Simple Metadevices and Starting Blocks

When you create a simple metadevice of more than one slice, any slice except the first skips the first disk cylinder, if the slice starts at cylinder 0. For example, consider this output from the metastat(1M) command:

# metastat d0
 
d0: Concat/Stripe
    Size: 3546160 blocks
    Stripe 0: (interface: 32 blocks)
        Device              Start Block  Dbase
        c1t0d0s0                   0     No
        c1t0d1s0                   1520  No
        c1t0d2s0                   1520  No
        c1t0d2s0                   1520  No
        c1t1d0s0                   1520  No
        c1t1d1s0                   1520  No
        c1t1d2s0                   1520  No

In this example, stripe d0 shows a start block for each slice except the first as block 1520. This is to preserve the disk label in the first disk sector in all of the slices except the first. The metadisk driver must skip at least the first sector of those disks when mapping accesses across the stripe boundaries. Because skipping only the first sector would create an irregular disk geometry, the entire first cylinder of these disks is skipped. This enables higher level file system software (UFS) to optimize block allocations correctly. Thus, DiskSuite protects the disk label from being overwritten, and purposefully skips the first cylinder.

The reason for not skipping the first cylinder on all slices in the concatenation or stripe has to do with UFS. If you create a concatenated metadevice from an existing file system, and add more space to it, you would lose data because the first cylinder is where the data is expected to begin.