13.6 Some Examples

13.6.1 The Configuration File

The configuration file describes individual Vinum objects. The definition of a simple volume might be:

    drive a device /dev/da3h
    volume myvol
      plex org concat
        sd length 512m drive a

This file describes four Vinum objects:

• The drive line describes a disk partition (drive) and its location relative to the underlying hardware. It is given the symbolic name a. This separation of the symbolic names from the device names allows disks to be moved from one location to another without confusion.

• The volume line describes a volume. The only required attribute is the name, in this case myvol.

• The plex line defines a plex. The only required parameter is the organization, in this case concat. No name is necessary: the system automatically generates a name from the volume name by adding the suffix .px, where x is the number of the plex in the volume. Thus this plex will be called myvol.p0.

• The sd line describes a subdisk. The minimum specifications are the name of a drive on which to store it, and the length of the subdisk. As with plexes, no name is necessary: the system automatically assigns names derived from the plex name by adding the suffix .sx, where x is the number of the subdisk in the plex. Thus Vinum gives this subdisk the name myvol.p0.s0.

After processing this file, vinum(8) produces the following output:

      # vinum -> create config1
      Configuration summary
      Drives:         1 (4 configured)
      Volumes:        1 (4 configured)
      Plexes:         1 (8 configured)
      Subdisks:       1 (16 configured)
     
    D a                     State: up       Device /dev/da3h        Avail: 2061/2573 MB (80%)
    
    V myvol                 State: up       Plexes:       1 Size:        512 MB
    
    P myvol.p0            C State: up       Subdisks:     1 Size:        512 MB
    
    S myvol.p0.s0           State: up       PO:        0  B Size:        512 MB

This output shows the brief listing format of vinum(8). It is represented graphically in Figure 13-4.

This figure, and the ones which follow, represent a volume, which contains the plexes, which in turn contain the subdisks. In this trivial example, the volume contains one plex, and the plex contains one subdisk.

This particular volume has no specific advantage over a conventional disk partition. It contains a single plex, so it is not redundant. The plex contains a single subdisk, so there is no difference in storage allocation from a conventional disk partition. The following sections illustrate various more interesting configuration methods.

13.6.2 Increased Resilience: Mirroring

The resilience of a volume can be increased by mirroring. When laying out a mirrored volume, it is important to ensure that the subdisks of each plex are on different drives, so that a drive failure will not take down both plexes. The following configuration mirrors a volume:

   drive b device /dev/da4h
    volume mirror
      plex org concat
        sd length 512m drive a
      plex org concat
        sd length 512m drive b

In this example, it was not necessary to specify a definition of drive a again, since Vinum keeps track of all objects in its configuration database. After processing this definition, the configuration looks like:

   Drives:         2 (4 configured)
    Volumes:        2 (4 configured)
    Plexes:         3 (8 configured)
    Subdisks:       3 (16 configured)
    
    D a                     State: up       Device /dev/da3h        Avail: 1549/2573 MB (60%)
    D b                     State: up       Device /dev/da4h        Avail: 2061/2573 MB (80%)

    V myvol                 State: up       Plexes:       1 Size:        512 MB
    V mirror                State: up       Plexes:       2 Size:        512 MB
  
    P myvol.p0            C State: up       Subdisks:     1 Size:        512 MB
    P mirror.p0           C State: up       Subdisks:     1 Size:        512 MB
    P mirror.p1           C State: initializing     Subdisks:     1 Size:        512 MB
  
    S myvol.p0.s0           State: up       PO:        0  B Size:        512 MB
    S mirror.p0.s0          State: up       PO:        0  B Size:        512 MB
    S mirror.p1.s0          State: empty    PO:        0  B Size:        512 MB

Figure 13-5 shows the structure graphically.

In this example, each plex contains the full 512 MB of address space. As in the previous example, each plex contains only a single subdisk.

13.6.3 Optimizing Performance

The mirrored volume in the previous example is more resistant to failure than an unmirrored volume, but its performance is less: each write to the volume requires a write to both drives, using up a greater proportion of the total disk bandwidth. Performance considerations demand a different approach: instead of mirroring, the data is striped across as many disk drives as possible. The following configuration shows a volume with a plex striped across four disk drives:

   drive c device /dev/da5h
    drive d device /dev/da6h
    volume stripe
    plex org striped 512k
      sd length 128m drive a
      sd length 128m drive b
      sd length 128m drive c
      sd length 128m drive d

As before, it is not necessary to define the drives which are already known to Vinum. After processing this definition, the configuration looks like:

   Drives:         4 (4 configured)
    Volumes:        3 (4 configured)
    Plexes:         4 (8 configured)
    Subdisks:       7 (16 configured)
  
    D a                     State: up       Device /dev/da3h        Avail: 1421/2573 MB (55%)
    D b                     State: up       Device /dev/da4h        Avail: 1933/2573 MB (75%)
    D c                     State: up       Device /dev/da5h        Avail: 2445/2573 MB (95%)
    D d                     State: up       Device /dev/da6h        Avail: 2445/2573 MB (95%)
  
    V myvol                 State: up       Plexes:       1 Size:        512 MB
    V mirror                State: up       Plexes:       2 Size:        512 MB
    V striped               State: up       Plexes:       1 Size:        512 MB
  
    P myvol.p0            C State: up       Subdisks:     1 Size:        512 MB
    P mirror.p0           C State: up       Subdisks:     1 Size:        512 MB
    P mirror.p1           C State: initializing     Subdisks:     1 Size:        512 MB
    P striped.p1            State: up       Subdisks:     1 Size:        512 MB
  
    S myvol.p0.s0           State: up       PO:        0  B Size:        512 MB
    S mirror.p0.s0          State: up       PO:        0  B Size:        512 MB
    S mirror.p1.s0          State: empty    PO:        0  B Size:        512 MB
    S striped.p0.s0         State: up       PO:        0  B Size:        128 MB
    S striped.p0.s1         State: up       PO:      512 kB Size:        128 MB
    S striped.p0.s2         State: up       PO:     1024 kB Size:        128 MB
    S striped.p0.s3         State: up       PO:     1536 kB Size:        128 MB

This volume is represented in Figure 13-6. The darkness of the stripes indicates the position within the plex address space: the lightest stripes come first, the darkest last.

13.6.4 Resilience and Performance

With sufficient hardware, it is possible to build volumes which show both increased resilience and increased performance compared to standard UNIX® partitions. A typical configuration file might be:

   volume raid10
      plex org striped 512k
        sd length 102480k drive a
        sd length 102480k drive b
        sd length 102480k drive c
        sd length 102480k drive d
        sd length 102480k drive e
      plex org striped 512k
        sd length 102480k drive c
        sd length 102480k drive d
        sd length 102480k drive e
        sd length 102480k drive a
        sd length 102480k drive b

The subdisks of the second plex are offset by two drives from those of the first plex: this helps ensure that writes do not go to the same subdisks even if a transfer goes over two drives.

Figure 13-7 represents the structure of this volume.