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RAID 5
A RAID 5 uses block-level striping with parity data distributed
across all member disks. Generally, RAID 5 is implemented with
hardware support for parity calculations.
Every
time a block is written to a disk in a RAID 5, a parity block is
generated within the same stripe. A block is often composed of
many consecutive sectors on a disk. A series of blocks (a block
from each of the disks in an array) is collectively called a
"stripe".
A hardware RAID 5 array presents itself to the end user and the
operating system as one large single disk. This disk can be
treated as such and can be divided in a single or multiple
partitions.
Data recovery from an intact array
Damage to logical structures on an intact RAID array
resulting in loss of data can be treated as logical damage on a
single disk. So, if for example a file is deleted from a volume
residing on a RAID 5 array, file recovery can be attempted using conventional
undelete software. Many software vendors advertise their software
to be RAID 5 capable if they can perform data recovery under such
conditions, which is basically a scam because they do not have to
anything at all to achieve this so called compatibility. Even
the simplest undelete software can recover deleted files as long
as an array is intact.
Data recovery from a broken array
Due to the parity information a RAID 5 array can survive one
single disk failing, RAID 5 is 'fault tolerant'. The falling disk
can be replaced (hot swapped) and the data on the disk is rebuild
using the parity information. However due to circumstances this
may fail. It can also happen that the RAID adapter itself fails
and very often it is not possible to migrate an existing array to
another RAID adapter. As a result you have a bunch of disks, all containing
bits of your data but you can not access that data. This is where
you will need true RAID capable data recovery software.
You will need software that can treat the separate disks as one
single array. The software should enable the user to add disks
that were part of the array and to configure RAID parameters such
as stripe size. However many will not be knowledgeable enough to
provide the parameter sets thus ideally the software should be
able to detect RAID parameters such as stripe size and rotation
(for example, in above illustration you see forward rotation,
inverted rotation is also possible).
As soon as the software has virtually recreated the array data
recovery proceeds normally; the disk is being scanned for file
system structures and a virtual file system is created from which
data can be recovered. DIY DataRecovery iRecover
follows this procedure. Alternatively the reconstructed array is
copied entirely to another disk or raw image file. The destination
disk can then be analysed with any data recovery software, an
image can be analyzed with any software that is capable of
processing a raw image file. The latter is the method used by RAID
Reconstructor from Runtime Software (www.runtime.org)
RAID recovery using DIY DataRecovery iRecover
Note 1: As multiple disks have to be scanned the
process can be a lengthy one.
Note 2: This procedure is only required if the array is
damaged. For a functional array select simple volume recovery!
Select 'reconstruct RAID layout, then recover data' in the
Recovery type selection dialog.
In the next step select disks which are members of the array. If
the array is RAID5 with one disk missing, select all available
array members, the missing disk will be reconstructed based on the
RAID5 parity data.
The procedure is fully automated. Expect the following progress
indications:
Checking RAID5
parity: early into the process, iRecover checks to see if it
needs to reconstruct a missing RAID5 member disk. This only
appears if RAID5 was marked as the possible layout.
Scanning disks:
during this stage all the array member disks are scanned and the
fragments of the file system structures and data files identified.
These fragments are then used to reconstruct the layout of the
array.
Reading sequence
numbers: during this stage objects marked as "worth
attention" during scan are read again and processed.
Calculation:
this is displayed when number-crunching is progressing to
determine the stripe size and build tables for the next stage to
use.
Probing [different
layouts]: during this stage, different hypotheses are
tried and their relevance measured.
Once RAID re-construction is completed the session will proceed as if
it is a
'simple disk' recovery session.
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