Storage RAID
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The objective of this article is to share the knowledge on RAID Technology and how the data will be written or restored when a disk fails. To know about components of a storage system environment, refer the link below
http://www.sanadmin.net/2015/10/storage-solutions.html
History of RAID:
In the late 1980’s, rapid growth of new applications and databases
created a high demand for storage capacity. At that time, data was stored
on a single large, expensive disk drive called Single Large Expensive Drive
(SLED).
Types of RAID:
There are two types of
RAID implementation, hardware and software. Both have their merits and demerits
and are discussed in this section.
Software RAID
Software RAID uses host-based software to provide RAID functions.
It is implemented at the operating-system level and does not use a dedicated
hardware controller to manage the RAID array.
Hardware RAID
A specialized hardware controller is implemented either on the
host or on the array. These implementations vary in the way the storage
array interacts with the host.
RAID Array Components
A RAID array is an enclosure that contains a number of HDDs and
the supporting hardware and software to implement RAID. HDDs inside a RAID
array are usually contained in smaller sub-enclosures. These sub-enclosures, or
physical arrays, hold a fixed number of HDDs, and may also include other
supporting hardware, such as power supplies. A subset of disks within a RAID
array can be grouped to form logical associations called logical arrays, also
known as a RAID set or a RAID group.
Logical arrays are comprised of logical volumes (LV). The
operating system recognizes the LVs as if they are physical HDDs managed by the
RAID controller. The number of HDDs in a logical array depends on the RAID
level used.
Components of a Raid Array |
Raid Levels
RAID levels are defined on the basis of striping, mirroring, and
parity techniques. These techniques determine the data availability and
performance characteristics of an array.
RAID 0: RAID 0 is also known as disk striping. All the data
is spread out in chunks across all the disks in the RAID set. RAID 0 is only
good for better performance, and not for high availability, since parity is not
generated for RAID 0 disks. RAID 0 requires at least two physical disks.
Raid 0 Striping |
Raid 1: RAID 1 is also known as disk
mirroring. All the data is written to at least two separate physical disks. The
disks are essentially mirror images of each other. If one of the disks fails,
the other can be used to retrieve data. Disk mirroring is good for very fast
read operations. It's slower when writing to the disks, since the data needs to
be written twice. RAID 1 requires at least two physical disks.
Raid 1 Mirroring |
RAID 5: RAID 5 uses disk striping with parity. The data
is striped across all the disks in the RAID set; it achieves a good balance
between performance and availability. RAID 5 requires at least three physical
disks.
Raid 5 Striping with single parity |
RAID 6: RAID 6 increases reliability by utilizing two parity
stripes, which allows for
two disk failures within the RAID set before data is lost. RAID 6 is seen in SATA environments,
and solutions that require long data retention periods, such as data archiving or disk-based
backup.
Raid 6 Striping with double parity |
RAID 1+0: RAID 1+0, which is also called RAID 10, uses a
combination of disk mirroring and disk striping. The data is normally mirrored
first and then striped. RAID 1+0 requires a minimum of four physical disks.
Raid 1 0 Stipe + Mirror |
RAID 0+1: RAID 0+1 also called as RAID 01, is a RAID level
using a mirror of stripes, achieving both replication and sharing of data
between disks.
Raid 0 1 Mirror + stripe |
RAID Comparison
Here we will discuss about the comparison between all RAID levels
such as read & write performance and min. disks required to build a Raid
and so on.
Raid comparison chart 1 |
Raid-comparison-Chart-2 |
Disk Details in Raid types |
Mini/Maxi drives in a Raid level |
Capacity utilization in Raid Level |
Application IOPS and RAID Configurations
When deciding the number of disks required for an application, it
is important to consider the impact of RAID based on IOPS generated by the
application. The total disk load should be computed by considering the type of
RAID configuration and the ratio of read compared to write from the host.
The following example illustrates the method of computing the disk
load in different types of RAID.
Consider an application that generates 5,200 IOPS, with 60 percent
of them being reads.
The disk load in RAID 5 is calculated as follows:
RAID 5 disk load = 0.6 × 5,200 + 4 × (0.4 × 5,200) [because the
write penalty for RAID 5 is 4]
= 3,120 + 4 × 2,080
= 3,120 + 8,320
= 11,440 IOPS
The disk load in RAID 1 is calculated as follows:
RAID 1 disk load = 0.6 × 5,200 + 2 × (0.4 × 5,200) [because every
write manifests as two writes to the disks]
= 3,120 + 2 × 2,080
= 3,120 + 4,160
= 7,280 IOPS
The computed disk load determines the number of disks required for
the application. If in this example an HDD with a specification of a maximum
180 IOPS for the application needs to be used, the number of disks required to
meet the workload for the RAID configuration would be as follows:
RAID 5: 11,440 / 180 = 64 disks
RAID 1: 7,280 / 180 = 42 disks (approximated to the nearest even
number)
Hot Spares
A hot spare refers to a spare HDD in a RAID array that temporarily
replaces a failed HDD of a RAID set. A hot spare takes the identity of the
failed HDD in the array.
Hot spares are of 2 types. Permanent and Temporary hot spare.
Permanent Hot Spare: The hot spare replaces the new HDD
permanently. This means that it is no longer a hot spare, and a new hot spare
must be configured on the array.
Temporary Hot Spare:
When a new HDD is added to the system, data from the hot spare is copied to it.
The hot spare returns to its idle state, ready to replace the next failed drive.
To know more details about intelligent storage system, refer the link below
http://www.sanadmin.net/2015/10/fc-storage.html
To know more details about intelligent storage system, refer the link below
http://www.sanadmin.net/2015/10/fc-storage.html
6 comments
commentswell documented
ReplyThanks Kaja. Happy to see your comment.
ReplyInformative post. Thank You!
ReplyReally useful
ReplyVery well explained. Thanks
ReplyExcellent info
Reply