Developing the SAP Data Center : General Storage Considerations

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In my experience, perhaps 85% of SAP performance problems are considered to be storage-related. This is not to say that 85% of all SAP problems can be traced back to the storage system—but by the time I am called in, the easy fixes are typically taken care of. So I rarely run into simple profile parameter problems, or issues with other hardware subsystems, or problems with the network, either. Instead, after a quick review of the entire solution stack I usually find myself drilling down into the following:

  • The model and features of the disk subsystem servicing the systems that exhibit the slow performance

  • Details as to how the disk subsystem has been configured

  • Database statistics, to quantify how well the database itself performs on the given disk subsystem platform

  • Specific transaction loads on the system (that is, batch processes or especially heavy online user transactions), especially the top 40 or so transactions identified by transaction ST03 as consuming the most database request time

  • Finally, the ABAP programs (or other code) that is actually being executed by these top 40 transactions

Although a variety of disk subsystems are deployed today in support of SAP solutions, including direct-attached SCSI, Channel-Channel Arbitrated Loop-based storage, and a few NAS (Network Attached Storage) systems, the discussions that follow focus on the predominant disk subsystems being deployed today—Storage Area Networks, or SANs.

Special Considerations for Storage Area Networks

Deploying Storage Area Networks, or SANs, for current and new SAP system landscapes has kept many SAP professionals quite busy for the last two years. The latest iterations represent not only some of the fastest disk subsystems ever manufactured, but also represent everything that customers need in a highly available and scalable disk solution. Not only does a SAN give us the ability to expand quickly, but it also allows us to move data around with ease. Snapshots, disk clones, data replication, and more satisfy high-availability requirements, and help us to address disaster recovery, too.

With these new capabilities comes new complexity, of course, and certainly new paradigms. In some cases, we also have a new set of issues that must be addressed when deploying the latest in SAN technology—switched fabric designs, planning for different data access models within the same SAN, providing connectivity to new shared resources like tape drives and tape libraries, and all of the complexity that comes with deploying new software solution sets that interoperate seamlessly (or nearly so!) within the SAN environment. Moving on, let’s sneak a quick look at some basic best practices for implementing SAP in a SAN environment, with the understanding that the newer “virtual” SANs will be covered in detail later.

Deploying SAP on a SAN—General Best Practices and Observations

To approach this in an organized manner, we will start with some general SAN observations, and then cover the servers that play a role in the SAN, move to the SAN infrastructure, and finally wrap up with the disk subsystem itself. The following list has been assembled as a result of more than a hundred SAP-on-SAN design or deployment engagements:

  • As the SAN is a special network encompassing all components from the HBA to the disk, it is incorrect to call each cabinet of controllers and disks a SAN. Rather, the entire connected solution is the SAN. If the development environment is off by itself and is not connected in any way to production, for example, we have in place a Development SAN and a Production SAN. After these SANs are connected (that is, via a fibre cable connecting one SAN’s switch to the other SAN’s switch), it then becomes a single SAN.

  • Each server needs at least a single Host Bus Adapter, or HBA. Because this represents a glaring single point of failure, it is always recommended to install and configure two where high availability is important. Both HBAs are cabled to the same SAN, but to redundant switches (discussed later).

  • A server should not connect to two different SANs—connecting to multiple SANs concurrently is not supported. In other words, a single server with two HBAs should not be connected to two different models or implementations of a SAN.

  • If a server contains two HBAs, it actually now has two paths in which to access data on the SAN. To manage this condition, an OS or OS-supported software utility is typically employed on each server connected to the SAN. Hewlett-Packard’s line of StorageWorks SANs uses a utility called SecurePath, for example. SecurePath require a license for each server, but ships with all StorageWorks cluster kits—SecurePath can help to ensure that the HBAs and data access paths are balanced, too, from a performance perspective.

  • Each HBA requires what used to be called a GBIC, or gigabit interface connector, as might each port in your fibre switches. Today, GBICs are more commonly called transceivers.

  • Fibre cables then run from each GBIC in each HBA to a GBIC (or otherwise pre-enabled port) in each redundant set of fibre switches.

  • In regards to fibre switches, different switch vendors may have different rules regarding cascading their switches (to increase the port count available to the SAN, for example). I highly recommend that the specific vendor’s documentation is referenced for details, with the understanding that many SAN fabrics in use today are limited to very few levels of cascading, or hop counts.

  • Switched fabric switches should be configured with dual power supplies. Two switches are required at minimum to address high availability. A fully redundant SAN/server configuration consists of four fibre connections—two to the server, and two to the disk subsystem. These fibre connections should be carefully mapped to the switches so that each controller is connected to two switches, not one.

  • Either something referred to as “zoning” or something referred to as “selective storage presentation” is used to ensure that a specific server can only access a specific set of disk drives or virtual drives. The implementation of this varies with the vendor as well as the product set. Regardless, though, be sure to implement one of these access protocols. Failure to do so risks corrupting data.

  • Now that we have established connectivity from each server to the SAN, we need to carve up disk drives on the SAN. Until November of 2001, some of the highest-performing SANs on the market leveraged up to six separate SCSI controllers across six different SCSI busses, or drive shelves. Each drive shelf was capable of housing 14 drives, for 84 disk drives total. It is best to view the disks vertically, in that we stripe data “up and down” the disk subsystem. A maximum of six drives (when six shelves are deployed) can be configured per logical drive in this manner. By addressing our SAN disk configuration in this way, we mitigate risk inherent in a bus or shelf or shelf-power failure. This in turn prevents us from ever losing data in the event of one of these failures.

