Volumes’ Aggregation
Why?
improve performance.
prevent faults by adding redundancy.
gather free space from several drives.
Types:
RAID (Redundant Arrays of Inexpensive Disks).
common in high-performance systems has many variants: RAID-1, RAID-5, ...
can be implemented both in hardware, or software (usually at OS level).
Spanning.
creates a logical volume by adding free space from several smaller volumes.
just sums up space, it doesn’t have any redundancy or performance gains.
RAID
Levels
RAID-0 - has no redundancy, but may increase performance.
RAID-1 – consists of an exact copy (or mirror) of a set of data on two or more disks.
RAID-2 – stripes data at the bit (rather than block) level, and uses a Hamming code for error correction rarely used.
RAID-3 – consists of byte-level striping with a dedicated parity disk rarely used.
RAID-4 – consists of block-level striping with a dedicated parity disk, and provides good read performance.
RAID-5 – block-level striping with distributed parity among the drives.
RAID-6 – extends RAID 5 by adding another parity block and supports up to 2 drive failures.
RAID hybrid – a combination of more than one RAID level, e. g. RAID 10.
Schematics
By Hardware
Characteristics:
requires a specific controller.
guarantees the best performance.
but is more expensive.
may require the installation of drivers.
Data acquisition:
it’s easier to acquire at the logical level as if it were a single disc.
acquisition OS must support RAID controllers.
individual acquisition of RAID discs:
only when the OS doesn’t support RAID controllers.
analysis is more complex – the RAID volume must be rebuilt.
By Software
Characteristics:
implemented in the OS (supported by most modern OS).
less efficient, depending on the CPU to calculate the parity bits data splits.
Windows – Logical Disk Management (LDM).
requires dynamic volumes.
RAID volume configuration is stored on each drive.
supports RAID 0, 1, and 5.
Linux
uses Logical Volume Manager.
saves metadata of the volume inside the drives.
uses volumes on DOS partitions.
supports RAID 0, 1, 5 and 6.
supports Windows LDM (use of Logical Disk Manager may require kernel recompilation).
allows the creation of snapshots – records only the changes and can be reverted to a previous state.
Acquisition:
it’s easier to acquire at the logical level as if it were a single disc.
individual acquisition of RAID discs:
it’s easier than hardware RAID systems.
there are some tools to automatically rebuild the RAID volume.
Windows – with this OS a write blocker must be used.
Linux – it’s possible to do a read-only mount and it also supports LDM from Windows OS.
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