Virtual Object Data Organization (Volume Layouts)

Related Graphical User Interface (GUI) Terms

The following graphical user interface (GUI) terms refer to types of layered volumes created on the Storage Administrator:

Concatenation

Concatenation maps data in a linear manner onto one or more subdisks in a plex. To access all the data in a concatenated plex sequentially, data is first accessed in the first subdisk from beginning to end. Data is then accessed in the remaining subdisks sequentially from beginning to end until the end of the last subdisk.

The subdisks in a concatenated plex do not have to be physically contiguous and can belong to more than one VM disk. Concatenation using subdisks that reside on more than one VM disk is called spanning.

Figure 1-8 “Example of Concatenation” shows concatenation with one subdisk.

Figure 1-8 Example of Concatenation

You can use concatenation with multiple subdisks when there is insufficient contiguous space for the plex on any one disk. This form of concatenation can be used for load balancing between disks, and for head movement optimization on a particular disk.

For an example of a volume in a concatenated configuration, see Figure 1-9 “Example of a Volume in a Concatenated Configuration”

Figure 1-9 Example of a Volume in a Concatenated Configuration

In the example shown in Figure 1-10 “Example of Spanning”, the first six blocks of data (B1 through B6) use most of the space on the disk that VM disk disk01 is assigned to. This requires space only on subdisk disk01-01 on VM disk disk01. However, the last two blocks of data, B7 and B8, use only a portion of the space on the disk that VM disk disk02 is assigned to.

The remaining free space on VM disk disk02 can be put to other uses. In this example, subdisks disk02-02 and disk02-03 are available for other disk management tasks.

Figure 1-10 “Example of Spanning” shows data spread over two subdisks in a spanned plex.

Figure 1-10 Example of Spanning

	CAUTION: Spanning a plex across multiple disks increases the chance that a disk failure results in failure of the assigned volume. Use mirroring or RAID-5 (both described later) to reduce the risk that a single disk failure results in a volume failure.

Striping (RAID-0)

Striping (RAID-0) maps data so that the data is interleaved among two or more physical disks. A striped plex contains two or more subdisks, spread out over two or more physical disks. Data is allocated alternately and evenly to the subdisks of a striped plex.

The subdisks are grouped into "columns," with each physical disk limited to one column. Each column contains one or more subdisks and can be derived from one or more physical disks. The number and sizes of subdisks per column can vary. Additional subdisks can be added to columns, as necessary.

	CAUTION: Striping a volume, or splitting a volume across multiple disks, increases the chance that a disk failure will result in failure of that volume. For example, if five volumes are striped across the same five disks, then failure of any one of the five disks will require that all five volumes be restored from a backup. If each volume is on a separate disk, only one volume has to be restored. Use mirroring or RAID-5 to substantially reduce the chance that a single disk failure results in failure of a large number of volumes.

Data is allocated in equal-sized units (stripe units of size called stripe unit size) that are interleaved between the columns. Each stripe unit is a set of contiguous blocks on a disk. The default stripe unit size is 64 kilobytes.

For example, if there are three columns in a striped plex and six stripe units, data is striped over three physical disks, as shown in Figure 1-11 “Striping Across Three Disks (Columns)”. In Figure 1-11 “Striping Across Three Disks (Columns)”:

Figure 1-11 Striping Across Three Disks (Columns)

A stripe consists of the set of stripe units at the same positions across all columns. In Figure 1-11 “Striping Across Three Disks (Columns)”, stripe units 1, 2, and 3 constitute a single stripe.

Viewed in sequence, the first stripe consists of:

The second stripe consists of:

Striping continues for the length of the columns (if all columns are the same length) or until the end of the shortest column is reached. Any space remaining at the end of subdisks in longer columns becomes unused space.

Striping is useful if you need large amounts of data written to or read from the physical disks quickly by using parallel data transfer to multiple disks. Striping is also helpful in balancing the I/O load from multi-user applications across multiple disks.

Figure 1-12 “Example of a Striped Plex with One Subdisk per Column” shows a striped plex with three equal-sized, single-subdisk columns. There is one column per physical disk.

Figure 1-12 Example of a Striped Plex with One Subdisk per Column

The example in Figure 1-12 “Example of a Striped Plex with One Subdisk per Column” shows three subdisks that occupy all of the space on the VM disks. It is also possible for each subdisk in a striped plex to occupy only a portion of the VM disk, which leaves free space for other disk management tasks.

