Intel SSD DC S3500 Series Workload
Characterization in RAID Configurations
White Paper
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
Contents
2.0
3.0
Supporting Documentation................................................................................... 5
About This Guide .................................................................................................. 5
Overview.............................................................................................................. 5
6.3
RAID 5 Performance Consistency...................................................................16
Summary............................................................................................................ 18
Appendix............................................................................................................ 19
RAID Levels................................................................................................19
December 2013
329903-001US
White Paper
3
Intel SSD DC S3500 Workload Characterization in RAID Configurations
Tables
Table 1
Typical Mixed Workloads in Data Center Applications....................................... 7
Figures
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
RAID 1 Random 100% Write @ 4KB Transfer Size with Average Latency............11
RAID 1 Random 70% Read @ 4KB Transfer Size with Average Latency .............11
RAID 1 Random 90% Read @ 4KB Transfer Size with Average Latency .............12
RAID 1 Random 100% Read @ 4KB Transfer Size with Average Latency ...........12
RAID 1 Maximum Latency for 2-drive and 8-drive Configurations ......................13
RAID 5 Random 100% Write @ 4KB Transfer Size with Average Latency ...........15
RAID 5 Random 70% Read @ 4KB Transfer Size with Average Latency .............15
RAID 5 Random 90% Read @ 4KB Transfer Size with Average Latency .............16
RAID 5 Random 100% Read @ 4KB Transfer Size with Average Latency ...........16
Figure 10 RAID 5 Maximum Latency for 3-drive and 8-drive Configurations ......................17
White Paper
4
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
1.0 Revision History
Document
Number
Revision
Number
Description
Revision Date
December 2013
Initial release
329903
001
2.0 Supporting Documentation
For more information on Intel SSDs, see the corresponding documentation.
Document
Document No./Location
328860
Intel® Solid-State Drive DC S3500 Series Product Specification
3.0 About This Guide
This guide describes Intel® SSD DC S3500 Series performance characteristics in RAID
configurations across multiple workloads, and provides analysis to help optimize
performance.
The audience is technical IT professionals: Systems, Storage, Database, and Application
Engineers.
4.0 Overview
The Intel SSD DC S3500 Series provides high random read and write storage
Input/Output Operations per Second (IOPS) across mixed read and write workloads. This
high random performance and the consistency of IOPS under workload deliver robust
and scalable operation when used behind a RAID controller. Data centers can benefit in
both performance and TCO by using the Intel SSD DC S3500 Series in the appropriate
applications.
Compared to the approximately 200-300 random IOPS that a single 15K SAS hard disk
drive (HDD) can provide, an Intel SSD DC S3500 Series operates at much higher IOPS;
up to 75,000 IOPS for random 4KB reads and up to 11,500 IOPS for random 4KB writes,
over the entire span of the SSD. The Intel SSD performance numbers are based on the
Intel product specification sheet, as derived from internal Intel testing. With real-world
workloads, the IOPS that any particular device can produce will vary depending on
several factors: the application’s ability to produce IOPS, the ratio of random to
sequential access, the block transfer size, the queue depth, the read/write mix of the
workload, and overall resource utilization in the server running the workload.
This guide presents data for RAID 1 and RAID 5 configurations due to their prominence
in the datacenter. Additional RAID levels are currently being tested and will be
presented in future revisions, or as separate papers.
December 2013
329903-001US
White Paper
5
Intel SSD DC S3500 Workload Characterization in RAID Configurations
A selection of workloads that represent both best-case performance and real-world
performance are presented in this white paper. These scenarios give the IT professional
a better understanding of the capabilities of the Intel® SSD DC S3500 Series drive when
used in conjunction with a hardware RAID controller. More importantly, it helps the IT
professional understand a variety of workloads and circumstances in which Intel SSD
technologies will accelerate those workloads and provide business value for their
organization.
4.1 What Impacts SSD IO Performance
Although Intel SSDs excel in delivering random read and write IOPS, it is important to
remember that more IO activity at the application level results in higher CPU utilization
in the applications’ host. In addition to the abilities of the SSD, IO performance in any
particular situation is dictated by how the particular application scales, and the IO profile
of the workload produced by the application.
The following workload-specific characteristics have a direct impact on the ability of the
SSD to produce IO:
•
•
Read/Write Mix – NAND programming (writes) and read timing (reads) differ
significantly at the hardware level. Because of the higher controller overhead
required for processing writes, the number of read IOPS are often higher than
write IOPS. Real world workloads are most often a mix of read and write.
