User`s guide

Because SQL Server is extremely disk-write intensive, performance suffers when SQL is configured with
RAID 5. Understanding the RAID levels can help SQL DBAs configure the disk IO subsystem in the most
efficient manner.
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RAID 0. “Striping Without Parity”. In this configuration, each block of data is written to each disk in
the array in a “round robin” fashion, which means each disk in the array holds only a portion of the
total data written. Depending on the array configuration, this drastically improves read performance
because data can be read in small parallel chunks. This method also provides improved write
performance because data can be written in parallel. However, time is required to break the data into
the “stripe” that will be written. Because no fault-tolerance exists in this model, when a drive fails in the
array, the entire array fails. A minimum of 2 drives is required for RAID 0, and the resulting size of the
array is calculated by adding the sizes of the drives together.
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RAID 1. “Disk Mirroring” or “Disk Duplexing”, which means mirroring on a single channel, duplexing
when multiple channels are used. In this configuration, each bit of data that is written to a single disk is
duplicated on the second disk in the array. RAID 1 is limited to 2 physical disks, which means the array
is capable of increasing the read performance. In a duplexed environment, the performance is
theoretically doubled, while at the same time providing fault tolerance in case a drive fails. Write
performance is not affected by RAID 1. Only 2 drives can participate in a RAID 1 array, and the size of
the array is the same as a single disk.
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RAID 5. “Disk Striping with Parity”. As with RAID 1, data is written to each disk in the array in a
“round robin” fashion, but an additional block of data written as “parity” also exists. This parity
information can be used to rebuild the array in case of a disk failure. RAID 5 is the most popular RAID
configuration in data centers and represents an effective compromise between read performance and
fault tolerance. Because time is required to calculate the parity stripe, write performance is not as good
as RAID 0. A minimum of 3 disks is required for RAID 5. The size of the array is calculated by taking
the added size of the total disks and subtracting the size of 1 disk. For example, 80GB + 80GB + 80GB is
equal to the total array size of 160GB.
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RAID 0+1. “Mirror of Stripes”. In this configuration, 2 RAID 0 arrays are mirrored with RAID 1, which
provides the fast read and write performance of RAID 0 and the fault tolerant features of RAID 1,
which addresses performance first and then fault tolerance.
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RAID 10. “Stripe of Mirrors”. In this configuration, multiple RAID 1 arrays are also striped, which
addresses fault tolerance first and then performance.
When you examine the RAID levels for use with SQL Server, follow these guidelines.
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SQL Server Log Files work best on RAID 10, and should never be used on RAID 5. If RAID 10 is not
available, RAID 1 should be used.
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SQL Server Data Files work best on RAID 0+1, but can be used on RAID 5 with little degradation in
performance.
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Multiple Disk channels are preferred. At the minimum, SQL Server Log files should be on a separate
physical channel from the SQL Server Data files, and if possible, neither should be mixed with OS or
Application files. For example, at a minimum SQL Server prefers 3 separate disk channels.
In addition to selecting the appropriate RAID configuration, consider disk interface and disk drive
performance. VCM data storage needs are usually low enough relative to commonly available drives that
the smallest drives are sufficient. Fast drives that have fast interfaces are important, along with having a
good number of “spindles” (drives) per RAID to distribute read, write, and seek activity across devices.
Most high-end drives are available in 10,000 RPM or 15,000 RPM spin rates. The faster spinning drives
usually seek faster and can achieve a higher sustained data throughput because more of the platter surface
area passes under the heads in each second. Two primary interface technologies are suitable for use in
high-throughput RAIDS:
vCenter Configuration Manager Hardware and Software Requirements Guide
50 VMware, Inc.