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SQL Server and SANs
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Don t make the mistake of thinking that because SANs are big and expensive you ll be guaranteed to get good I/O performance. SAN storage design goals are often in conflict with those of SQL Server. SANs are effective at maximizing disk utilization by sharing a large central store of disks between many servers. In contrast, SQL Server benefits from striping over dedicated disks, with an emphasis on disk quantity rather than utilization. This section addresses some of the common issues that DBAs face in SAN-based environments, including the relationship with the SAN administrator, LUN configuration, performance tuning, and disaster-recovery options.
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If you re like me, you like being in control. You like having operating system administrator privileges to all database servers, despite best practice, and you like having direct control over disk configuration. In environments with multiple servers and different applications sharing SAN storage, such control is unlikely, particularly when the DBA and SAN administration roles are separated. In such sites, it s not uncommon for the SAN administrator to be dismissive of the DBA s concerns about SAN configuration; often with a let the SAN take care of it attitude. Like any other storage system, SAN disks that are presented to SQL Server need to be configured in line with the storage practices we ve already covered. Given the complex, shared nature of SANs and the difficulty of changing a design after deployment, it s critical for you to become involved in the SAN configuration as early as possible and present storage requirements from a DBA s perspective. I ve been fortunate to work in environments with highly skilled SAN administrators who were receptive to the unique storage requirements of SQL Server. Through
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our good working relationship, we were able to combine our skill sets to deliver reliable, high-performance SAN storage for SQL Server. Unfortunately, such outcomes aren t always achieved; the most common problem is LUN configuration, which we ll look at next.
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A LUN is a logical unit of SAN disk created by the SAN administrator and presented to an attached server s operating system. The server is unaware of the physical makeup of the disks involved in the LUN, and sees it as a single locally attached disk. As shown in figure 2.7, each physical disk in Disk 1 Disk 2 Disk 3 Disk 4 the SAN can be carved up into parts and used in the creation of separate LUNs. As a result, LUNs from many servers can all be using difLUN 1 ferent parts of the same physical disk. LUN 2
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Disk 5 Disk 6 Disk 7 Disk 8 LUN 3 LUN 4
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Figure 2.7 A sample LUN composition. Physical disks are broken up into slices, or hypers. A LUN is constructed by combining hypers from several disks.
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When troubleshooting performance problems involving SAN storage, I ve found it useful to ask the SAN administrator a series of questions involving the makeup of the LUNs: How many individual physical disks are included in the SQL Server LUNs Remembering the principle of striping across many physical disks, if a LUN consists of a small number of physical disks, then performance may be less than ideal. What other servers are sharing the physical disks in the LUNs, and what is their I/O profile If many servers LUNs share the same physical disks, performance may be reduced.2 This is particularly important for transaction log LUNs. Transaction log I/O is sequential in nature, and dedicated physical disks mean the disk heads are able to stay in position, with writes proceeding in a sequential manner. This is obviously not possible if the transaction log LUNs are created on disks containing other LUNs. For SQL Server applications with high transaction log rates, this can have a large impact on transaction response time, leading to decreased performance and throughput. What are the RAID levels of the LUNs Earlier in the chapter, we covered the various RAID levels, and noted that RAID 5 has a disk write overhead and is therefore not an ideal choice for SQL Server applications with a high percentage of disk writes. Given the SAN goal of increased disk utilization, RAID 5 is often chosen as the default RAID level. The SAN administrator should be able to tell you the current RAID level and the different levels supported in the SAN.
Some SAN solutions use a balancing mechanism whereby hypers are moved between physical disks for better I/O balancing.
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