Random/Sequential Mix – IOPS can vary depending on the ratio of sequential
versus random accesses. With higher random write workloads, more data
movement and greater data management activity occurs in the drive. As random
write activity increases, the IOPS serviceable to the host typically decreases.
•
•
Queue Depth - Higher queue depths typically allow the SSD to generate higher
IOPS through concurrent processing of commands. However, as the queue size
increases, latency will be negatively impacted.
Random Transfer/Block Size - With a smaller transfer size, the SSD controller has
to work harder to maintain the logical-to-physical address mappings. In addition,
the smaller the transfer size, the larger the logical space needed for its mapping.
Once logical space constraint is reached, background re-mapping will take place.
These frequent events slow IOPS.
•
Available Spare Area – A larger spare area directly impacts random write and
mixed read/write performance by minimizing the frequency of reclaim activities
and freeing up processor cycles to support more host read/write requests. You can
increase the spare area by over-provisioning the SSD. See the Intel® High
Performance SATA Solid-State Drive Over-Provisioning an Intel® SSD White Paper
for more information.
In summary, the following principles of storage are often true concerning queue depth,
block size, randomness, and per-IO transactional latency:
•
•
As queue depth increases, IOPS increase, and latency increases.
As block size increases, throughput increases, and latency increases.
•
As randomness increases, IOPS decreases, and latency increases.
White Paper
6
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
4.2 Queue Depth and Latency
Latency – The amount of time needed to service one outstanding IO to the drive,
measured in milliseconds (ms) or, with SSDs, microseconds (µs).
The Intel® SSD DC S3500 Series supports a maximum queue depth of 32 per drive. In a
RAID array, the queue depth is multiplied by the number of drives in the RAID set.
Example: In a RAID 5 set of 8 drives, the maximum total queue depth would be 256 (8
X 32). As more commands are queued in the SSD, average latency is impacted. Our
internal testing indicates that average latency increases sharply with queue depths
beyond 8. However, these high queue depths can increase IOPS with read intensive
workloads.
Obtaining the best performance for a particular application requires balance. The
challenge is to achieve high speed or IOPS at an acceptable latency level. This white
paper presents lower queue depths of 1, 2, 4 and 8 per drive. The results shown
demonstrate favorable speed and IOPS generation without pushing latency to extreme
levels.
4.3 Why Mixed Workload Is Important
Mixed random workloads are predominant in data center and enterprise applications.
Intel SSDs have been deployed in a variety of these applications ranging from content
delivery and video on demand networks, to Internet datacenter portals and database
management servers. Although these applications see unique IO traffic across the
storage drive, there are commonalities in their usage of random and read/write mixed
workloads.
Table 1 shows an overview of transfer sizes, read/write mixes and randomness in
commonly used workloads in data center applications. These are based on commonly
available industry information and information available through such benchmarks as,
TPC-C, TPC-E, TPC-H, and TPOX, which attempt to mimic these real world applications.
Table 1.
Typical Mixed Workloads in Data Center Applications
Application
Transfer Size
4KB/8KB/16KB+
4KB
%Random
~75%
%Read
~95%
~70%
~70%
~95%
~95%
~95%
~95%
Web-Servers
Exchange Email
Database OLTP
Decision Support
Video On Demand
Search Engine
~95%
~95%
~95%
~95%
~95%
~95%
4KB/8KB
16KB+
16KB+
4KB/8KB/16KB
16KB+
Cache
Content Delivery Network
16KB+
~95%
~70%-95%
Based on these usage trends, small transfer sizes–ranging from 4KB to 16KB and
above–are common in enterprise and data center applications. Also, much emphasis is
placed on random accesses, and although there are varied levels of read and write
December 2013
329903-001US
White Paper
7
Intel SSD DC S3500 Workload Characterization in RAID Configurations
mixes, read-intensive workloads are more prominent. In summary, it is important to
select the proper SSD for a particular workload.
The examples presented here use 100% write and 100% read workloads to show the
maximum performance in these areas. Also, 70%/30% read/write and 90%/10%
read/write are used in order to simulate typical workloads in the datacenter.
4.4 Drive Endurance
Drive endurance, or wear, is an important consideration when selecting an SSD for a
particular application. The Intel® SSD DC S3500 Series drive is a standard endurance,
enterprise class drive, designed for read-heavy workloads. It is important to understand
how drive wear is affected by the RAID level.
