| RAID what it means?
RAID - Using various methods of data protection an array of disks
can continue operation after one or more members fail. How the
redundency is provided and how many extra device are needed to
provide the redundency are among the things that define the
RAID levesl.
RAID-0 - Disk striping. Data is interleaved among the member
disks. Striping is often called RAID-0 because there is no
redundency provided. If you lose a disk you lose the array.
The advantages are low cost (no extra disks) and performance.
RAID-1 - Disk Shadowing. The data is duplicated among two or
devices in the array. Implementations will typically optimize
reads by distributing the read load among the members. Writes
have to be duplicated to all the members and all write must
complete before the write to the array is considered complete.
This provides good read performance and write performance that
is likely to slightly worse than single disk. In hardware this
write penalty can be hidden by the controller. The cost is the
duplication of disks and possibly the duplication of paths to
the disk (extra adapters, housings, etc).
RAID-2 - Data is interleaved among many disks at the bit, byte
or other small size. A single 512 byte sector will be spread
across all the data members of the array. I think a Hamming
code is used to encode the redundent information on other disks
of the array. If a single disks fails the other disks can be
read to reconstruct the missing data. Writes have to update all
the disks.
RAID-3 - Like RAID-2 a sector is spread across all members. In
this case a single disk is the protection disk and it contains
the XOR of the other members. Some RAID-3 systems interleave
at the sector level of the members and then require the array
present a larger "sector". For example a 4+1 array (4
data members,
1 XOR) may present a 2048 sector size to the host.
RAID-2 and RAID-3 can provide exceptional sequential I/O performance
because all data members of the array participate in the transfer.
If the host to controller data path will support the bandwidth,
the
bandwidth of the array is the sum of that of the members (or close).
If the I/O load is small random transfers, then it may be little
better than a single disk.
RAID-{4,5,6} - Data is interleaved in chunks larger than the minimum
transfer size. For reads a transfer may touch from one to all the
members. If the interleave size is correctly chosen for the I/O
load these levels can provide good random I/O access for multiple
I/O streams; each member can satisfy a different I/O request.
For writes and reads when the array is reduced (one member broken)
these three level offer quite different performance due to the
way the redundency is provided.
RAID-4 - One disk in the array has all the XOR data on it. Writes
often have to be performed as a read-modify-write operation (read
the old data that won't be touched by the transfer, calculate the
XOR with this and the new data, write the new and new XOR). Since
the XOR disk has the particpate in EVERY transfer, it becomes the
bottleneck unless you make it VERY fast.
When the array is reduced the XOR disk again becomes the bottleneck
because it accessed by every transfer.
RAID-5 - The XOR is spread among all the disks. This allows multiple
writes and reduced reads to be satisfied by the array at the same
time when if different data and XOR members are involved. The
schemes can be used to distribute the parity among the members.
RAID-6 - Multiple XOR members are used. This allows the array to
remain redundent when a single member is lost and it can will
regenerate data when two members are lost.
RAID-7 - This is marketing name used by a particular company. If
one distinguishes the different RAID levels by the topological
arrangment of data, XOR and chunk size, then they claim to have
a fast RAID-4. They will claim that the distinguishing feature
of the different RAID levels is by performance. What really
matters is whether their product provides you the required
performance, data availability and reliability at the price
you are willing to pay.
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