Figure 1-13 “Example of a Striped Plex with Concatenated Subdisks per Column” shows a striped plex with three columns containing subdisks of different sizes. Each column contains a different number of subdisks. There is one column per physical disk. Striped plexes can be created by using a single subdisk from each of the VM disks being striped across. It is also possible to allocate space from different regions of the same disk or from another disk (for example, if the plex is grown). Columns can contain subdisks from different VM disks if necessary.

Figure 1-13 Example of a Striped Plex with Concatenated Subdisks per Column

RAID-5

	NOTE: You may need an additional license to use this feature.

RAID-5 provides data redundancy by using parity. Parity is a calculated value used to reconstruct data after a failure. While data is being written to a RAID-5 volume, parity is calculated by doing an exclusive OR (XOR) procedure on the data. The resulting parity is then written to the volume. If a portion of a RAID-5 volume fails, the data that was on that portion of the failed volume can be recreated from the remaining data and parity information.

RAID-5 volumes maintain redundancy of the data within a volume. RAID-5 volumes keep a copy of the data and calculated parity in a plex that is "striped" across multiple disks. In the event of a disk failure, a RAID-5 volume uses parity to reconstruct the data. It is possible to mix concatenation and striping in the layout.

RAID-5 volumes can do logging to minimize recovery time. RAID-5 volumes use RAID-5 logs to keep a copy of the data and parity currently being written. RAID-5 logging is optional and can be created along with RAID-5 volumes or added later.

Figure 1-14 “Parity Locations in a RAID-5 Model” shows parity locations in a RAID-5 array configuration. Every stripe has a column containing a parity stripe unit and columns containing data. The parity is spread over all of the disks in the array, reducing the write time for large independent writes because the writes do not have to wait until a single parity disk can accept the data.

Figure 1-14 Parity Locations in a RAID-5 Model

For more information, see “Volume Manager and RAID-5”.

Mirroring (RAID-1)

	NOTE: You may need an additional license to use this feature.

Mirroring uses multiple mirrors (plexes) to duplicate the information contained in a volume. In the event of a physical disk failure, the plex on the failed disk becomes unavailable, but the system continues to operate using the unaffected mirrors. Although a volume can have a single plex, at least two plexes are required to provide redundancy of data. Each of these plexes must contain disk space from different disks to achieve redundancy.

When striping or spanning across a large number of disks, failure of any one of those disks can make the entire plex unusable. The chance of one out of several disks failing is sufficient to make it worthwhile to consider mirroring in order to improve the reliability (and availability) of a striped or spanned volume.

Mirroring Plus Striping (RAID-1 + RAID-0)

	NOTE: You may need an additional license to use this feature.

The Volume Manager supports the combinations of mirroring plus striping. When used together on the same volume, mirroring plus striping offers the benefits of spreading data across multiple disks (striping) while providing redundancy (mirror) of data.

For mirroring plus striping to be effective when used together, the mirror and its striped plex must be allocated from separate disks. The layout type of the mirror can be concatenated or striped.

Striping Plus Mirroring (RAID-0 + RAID-1)

	NOTE: You may need an additional license to use this feature.

The Volume Manager supports the combination of striping with mirroring. In previous releases, whenever mirroring was used, the mirroring had to happen above striping. Now there can be mirroring both above and below striping.

By putting mirroring below striping, each column of the stripe is mirrored. If the stripe is large enough to have multiple subdisks per column, each subdisk can be individually mirrored. This layout enhances redundancy and reduces recovery time in case of an error.

In a mirror- stripe layout, if a disk fails, the entire plex is detached, thereby losing redundancy on the entire volume. When the disk is replaced, the entire plex must be brought up to date. Recovering the entire plex can take a substantial amount of time. If a disk fails in a stripe-mirror layout, only the failing subdisk must be detached, and only that portion of the volume loses redundancy. When the disk is replaced, only a portion of the volume needs to be recovered.

Compared to mirroring plus striping, striping plus mirroring offers a volume more tolerant to disk failure. If a disk failure occurs, the recovery time is shorter for striping plus mirroring. See “Layered Volumes” for more information.