haven't changed the partition table yet.   This is your last chance, parameters extracted from in-core disklabel are: 
cylinders=13726 heads=13 sectors/track=47 (611 blks/cyl)

Figures below won't work with BIOS for partitions not in cyl 1 parameters to be used for BIOS calculations are:
cylinders=13726 heads=13 sectors/track=47 (611 blks/cyl)

Information from DOS bootblack is:
1: sysid 6,(Primary 'big' DOS (> 32MB))
    start 0, size 2091453 (1021 Meg), flag 0
      beg: cyl 0/ head 0/ sector 1;
      end: cyl 350/ head 12/ sector 47 
2: sysid 165,(FreeBSD/NetBSD/386BSD)
    start 2091453, size 6295133  (3073 Meg), flag 80 (active)
      beg: cyl 351/ head 0/ sector 1;
      end: cyl 413/ head 12/ sector 47 
3: <UNUSED> 
4: <UNUSED>
Should we write new partition table?  [n] y

You'll notice a couple of things here:

Labeling the disk

Once we have a valid PC BIOS partition table, we need to create the file systems. We won't look at the Microsoft partition in anymore detail, but we still need to do some more work on our FreeBSD slice (slice or PC BIOS partition 2). It'll make life easier here to remember a couple of things:

The first thing we need is the disk (slice) label, which supplies general information about the slice:

The only information we need to input is the kind, size and locations of the partitions. In this case, we have decided to create a file system on partition h (/dev/da1s2h) and swap space on partition b (/dev/dalslb). The swap space will be 512 MB, and the file system will take up the rest of the slice. This is mainly tradition: traditionally data disks use the h partition and not the a partition, so we'll stick to that tradition, though there's nothing to stop you from using the a partition if you prefer. In addition, we need to define the c partition, which represents the whole slice. In summary, the FreeBSD slice we want to create looks like:

bsdlabel

The program that writes the disk label used to be called disklabel As FreeBSD migrated to multiple platforms, this proved to be too generic: many hardware platforms have their own disk label formats. For example, FreeBSD on SPARC64 uses the Sun standard labels. On platforms which use the old BSD labels, such as the PC, the name was changed to bsdlabel. On SPARC64 it is called sunlabel. On each platform, the appropriate file is linked to the name disklabel, but some of the options have changed. In addition, the output format now normally ignores a number of historical relics. It's not as warty as fdisk, but it can still give you a run for your money. You can usually ignore most of the complexity, though. You can normally create a disk label with the single command:

# bsdlabel -w /dev/da1s2 auto

This creates the label with a single partition c. You can look at the label with bsdlabel without options:

#bsdlabel /dev/da1s2
#/dev/da0s2:
8partitions:
#       size  offset  fstype  [fsize bsize bps/cpg]
 c:  6295133       0  unused     0     0          # "raw" part, don't edit

At this point, the only partition you have is the "whole disk" partition c. You still need to create partitions b and h and specify their location and size. Do this with bsdlabel -e, which starts an editor with the output you see above. Simply add additional partitions:

8partitions:
#       size   offset  fstype   [fsize bsize bps/cpg]
 c:  6295133        0  unused      0     0  # "raw" part, don't edit
 b:  1048576        0  swap        0     0
 h:  5246557  1048576  unused      0     0

You don't need to maintain any particular order, and you don't need to specify that partition h will be a file system. In the next step , newfs does that for you automatically.

Problems running biddable

Using the old disklabel program used to be like walking through a minefield. Things have got a lot better, but it's possible that some problems are still hiding. Here are some of the problems that have been encountered in the past, along with some suggestions about what to do if you experience them:

# bsdlabel -w da1s2
bsdlabel: /dev/da1s2c: Undefined error: 0

# dd if=/dev/zero of=/dev/da1 bs=128k count=

fixlabel: raw partition size > slice size
or
fixlabel: raw partitions offset != slice offset

Creating file systems

Once we have a valid label, we need to create the file systems. In this case, there's only one file system, on /dev/da1s2/z. Mercifully, this is easier:

# newfs -U /dev/da1s2h
/dev/vinum/da1s2h: 2561.8MB (5246556 sectors) block size 16384, fragment size 2048 
    using 14 cylinder groups of 183.77MB, 11761 blks, 23552 inodes.
    with soft updates
super-block backups (for fsck -b #)at:
160, 376512, 752864, 1129216, 1505568, 1881920, 2258272, 2634624, 3010976, 3387328,
3763680, 4140032, 4516384, 4892736

The -U flag tells newfs to enable soft updates, which we looked at on page 191.

