RAID Server Recovery
RAID 0
Also known as a striped set, RAID 0 splits data evenly across two or more disks
with no parity information for redundancy. We can recover data from these striped
sets. Regardless of the problem, if you have suffered a drive failure, controller
failure, or file system corruption, we can recover data from your RAID 0 array.
Many customers are utilizing this technology and they don't even realize it. RAID
0 is commonly used in 500GB+ external drives. Some of the most common of these that
we get in are LaCie Big Disk and Maxtor One Touch drives. It should be noted that
for any RAID 0 recovery to be successful, ALL drives must be accessible. If one
drive has physically failed, then we must first get that drive funtional again so
that we can image and destripe the set. If we cannot image all of the drives within
the array then data corruption will be prevalent.
It is important to note that RAID 0 was not one of the original RAID levels, and
is not redundant. RAID 0 is normally used to increase performance, although it can
also be used as a way to create a small number of large virtual disks out of a large
number of small physical ones. A RAID 0 can be created with disks of differing sizes,
but the storage space added to the array by each disk is limited to the size of
the smallest disk—for example, if a 120 GB disk is striped together with a 100 GB
disk, the size of the array will be 200 GB. Although RAID 0 was not specified in
the original RAID paper, an idealized implementation of RAID 0 would split I/O operations
into equal-sized blocks and spread them evenly across two disks. RAID 0 implementations
with more than two disks are also possible, however the reliability of a given RAID
0 set is equal to the average reliability of each disk divided by the number of
disks in the set. That is, reliability (as measured by mean time to failure (MTTF)
or mean time between failures (MTBF)) is roughly inversely proportional to the number
of members—so a set of two disks is roughly half as reliable as a single disk. The
reason for this is that the file system is distributed across all disks. When a
drive fails the file system cannot cope with such a large loss of data and coherency
since the data is "striped" across all drives.
While the block size can technically be as small as a byte it is almost always a
multiple of the hard disk sector size of 512 bytes. This lets each drive seek independently
when randomly reading or writing data on the disk. If all the accessed sectors are
entirely on one disk then the apparent seek time would be the same as a single disk.
If the accessed sectors are spread evenly among the disks then the apparent seek
time would be reduced by half for two disks, by two-thirds for three disks, etc.,
assuming identical disks. For normal data access patterns the apparent seek time
of the array would be between these two extremes. The transfer speed of the array
will be the transfer speed of all the disks added together.
RAID 0 is useful for setups such as large read-only NFS servers where mounting many
disks is time-consuming or impossible and redundancy is irrelevant. Another use
is where the number of disks is limited by the operating system. In Microsoft Windows,
the number of drive letters for hard disk drives may be limited to 24, so RAID 0
is a popular way to use more disks. It is also a popular choice for gaming systems
where performance is desired, data integrity is not very important, but cost is
a consideration to most users. However, since data is shared between drives without
redundancy, hard drives cannot be swapped out as all disks are dependent upon each
other.
RAID 1
RAID 1 creates an exact copy (or mirror) of a set of data on two or more disks.
This is useful when write performance is more important than minimizing the storage
capacity used for redundancy. This is thought to be a foolproof method of data protection,
but we commonly receive RAID 1 arrays that have failed due to:
corrupted mirrors
bad data from one drive moves to the other drive
mirror breaks, and does not allow system to boot
improper rebuild
The array can only be as big as the smallest member disk, however. A classic RAID
1 mirrored pair contains two disks, which increases reliability by a factor of two
over a single disk, but it is possible to have many more than two copies. Since
each member can be addressed independently if the other fails, reliability is a
linear multiple of the number of members. To truly get the full redundancy benefits
of RAID 1, independent disk controllers are recommended, one for each disk. Some
refer to this practice as splitting or duplexing.
When reading, both disks can be accessed independently. Like RAID 0 the average
seek time is reduced by half when randomly reading but because each disk has the
exact same data the requested sectors can always be split evenly between the disks
and the seek time remains low. The transfer rate would also be doubled. For three
disks the seek time would be a third and the transfer rate would be tripled. The
only limit is how many disks can be connected to the controller and its maximum
transfer speed. Many older IDE RAID 1 cards read from one disk in the pair, so their
read performance is that of a single disk. Some older RAID 1 implementations would
also read both disks simultaneously and compare the data to catch errors. The error
detection and correction on modern disks makes this less useful in environments
requiring normal commercial availability. When writing, the array performs like
a single disk as all mirrors must be written with the data.
RAID 1 has many administrative advantages. For instance, in some 365*24 environments,
it is possible to "Split the Mirror": declare one disk as inactive, do
a backup of that disk, and then "rebuild" the mirror. This requires that
the application support recovery from the image of data on the disk at the point
of the mirror split. This procedure is less critical in the presence of the "snapshot"
feature of some filesystems, in which some space is reserved for changes, presenting
a static point-in-time view of the filesystem. Alternatively, a set of disks can
be kept in much the same way as traditional backup tapes are.
RAID 5
When a RAID 5 fails, many of our customers come to us desperate for help. In most
instances, a person's job or business can be on the line when this type of data
loss occurs. Many corporate executives don't understand why there wasn't a backup
of the data prior to the crash, and that can be a hard thing to explain. We understand
how difficult this time can be, and we work with you every step of the way in order
to get your data back as quickly and efficiently as possible.
One of the biggest concerns we have from potential customers is: "will your
services make matters worse?" The answer is a resounding NO. We start off with
an initial evaluation just to determine the integrity of the drives, and confirm
they are functioning as they should. This usually consists of just a quick power
up to verify there is no unusual noises. If the customer has specified that a drive
is making unusual noises, then we do not risk powering it up at all. There are a
series of tests we can perform in order to determine how serious the failure is,
without risking further damage.
In situations where there is a physical problem with the drives, then we will proceed
with making whatever repairs are necessary in order to get the drive functioning,
in order that we may be able to image it. While we can technically rebuild the array
in our emulator with one drive missing, rarely have we seen this work effectively
enough to garner the best results in the recovery process.
If all of the drives power up and appear to be functioning properly, then we start
off by creating an image of each drive in the array. These are sector-by-sector
clones. From there we analyze the strip, and determine a number of parameters associated
with the array that will allow us to recover as much data as possible.
Data Recovery Services Offered:
