Fixed-disk storage management performance

The operating system uses a hierarchy of structures to manage fixed-disk storage.

Each individual disk drive, called a physical volume (PV), has a name, such as /dev/hdisk0. If the physical volume is in use, it belongs to a volume group (VG). All of the physical volumes in a volume group are divided into physical partitions (PPs) of the same size (by default, 4 MB in volume groups that include physical volumes smaller than 4 GB; 8 MB or more with bigger disks).

For space-allocation purposes, each physical volume is divided into five regions. See Position on physical volume for more information. The number of physical partitions in each region varies, depending on the total capacity of the disk drive.

Figure 1. Organization of Fixed-Disk Data (Unmirrored). The illustration shows the hierarchy of a physical volume that is partitioned into one or more logical volumes. These partitions or logical volumes contain file systems with directory structures which contain individual files. Files are written to blocks contained in tracks on the storage media and these blocks are usually not contiguous. Disk fragmenting occurs when data gets erased and new data files are written to the empty blocks that are randomly scattered around multiple tracks on the media.

Within each volume group, one or more logical volumes (LVs) are defined. Each logical volume consists of one or more logical partitions. Each logical partition corresponds to at least one physical partition. If mirroring is specified for the logical volume, additional physical partitions are allocated to store the additional copies of each logical partition. Although the logical partitions are numbered consecutively, the underlying physical partitions are not necessarily consecutive or contiguous.

Logical volumes can serve a number of system purposes, such as paging, but each logical volume that holds ordinary system data or user data or programs contains a single journaled file system (JFS or Enhanced JFS). Each JFS consists of a pool of page-size (4096-byte) blocks. When data is to be written to a file, one or more additional blocks are allocated to that file. These blocks may or may not be contiguous with one another and with other blocks previously allocated to the file.

For purposes of illustration, the previous figure shows a bad (but not the worst possible) situation that might arise in a file system that had been in use for a long period without reorganization. The /op/filename file is physically recorded on a large number of blocks that are physically distant from one another. Reading the file sequentially would result in many time-consuming seek operations.

While an operating system's file is conceptually a sequential and contiguous string of bytes, the physical reality might be very different. Fragmentation may arise from multiple extensions to logical volumes as well as allocation/release/reallocation activity within a file system. A file system is fragmented when its available space consists of large numbers of small chunks of space, making it impossible to write out a new file in contiguous blocks.

Access to files in a highly fragmented file system may result in a large number of seeks and longer I/O response times (seek latency dominates I/O response time). For example, if the file is accessed sequentially, a file placement that consists of many, widely separated chunks requires more seeks than a placement that consists of one or a few large contiguous chunks. If the file is accessed randomly, a placement that is widely dispersed requires longer seeks than a placement in which the file's blocks are close together.

The effect of a file's placement on I/O performance diminishes when the file is buffered in memory. When a file is opened in the operating system, it is mapped to a persistent data segment in virtual memory. The segment represents a virtual buffer for the file; the file's blocks map directly to segment pages. The VMM manages the segment pages, reading file blocks into segment pages upon demand (as they are accessed). There are several circumstances that cause the VMM to write a page back to its corresponding block in the file on disk; but, in general, the VMM keeps a page in memory if it has been accessed recently. Thus, frequently accessed pages tend to stay in memory longer, and logical file accesses to the corresponding blocks can be satisfied without physical disk accesses.

At some point, the user or system administrator can choose to reorganize the placement of files within logical volumes and the placement of logical volumes within physical volumes to reduce fragmentation and to more evenly distribute the total I/O load. Logical volume and disk I/O performance contains further details about detecting and correcting disk placement and fragmentation problems.