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Performance tuning and scalability
Scaling a storage system also means addressing performance bottlenecks. This list highlights potential spots where performance bottlenecks can occur. Keep in mind that this entire stack is a tightly coupled chain--a change in any of the links will be felt throughout the

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  • Hosts and applications
  • Application (layout of application objects across multiple file systems or devices, number of spindles made available)
  • File system (type of file system, journaling impact, file-system parameters, direct vs. asynchronous I/O, special access handling such as Quick I/O or Oracle Disk Manager)
  • Volume manager (layout to avoid or minimize spindle contention, stripe sizes, mirror policies, read/write optimization)
  • Operating system setup (SCSI and IP/network parameters, multipathing software)
  • Host bus adapters (fan-in ratios, number of HBAs per host, vendor-recommended settings, binding)

  • Network
  • Switch ports (speed settings, fabric parameters)
  • Fabric (inter-switch links, fabric parameters, Fabric Shortest Path First, oversubscription)

  • On the array
  • Front-end host directors or cards (fan-out ratios, port settings)
  • Cache (read vs. write cache, cache hit ratios, utilization)
  • Back-end disk directors or cards (I/O balance and spreading, disk geometry mismatch, RAID layout and access)
  • Disks (type, speed and size of disks; SATA vs. Fibre Channel; spindle contention)

Virtualization scales beyond the array
Virtualization provides additional flexibility in a storage infrastructure. It allows you to scale beyond a single storage array with seamless data mobility. Data replication and migrations can be performed across multiple storage systems (including heterogeneous environments) using a single interface. It is, however, an evolving technology and one has to pay careful attention while designing a solution. Virtualization standards (such as the Fabric Application Interface Standard) haven't been widely adopted, so a storage team may have to perform provisioning manually using individual point tools. But virtualization is here to stay, and as it matures some of its current limitations will be overcome, allowing the effortless provisioning of virtualized storage.

An array's architectural limitations are a key limiting factor when attempting to scale that array--hitting that wall essentially means that there are no longer any scalability options remaining. Virtualization allows users to mask the limitations and move data onto other storage subsystems without costly downtime. This enables performance and capacity-hungry applications to be satisfied from the virtual storage pool and, conversely, apps whose I/O and capacity requirements are reduced can be scaled down to slower, cheaper storage.

A growing list of vendors tout some type of virtualization in their arrays or other products. Products such as Hewlett-Packard Co.'s StorageWorks XP series, Hitachi Data Systems' TagmaStore Universal Storage Platform and Sun Microsystems Inc.'s StorEdge 6920 provide in-band virtualization in the array itself. That basically means all virtualization of existing storage is performed by the new array in a manner that's totally transparent to the network or host.

This was first published in June 2006

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