How to design your disk
system for maximum performance at the database level.
Disk
input/output (I/O) performance is the single biggest constraint on database
performance in the SharePoint world. Running SharePoint on an underpowered or
poorly designed SQL Server environment will slow performance throughout the
farm.
Because
it's equally important to make sure SQL Server databases are kept highly
available, some level of disk redundancy is usually factored into physical
designs for database servers. Disk redundancy is often classified by RAID
levels. After I explain how the various RAID levels can help performance and
availability, I'll share some tips on how to design your disk system for
maximum performance at the database level.
RAID Fundamentals
RAID
technologies predate PCs and modern x86 server designs. Three of the most
common levels are RAID 1, RAID 5, and RAID 10, so let's take a closer look at
them.
RAID
1. In a RAID 1
design, you have mirrored pairs of disks. If one disk fails, the other has a
full copy of all the data and goes on operating. RAID 1 is a high-performance
technology but requires purchasing 2GB of raw disk space for every 1GB of data
because you need two sets of identical data. So, although RAID 1 might
initially seem like an inexpensive solution because there are only two disks
involved, it can be an expensive one if you have a lot of data to store.
RAID
5. In this
RAID level, you use groups of disks to create a large synthetic unit. For
example, five 150GB disks could be combined to create one hybrid 600GB storage
pool.
At
this point, you might thinking, "Wait a minute, that's not correct!"
Although I'm not great at math, I know that 150GB ´ 5 should equal 750GB, not
600GB. However, in RAID 5, one disk is dedicated to calculating a checksum, or parity bit, every time
data is written to the group. That way, if any of the single disks fail, the
data can be redistributed to other disks by reversing the parity calculations.
As a result, RAID 5 is highly efficient at optimizing the volume of data
stored. It's also great at handling high-volume read operations -- more disks
working together means more systems to read bits from a disk platter and fewer
chances for bottlenecks.
Write
operations are a different story. Because every write operation requires a
calculation, high-volume write operations are slower than they would be in a
traditional disk system. However, for SharePoint, that's not always the worst
thing, because SharePoint is a read-heavy database consumer.
RAID
10. Some
people make a distinction between RAID 1+0 (stripes across mirrors) and RAID
0+1 (mirrors of stripes). This debate doesn't really matter in SharePoint
designs. What does matter is that out of all the RAID levels, RAID 10 offers
the highest availability and highest performance possible. However, RAID 10
requires purchasing extra disks for parity and mirroring -- more than 2GB of
raw disk space for every 1GB of data, effectively more than doubling storage
costs.
Design Tips
SQL
Server databases are the largest consumers of disk space in SharePoint. Thus,
designing your disk system for maximum performance at the database level is
more crucial than for any other level. Here are some general design tips.
Don't
virtualize SQL Server. If
possible, don't virtualize SQL Server because it's already an integration
platform. If you have to virtualize SQL Server, try to limit the virtualization
to test or development systems. Avoid using it for production systems because
it raises the bar in disk engineering to get good performance. SQL Server
databases stored in virtualized disk files are inherently slow compared with
dedicated physical disks.
Use
multiple logical drive letters.
It's usually a good idea to break up SQL Server databases into multiple logical
drive letters because database files, transaction logs, backup files, and
temporary databases (tempdb) benefit from having multiple independent sets of
disk spindles. Adding more spindles spreads the load across multiple parallel
operations when data is being written to the database.
If
you're going to use a SAN or virtualized environment, make sure you understand
where those logical drive letters will be mapped. For example, if the D, E, and
F drives will all point back to separate LUNs on your SAN, but those LUNs will
be part of the same storage group and same set of physical disks, splitting
those files into multiple drive letters will add complexity without significant
performance gains.
Use
RAID 10 judiciously.
RAID 10 is great, but you might not be able to justify it for all applications.
For example, it might be overkill for backup files. Balancing disk performance
and cost is a reasonable trade-off. One possible design is to use:
- RAID 1 on boot disks
- RAID 5 on data disks
- RAID 10 on log disks
- No RAID or RAID 5 on backup disks
Break
large content databases into multiple database files. If you have large content
databases, you can engineer better performance by breaking each large database
into multiple database files. Each database file should be on a separate disk.
Presize
SQL Server databases. SQL
Server databases can be set to automatically grow as needed, but this can lead
to massive file fragmentation. Presizing the databases to a sufficient size at
the outset helps ensure contiguous file allocations. Note that SQL Server's
tempdb database is heavily used by SharePoint, so you should presize it to
about 20 percent of the size of the single largest content database.
If
you want to use automatic database growth settings instead of presizing your
databases, you should set the databases to grow in 50MB to 100MB clumps and not
by percentage. Setting a 100GB database to grow in 10 percent increments means
the database essentially stops to add 10GB or more on each increment. Using a
small clump size will lead to more frequent, but smoother, steady state (i.e.,
continuous) growth.
Design a High-Performance Database Environment
If
you use RAID and follow the general design tips, you can design a
high-performance database environment that's also highly available. This is
essential to a smoothly running SharePoint system.
With permissions from SharePoint Pro.
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