A RAID level that uses dedicated parity, such as RAID 4, will write all parity to a single
drive. This can potentially cause the parity drive to wear faster than the other drives in
the set. Distributed parity RAID levels (RAID 5 and RAID 6) reduce this issue.
RAID 1 and RAID 5, as tested in this example, shows very consistent wear across all
drives in the RAID sets. This is due to the tests using the full LBA space of the RAID set,
thereby not creating any hotspot activity.
4.5 Selection of RAID Controller
There are many quality RAID controllers on the market today, with varying levels of
performance, features and price points. Below are the important features considered in
selecting the RAID controller for this sample test:
•
•
•
•
•
•
RAID levels available
Controller chipset
PCIe* version
SAS/SATA speed
Internal/External ports
Compatibility with SSDs
Within these categories, the LSI* MegaRAID 9265-8i was chosen for the following
reasons:
•
•
•
•
•
•
•
•
RAID 0, 1, 5, 6, 10, 50, 60 are supported
LSISAS2208 Dual-Core RAID on Chip (ROC)
1 GB 1333MHz DDR3 SDRAM Cache
x8 PCIe 2.0
6Gb/s per port
8 internal SAS ports
SSD support
Relative availability/popularity in the industry
White Paper
8
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
5.0 RAID 1
5.1 Test System Specifications¹
The system used for RAID 1 testing include the following:
•
•
•
•
•
•
•
•
Intel® R2208GZ4GC-IDD 2U rack mount server
Intel® S2600GZ server board
2x Intel® Xeon® E5-2690 8-core CPUs (2.9 GHz)
Intel® C602 chipset
192GB DDR3-1333 memory
Microsoft Windows Server 2008 R2*, 64-bit
LSI MegaRAID 9265-8i* controller card
2x up to 8x Intel® SSD DC S3500 Series 480GB drives
BIOS configuration changes:
Hyper-Threading disabled²
•
RAID controller configuration:
•
•
•
•
256KB Striping (default)
No Read-Ahead³
Write-Through³
Direct I/O
Windows Drive Configuration:
•
•
•
•
•
Basic disk
GUID partition table
Simple volume
Use full available space
NTFS format
Test Software configuration:
•
•
IOMeter 2009.10.22
1x worker per drive in RAID set⁴
Notes:
1.
The system was selected to make sure the performance of the RAID card and the SSDs would not
be inhibited by the server.
2.
3.
Hyper-Threading is disabled in this test system specifically due to additional latency introduced
during benchmark testing. In any practical application, Hyper-Threading would NOT be disabled.
In the configuration of the RAID set, No Read Ahead and Write-through are used due to the speed
of the SSDs. Read and write caching was designed for use with HDDs. Caching with SSDs introduces
additional overhead thus interfering with the SSDs performance.
4.
One thread, or worker, per drive was used in order to simulate the manner in which many
applications utilize storage and also to attempt to saturate the communication channels to the SSDs.
December 2013
329903-001US
White Paper
9
Intel SSD DC S3500 Workload Characterization in RAID Configurations
5.2 Intel® SSD DC S3500 Series in RAID 1 Performance
Characterization Data
This section provides performance characterization data for the Intel® SSD DC S3500
Series in RAID 1 configurations.
To establish baseline expectations for IOPS, the Intel SSD DC S3500 Series 480GB
drives were evaluated in RAID 1 sets of 2, 4, 6 & 8 drives. The data collected was based
on a different mix of read and write random and sequential workloads. Since higher
queue depths can sometimes yield higher IOPS, queue depths of 1, 2, 4, & 8 per drive
were chosen in the test setup. Multiple transfer sizes were tested, however, only
selected data is presented here. All tests were done using the entire LBA range of the
virtual drive. Tests were repeated at least twice to validate results.
Drives were prepared using IOMeter to fill the entire user area of the drive with data.
Then, the first workload of each type (Sequential or Random) was 100% write
performed for 120 minutes. Each subsequent workload was run for 12 minutes, with
average IOPS collected over the last 10 minutes of the run.
The following figures show the different levels of performance for a selection of
configurations.
Note: The scale of the IOPS charts is variable to clarify the changes that occur as drives are
added.
White Paper
10
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
Figure 1.