  • Whenever possible, I promote the idea of using a graphical user interface to address day-to-day SAN management, and a scripted command-line approach for actually configuring a SAN (to allow for rapid cookie-cutter standard SAN installations).

  • For SAP we need several sets of disks, depending on which database vendor has been selected. All data and log files (including redo logs, archive logs, SQL transaction logs, TempDB, and so on) must reside on the SAN. Further, as long as we are not using a clustering technology that requires local access to database executables, the Oracle or SQL Server (or Informix, DB2, SAPDB, and so on) executables can be located on the SAN as well.

  • If you are clustering, you need to add a Quorum drive or similar such drive as well (actually, a mirrored disk pair distributed across two different busses is recommended). The quorum must be located on the SAN. Also, clustering SQL Server or Oracle means that these database executables must be installed on each node in the cluster, not out on the SAN.

  • For the production database, I nearly always recommend that the production datafiles reside on LUNs (drive partitions) set up for hardware-based mirroring and striping (RAID 1+0 or 0+1, depending on the storage vendor’s implementation). For simplicity, I like to create LUNs where the physical drives are vertically situated one on top of the other.

  • SAP BW represents a potential exception to this general configuration rule of thumb for production systems. In the fast disk subsystems available today, the trade-off between performance and disk space between the different RAID levels is not as significant as it has been in the past.

  • If all of the preceding items are in place, you are well-prepared to address database growth. As the database grows you can add another RAID 1+0 set, for example. The OS will immediately recognize this additional disk space (if not, go to Disk Administrator and execute the option to “scan for new devices”), and allow us to format this space and create a new drive letter. Then, in order to increase your effective database size, the database administrator simply needs to create more table spaces or data files on the new disk partitions.

A good example of what a traditional SAN deployment might look like from a disk-layout perspective can be seen in Figure 1.

Figure 1. A well-performing and highly available disk subsystem requires that no two drives share the same storage shelf, power cable, or SCSI bus.

With your new understanding of how to take advantage of the features and capabilities a classic SAN environment can provide in regards to our SAP environment, let’s move ahead and discuss the latest in SAN technology—the virtual SAN.

The Latest in SAN Technology—Leveraging Storage Virtualization

In review, Storage Virtualization, or SV, is defined as “the transparent abstraction of storage at the block level.” Regarding SV, the idea is to minimize the need for parameters that allow fine-tuning, low-level optimization, and “tweaking,” in favor of allowing the intelligent virtual disk subsystem to take care of everything. It’s a paradigm shift in the biggest sense of the word—hands-off!

The ultimate in high-performance SV equates to striping and then mirroring all disk partitions across all 84–168 or more disk drives in a virtual storage disk subsystem. The ultimate in storage capacity equates to creating a RAID 5 partition across all of these drives. In either case, multiple partitions, or drives, can be created and presented to the operating system, or a single enormous drive can be created and presented instead. And many storage virtualization technologies automatically create optimally sized and configured RAID drives based on the space available in conjunction with historical data. It’s that simple. The various block size parameters, read/write caching algorithms, read-ahead logic, and so on are all under the covers, so to speak. The best SV disk subsystems allow the few remaining configuration choices to be made via a browser-based graphical user interface, too.

Virtual storage eliminates the physical one-to-one relationship between servers and storage devices, or databases and physical drives. Some important, albeit limited, configuration options still exist, however, and it is these options that concern us next.

Storage Virtualization Best Practices

When it comes to implementing and configuring a Virtual Array, you have control over the following options:

  • As with a traditional SAN, you can still design and implement high-availability SAN switch architectures, including the ability to create zones for effectively segregating specific servers and their storage across multiple storage systems.

  • You can create one or more groups within a physical Virtual Array storage system. Groups represent another way to segregate storage and servers, though this capability is limited to disks that are physically attached to the same disk controller subsystem (normally in one or more “cabinets” or “disk enclosures”). Thus, in an 84-drive Virtual Array, you might choose to create two groups of 42 physical drives each, three groups of varying sizing, or just a single group containing all 84 drives.

  • You can manually create LUNs, or disk partitions, and select the group (and therefore disk drives) over which this LUN will be created. You can create many LUNs, or one LUN, whatever is optimal for the need at hand. For example, you might create three 100GB LUNs in a single group consisting of 42 drives, and place your SAPDATA files on these.

  • Upon creating the LUNs, you can typically specify what RAID level each LUN is to use, and whether caching should be enabled or disabled (although in some virtualization product sets, this is simply not possible). In this way, if required of your solution, you can mix high-density RAID 5 LUNs (for database disk dumps, for example) with high-performance RAID 1+0 LUNs (for database data files and logs).

Some of the preceding points defy the preaching of many a database administrator or basis consultant. I can hear them now. “What, one giant virtual disk chopped up into pieces? You can’t do that, we’re supposed to keep our logs separate from our data! What happened to best practices?” Let them know that best practices differ now, based on the storage solution employed. And recommend that they read the PDF files found on the Planning CD that relate to configuring and optimizing, for example, HP’s Enterprise Virtual Array Storage System.

Finally, let the facts speak for themselves, as presented in Figure 2—the Virtual Array easily handles a variety of workloads, while other traditional high-end storage systems struggle.

Figure 2. This chart shows the results of actual I/O throughput tests (measured in Megabytes per second), comparing a Virtual SAN Array to both a legacy FCAL-based SAP Production disk subsystem and a traditional high-performance SAN.

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