Computer Memory and Storage

Unconnected lookup transformations in Informatica can only use static cache, not dynamic cache. This is because unconnected lookups are called from expressions using the :LKP reference and don't participate in the data flow the same way connected lookups do - they receive input via function call arguments rather than flowing through input ports, so the cache doesn't need to be updated dynamically during the session.

Descriptive variable names designed to assist human memory about the stored data are known as mnemonic variables. Although they are types of identifiers, the term 'mnemonic' specifically denotes this memory-aiding characteristic, whereas identifiers is a broader category.

Mnemonic variables are named meaningfully to describe the data they store (like 'student_age' instead of just 'x' or 'a'). Identifiers are general names for any programming element, constant variables cannot change, and floating point refers to a data type.

Variables named to describe their content are called mnemonic variables, helping programmers remember their purpose. Identifiers refer generally to any user-defined name, constant variables hold fixed values, and floating point variables represent fractional numbers. Mnemonic is the most specific descriptor here.

The Working Storage Section in a COBOL program that interfaces with IMS/DL/I contains dynamic data areas needed during execution. This includes function codes (what operation to perform), I/O areas (buffers for data being transferred), and Segment Search Arguments (SSAs that describe which segments to access). PCB masks are typically in the Linkage Section, not Working Storage.

DAS (Direct Attached Storage) refers to storage devices directly connected to a server without a network in between. NAS (Network Attached Storage) is storage connected over a network. SAN (Storage Area Network) is a dedicated high-speed network for storage. SLS is not a standard storage term.

NAS provides file-level access to clients, allowing them to access entire files over the network. Block-level access is provided by SAN. Direct disk access and logical block address access are characteristics of DAS, not NAS.

Parity in RAID systems (specifically RAID 5) provides fault tolerance by calculating and storing parity information across multiple drives. If a single drive fails, the parity information allows the data to be reconstructed without requiring a full mirror copy. Mirroring requires 2*n drives (complete duplication), striping provides no fault tolerance, and clustering provides high availability but not drive-level fault tolerance through parity.

When multiple HBAs are zoned to the same storage array port, LUN (Logical Unit Number) masking is the appropriate strategy to restrict which specific storage devices each HBA can access. LUN masking operates at the storage array level, allowing administrators to present only specific LUNs to each HBA's WWN (World Wide Name). WWN zoning operates at the switch level and is too broad. Port zoning also controls connectivity but at a different layer. LUN masking provides the granular device-level control needed.

Striping (RAID 0) spreads data across multiple disks to improve performance through parallel read/write operations. By dividing data into blocks and writing them simultaneously to different disks, I/O bandwidth increases. Parity provides fault tolerance, mirroring provides redundancy, clustering is unrelated.

Mirroring (RAID 1) writes identical data to two (or more) disks, creating redundancy. If one disk fails, the data remains available on the mirror disk. This is a simple but effective redundancy technique. Striping is for performance, parity is for fault tolerance without full duplication.

Mirroring (RAID 1) provides fault tolerance through redundancy without performance degradation during a drive failure. Since both disks have identical data, reads continue normally from the surviving disk. Parity RAID (5/6) requires reconstruction and slows down during failure.

Parity RAID uses parity information to rebuild data if a drive fails, providing fault tolerance without requiring full data duplication. This makes it more storage-efficient than mirroring while still protecting against drive failure. Performance improvement comes from striping, not parity itself.

RAID 5 needs at least 3 disks but uses only 1 disk's worth for parity, making it more storage-efficient than RAID 1 which requires 50% overhead for mirroring. RAID 5 has worse write performance than RAID 1 and similar redundancy levels.

A hot spare is a standby drive that automatically replaces a failed drive in the array, rebuilding data without manual intervention. Hot swap allows manual replacement while running, and cold swap requires system shutdown.