RAID 1 Random 100% Write @ 4KB Transfer Size with
Average Latency
Intel internal testing, October 2013
Notes:
Figure 1 - The write performance of the two drive RAID 1 set matches the write performance of a
single Intel® DC S3500 drive. This indicates very low latency introduced by the RAID controller.
As more drives are added, the write performance scales linearly. At four drives, the performance is
2x that of a single drive, at six drives, it is 3x higher and at eight drives it is 4x. This is true at all
queue depths tested and at transfer sizes from 4KB to 128KB. In this case, queue depth does not
affect performance significantly.
At a queue depth of 1, the average latency for 100% write at 4KB transfer size is less than 200 µs.
Latency increases as the queue deepens, ending at 1.4ms for a queue of 8. It is interesting to note
that latency is not affected by the number of drives.
Figure 2.
RAID 1 Random 70% Read @ 4KB Transfer Size with
Average Latency
Intel internal testing, October 2013
December 2013
329903-001US
White Paper
11
Intel SSD DC S3500 Workload Characterization in RAID Configurations
Figure 3.
RAID 1 Random 90% Read @ 4KB Transfer Size with
Average Latency
Intel internal testing, October 2013
Figure 4.
RAID 1 Random 100% Read @ 4KB transfer size with
Average Latency
Intel internal testing, October 2013
Notes:
Figures 2, 3 - In mixed workloads, 70% read and 90% read, IOPS increase with additional drives
and show slightly exponential growth with deeper queues.
Figures 2, 3, 4 - Average latency for 70% read starts out similar to 100% write, but the progression
is not as steep through deeper queues, ending between 500-600 µS. Average latency for 90% read
and 100% read continue to improve due to the higher speed of reads over writes.
Figures 2, 3, 4 - The latency of the two drive set is lower than other drive counts as read
percentage increases due to the manner in which the LSI controller deals with the additional drives.
This is expected behavior. For more information, please contact LSI for details.
Figure 4 - At 100% read the IOPS performance scales linearly. In other words, the IOPS for four
drives is double that of two drives, six drives is triple that of two drives, and eight drives is four
times that of two drives.
White Paper
12
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
5.3 RAID 1 Consistency
Consistency behind a RAID controller is very important because the performance of any
RAID set is limited by the lowest performing drive. As a RAID set increases in number of
drives, the probability of any given drive performing poorly increases. Therefore, if the
model of drive used is inconsistent in its performance, the inconsistency increases with
the size of the RAID set.
The Intel® SSD DC S3500 Series drive has shown excellent consistency when used as a
single drive. Figure 5 illustrates the consistent of DC S3500 in RAID sets. Notice that the
maximum latency is grouped very tightly for both the two-drive and eight-drive RAID
sets, indicating there is very little change in consistency as more SSDs are added.
Figure 5.
RAID 1 Maximum Latency for 2-drive and 8-drive
Configurations
Intel internal testing, October 2013
5.4 RAID 1 Performance Conclusions
RAID 1 is a very good choice for data needing robust replication. The RAID controller
used shows good bandwidth with low latency causing little to no effect on read and write
speeds of the SSDs. The linear scaling of read and write performance with additional
drives shows that adding more drives would provide good ROI in most applications. The
highest throughput seen in this test was 2300 MB/s during 100% read using eight drives
with transfer size of 128KB and a queue of 8 per drive. This means the theoretical
bandwidth limit of the PCIe lanes was not reached (4000 MB/s for x8 PCIe 2.0). It is
theoretically possible that more than eight drives could be used and obtain an increase
in performance, depending on the latency introduced by the necessary SAS expander.
The consistency of the drives is well demonstrated in these tests and shows that
Intel SSD DC S3500 Series drives provide high performance with excellent stability,
even behind a RAID controller.
December 2013
329903-001US
White Paper
13
Intel SSD DC S3500 Workload Characterization in RAID Configurations
6.0 RAID 5
6.1 Test System Specifications
The system used for RAID 5 testing was identical to the system used for RAID 1 testing
except the following changes:
• 2x Intel Xeon E5-2680 8-core CPUs (2.7 GHz)
• 3x up to 8x Intel SSD DC S3500 Series 800GB drives
Note:
For this test, 800GB drives were used. The rated performance of the Intel SSD DC S3500
Series drive in 800GB, 600GB, and 480GB capacities are nearly identical, per internal Intel testing.
6.2 Intel SSD DC S3500 Series in RAID 5 Performance
Characterization Data
This section provides performance characterization data for the Intel SSD DC S3500
Series in RAID 5 configurations.
