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A hot spare is a disk that sits in reserve, waiting for a critical disk to fail. The hot-sparing software agent migrates the data that was on the failed disk to the hot spare. Hot spares require some sort of RAID so that the contents of the failed disk can be re-created on the replacement. Hot sparing can be performed by either the disk array or by logical volume management software.
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Many disk arrays contain some specialized memory that acts as a staging area for disk writes. Since memory is much faster than disk, writes to this cache can complete many times faster than writes to disk. Write cache can increase the cost of a disk array, and unless the disk array has a reliable battery backup, it can cause data loss if the data written to the cache never makes it to disk. Data that s written to the write cache is flushed to disk on a regular basis, usually every few seconds.
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A storage area network is a new model for interconnecting storage and computing systems into a shared pool that many different hosts can access. Although many SANs are based on Fibre Channel, FC is not a requirement. SANs can be based on a number of different underlying technologies, including IP-based networks. We will discuss SANs in more detail in 8. The storage devices on the SAN are usually limited to disks and tape drives, although other devices, such as CD and DVD drives, are expected to be available in the near future. Through the use of protocol bridges, legacy SCSI-based storage devices can join a SAN as well. In Figure 7.5, we offer a diagram of a simple SAN.
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Fibre Channel Switches FC to DWDM bridge Tape drives and libraries FC to SCSI bridge Legacy SCSI disk array Remote SAN
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Figure 7.5 A simple storage area network.
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7
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In a SAN, newly purchased storage can be put into a storage pool and allocated as needed between the hosts on the SAN. As a result, storage purchasing becomes much more efficient. When you request a disk purchase, and management complains that you just bought 2TB of storage, you don t have to say, Yes, but we put that 2TB on that server over there; now I need disks for this server over here. All the storage can be shared and easily allocated among all of the servers on the SAN. (This argument assumes that some of the 2TB remains unused so that there is actually something to allocate between the hosts.) The following are other SAN advantages: Centralized management and allocation. The storage pool and its allocation can be centrally managed from one host in the SAN. This means that a single collection of disks can be allocated in chunks, generally allocated as whole disks or LUNs, are required, and can easily be reallocated among hosts as needs change. Intrinsic resilience and high availability. Disks can be centrally RAIDed and allocated to SAN hosts premirrored. This will help ease the storage management burden on each individual host. Storage will usually have dual connections to the SAN, so that the loss of a single controller or connector will not cause a failure. Disk pools can be spread out over multiple locations, so that the loss of a single location will not affect the availability of the data that is on that storage. Disks need not be colocated. Since the permitted distances for FC are so much greater than those for SCSI, disks can be mirrored (see RAID Technology later in this chapter) between multiple locations on the same SAN, or centrally located. The SAN model gives administrators much more flexibility in how they physically layout their storage, allowing them to balance availability with convenience. Smart and more efficient failover. More complex failover configurations, such as N-to-1, N-to-N, cascading, and service level failovers are much more practical in a SAN environment. We will discuss clustering configurations in much more detail in 17, Failover Configurations. Efficient resource deployment. The resources on a SAN are visible to all the members of the SAN, so a tape silo or other expensive backup device can be shared between all the members of the SAN. This makes the purchase of large, expensive backup devices much more practical and justifiable. LAN-free backups. With a tape drive on the SAN and some high-quality backup software that supports this capability, you can back up your server directly to tape by putting the data on the SAN rather than on the LAN, thus reducing the impact of backups on your user community.
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