Mounting the file systems

Finally the job is done. Well, almost. You still need to mount the file system, and to tell the system that it has more swap. But that's not much of a problem:

# mkdir /newhome             make sure we have a directory to mount on
# mount /dev/da1s2h/newhome  and mount it
# swapon /dev/da1s2b
# df                         show free capacity and mounted file systems
Filesystem    1024-blocks     Used    Avail  Capacity  Mounted on
/dev/ad0s1a         19966    17426      944     95%    /
/dev/ad0s1e       1162062   955758   113340     89%    /usr
procfs                  4        4        0    100%    /proc
presto:/            15823     6734     8297     45%    /presto/root
presto:/usr        912271   824927    41730     95%    /presto/usr
presto:/home      1905583  1193721   521303     70%    /presto/home
presto:/S         4065286  3339635   563039     86%    /S
/dev/da1s2h       2540316        2  2337090      0%    /newhome
# pstat -s                   show  swap usage
Device          1K-blocks     Used    Avail  Capacity  Type
/dev/ad0s4b        524160        0   524160      0%    Interleaved
/dev/da1s2b        524160        0   524160      0%    Interleaved
Total             1048320        0  1048320      0%

This looks fine, but when you reboot the system, /newhome and the additional swap will be gone. To ensure that they get mounted after booting, you need to add the following lines to /etc/fstab:

/dev/da1s2b  none      swap  sw  0  0
/dev/da1s2h  /newhome  ufs   rw  0  0

Moving file systems

Very frequently, you add a new disk to a system because existing disks have run out of space. Let's consider the disk we have just added and assume that currently the files in /home are physically located on the /usr file system, and that /home is a symbolic link to /usr/home. We want to move them to the new file system and then rename it to /home. Here's what to do:

#cd /home
#tar cf - .  |   (cd /newhome; tar xvf – 2>/var/tmp/tarerrors)

#  rm -rf /usr/home

#rm /home
#mkdir /home

/dev/da1s2h  /home  ufs  rw  0  0

#umount /newhome
#mount /home

Recovering from disk data errors

Modern hard disks are a miracle in evolution. Today you can buy a 200 GB hard disk for under $200, and it will fit in your shirt pocket. Thirty years ago, a typical disk drive was the size of a washing machine and stored 20 MB. You would need 10,000 of them to store 200 GB.

At the same time, reliability has gone up, but disks are still relatively unreliable devices. You can achieve maximum reliability by keeping them cool, but sooner or later you are going to run into some kind of problem. One kind is due to surface irregularities: the disk can't read a specific part of the surface.

Modern disks make provisions for recovering from such errors by allocating an alternate sector for the data. IDE drives do this automatically, but with SCSI drives you have the option of enabling or disabling reallocation. Usually reallocation is enabled when you buy the disk, but occasionally it is not. When installing a new disk, you should check that the parameters ARRE (Auto Read Reallocation Enable) and AWRE (Auto Write Reallocation Enable) are turned on. For example, to check and set the values for disk /dev/da1 , you would enter:

#  camcontrol modepage dal -m 1 -e

This command will start up your favorite editor (either the one specified in the EDITOR environment variable, or vi by default) with the following data:

AWRE (Auto Write Reallocation Enbld): 0
ARRE (Auto Read Reallocation Enbld): 0
TB (Transfer Block): 1
EER (Enable Early Recovery): 0
PER (Post Error): 1
DTE (Disable Transfer on Error): 0
DCR (Disable Correction): 0
Read Retry Count: 41
Write Retry Count: 24

The values for AWRE and ARRE should both be 1. If they aren't, as in this case, where AWRE is 0, change the data with the editor, write it back, and exit. camcontrol writes the data back to the disk and enables the option.

Note the last two lines in this example. They give the number of actual retries that this drive has performed. You can reset these values too if you want; they will be updated if the drive performs any additional retries.