To establish baseline expectations for IOPS, the Intel SSD DC S3500 Series 800 GB
drives were evaluated in RAID 5 sets of 3, 4, 5, 6, 7 and 8 drives. The data collected
was based on a different mix of read and write random and sequential workloads. Since
higher queue depths can sometimes yield higher IOPS, queue depths of 1, 2, 4, and 8
per drive were used in the test setup. Multiple transfer sizes were tested, however, only
selected data is presented here. All tests were done using the entire Logical Block
Address (LBA) range of the virtual drive. Tests were repeated at least twice to validate
results.
Drives were prepared using IOMeter to fill the entire user area of the drive with data.
Then, the first workload of each type (Sequential or Random) was 100% write
performed for 120 minutes. Each subsequent workload was run for 12 minutes, with
average IOPS collected over the final 10 minutes of the run.
The following figures show different levels of performance for a selection of
configurations.
Note:
The scale of the IOPS charts is variable in order to clearly show the change as drives are added.
White Paper
14
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
Figure 6.
RAID 5 Random 100% Write @ 4KB Transfer Size with
Average Latency
Intel internal testing, October 2013
NOTES: There are gains in write performance as drives are added to the RAID 5 set. The change at queue
depth 1 from three drives to six drives is approximately 58% increase in IOPS. For eight drives,
the change is 97% increase in IOPS over the three drive set.
At a queue depth of 1, latency increases as more drives are added to the RAID set, most likely
caused by the additional overhead of calculating parity and striping across more drives. This
increases as the queue deepens.
Figure 7.
RAID 5 Random 70% Read @ 4KB Transfer Size with
Average Latency
Intel internal testing, October 2013
December 2013
329903-001US
White Paper
15
Intel SSD DC S3500 Workload Characterization in RAID Configurations
Figure 8.
RAID 5 Random 90% Read @ 4KB Transfer Size with
Average Latency
Intel internal testing, October 2013
Figure 9.
RAID 5 Random 100% Read @ 4KB Transfer Size with
Average Latency
Intel internal testing, October 2013
Notes:
Figures 7, 8, 9 - As the workloads become more read intensive, there is a steady increase in
performance both as drives are added and as the queue deepens.
Figures 7, 8, 9 – As read percentage increases, the exponential increase in latency is not as
prominent with deeper queues. This is due to the speed at which reads are performed.
6.3 RAID 5 Consistency
Consistency behind a RAID controller is very important because the performance of any
RAID set is limited by the lowest performing drive. As the number of different drives in a
RAID set increases, so does the likelihood of any given drive performing poorly.
Therefore, if the model of drive used is inconsistent in its performance, the inconsistency
increases with the size of the RAID set.
The Intel® SSD DC S3500 Series drive has shown excellent consistency when used as a
single drive. Figure 10 illustrates that the DC S3500 is also very consistent in RAID sets.
The three-drive and eight-drive data shows the maximum latency is grouped very
tightly; indicating that adding more drives would have little impact on consistency.
White Paper
16
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
Figure 10. RAID 5 Maximum Latency for 3-drive and 8-drive
Configurations
Intel internal testing, October 2013
6.4 RAID 5 Performance Conclusions
The RAID 5 write performance data illustrates the additional processing power required
of the RAID controller to calculate parity and stripe data across multiple drives. There is
diminished performance gain be adding drives when compared to RAID 1. Intel’s data
also shows that in mixed workloads and in pure reads, RAID 5 performs well, reaching
over 300K IOPS in 100% read at a queue of 8 per drive on eight drives. As the
workloads become more read heavy, latency drops from a high of 2.2 ms (100% write)
to a low of 140 µs (100% read). The highest throughput achieved was 2400 MB/s with
eight drives, 100% read, 128KB transfer size, and queue depth of 8. This leaves room
for possible improvement by adding more drives to the RAID set.
In configurations where RAID 5 would traditionally be used, SSDs would provide
significant performance gain over HDDs. Additionally, RAID 5 with SSDs could be used in
situations where RAID 5 with HDDs would not perform well.
The consistency of the drives is well demonstrated in these tests and shows that
Intel® SSD DC S3500 Series drives consistently offer higher performance with excellent
stability, even behind a RAID controller.
