20.6. Some Examples

Vinum maintains a configuration database which describes the objects known to an individual system. Initially, the user creates the configuration database from one or more configuration files with the aid of the gvinum(8) utility program. Vinum stores a copy of its configuration database on each disk slice (which Vinum calls a device) under its control. This database is updated on each state change, so that a restart accurately restores the state of each Vinum object.

20.6.1. 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:

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

      # gvinum -> 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 gvinum(8). It is represented graphically in Rysunek 20-4.

Rysunek 20-4. A Simple Vinum Volume



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.

20.6.2. 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

Rysunek 20-5 shows the structure graphically.

Rysunek 20-5. A Mirrored Vinum Volume



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.

20.6.3. 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

Rysunek 20-6. A Striped Vinum Volume



This volume is represented in Rysunek 20-6. The darkness of the stripes indicates the position within the plex address space: the lightest stripes come first, the darkest last.

20.6.4. 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.

Rysunek 20-7 represents the structure of this volume.

Rysunek 20-7. A Mirrored, Striped Vinum Volume



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