December 2013
329903-001US
White Paper
17
Intel SSD DC S3500 Workload Characterization in RAID Configurations
7.0 Summary
The Intel® SSD DC S3500 Series drive has proven itself in many applications where
speed and reliability are essential. The data in this paper shows that this drive is very
robust behind a RAID controller.
In RAID 1 configurations, write performance is as expected for this setup; that is, a two
drive set matches the single drive specification, and increases linearly as drives are
added to the set. With eight drives at queue depth 1 per drive, this configuration
processes over 50K write IOPS with 4KB blocks. The RAID controller adds very little
latency, but latency does increase as the queue depth grows and, as more drives are
added to the array. Read performance increases with additional drives and with queue
depth, reaching over 200K read IOPS with eight drives, a queue of 8 per drive and 4KB
blocks. More importantly, the performance on mixed workloads was excellent and
increased as more drives were added. The 70% read workload topped out at close to
110K IOPS with 4KB blocks, and the 90% went to nearly 150K IOPS with 4KB blocks
(both at queue of 8 per drive). The highest throughput seen in this test was 2300 MB/s
during 100% read using eight drives with transfer size of 128KB and a queue of 8 per
drive. This means the bandwidth limit of the PCIe lanes was not reached (4000 MB/s for
x8 PCIe 2.0). It is theoretically possible that more than eight drives could be used and
an increase in performance obtained.
In RAID 5 configurations, write operations increase both as drives are added and as the
queue depth increases. The increase in write performance with queue depth is somewhat
surprising and is most likely attributed to the RAID controller and the scaling effect of
the cache in each drive. This may be due to the way the controller writes the stripes to
the drive set, possibly consolidating the 4KB blocks into the 256KB stripes. Read
performance is also very good, with eight drives reaching 300K IOPS with 4KB blocks
and queue depth 8 per drive. Mixed workloads show very good performance with 70%
read hitting 75K IOPS at queue of 8 per drive and 90% read coming in at almost 140K,
both with 4KB blocks. Latency on all workloads is very manageable, although, as the
queue depth increases, so does the latency. The graphs show that as queues grow,
latency increases at an increasingly higher rate.
To summarize:
•
•
•
•
In both RAID 1 and RAID 5, the Intel SSD DC S3500 Series drive shows
excellent scalability, performance, and consistency.
Very little latency was introduced by the RAID controller in RAID 1. In RAID 5,
the overhead and latency are slightly higher.
In random, mixed read/write workloads, SSDs perform significantly (as much as
100 times) better than HDDs in a similar situation.
With this RAID controller, there is the possibility of greater performance by
adding more than eight drives in both RAID 1 and RAID 5 configurations.
White Paper
18
December 2013
329903-001US
Intel SSD DC S3500 Workload Characterization in RAID Configurations
8.0 Appendix
8.1 RAID Levels
RAID (Redundant Array of Independent Disks), developed in 1988 to improve
performance, reliability and scalability of hard disk storage systems has become a
standard in datacenters because of these qualities. There are many types, or levels, of
RAID.
RAID 0 uses block level striping to span one or more drives. This does improve
performance, and increases capacity when more than one drive is used. However, there
is no fault tolerance, so failure of any one drive will cause full data loss.
RAID 1, also called mirroring, writes data identically to two drives, producing a mirrored
set. Reads can be serviced by either drive, and writes occur in unison on both drives. If
one drive has a hardware failure, the data is protected in the mirrored copy. RAID 1
requires two drives. Many modern RAID controllers support RAID 1 sets of more than
two drives, however, the original specification was for only two. Because of the 50%
overhead, RAID 1 is the most expensive RAID type.
RAID 2 uses bit-level striping with dedicated Hamming-code parity. This is a theoretical
model and not used in practice.
RAID3 uses byte-level striping with dedicated parity. This level is not commonly used.
RAID 4 uses block-level striping with dedicated parity. All parity data is on a single drive.
I/O requests are handled in parallel, increasing performance.
RAID 5 uses block-level striping with distributed parity. Data and parity are distributed
among all drives and requires all but one drive to be present. RAID 5 requires at least
three drives and can survive a single drive failure.
RAID 6 uses block-level striping with double distributed parity. Identical to RAID 5 in the
way it writes data, however, parity is written twice in different locations. RAID 6 can
survive 2 drive failures; therefore it is often used for larger sets of drives.
RAID levels can also be nested for improved performance or fault tolerance. RAID 10,
0+1, 50 and 60 are common combinations.
December 2013
329903-001US
White Paper
19
|