Stratus Continuous Process System COSC513 Presentation By Ying Li & Kunyu Zheng

Disk Name and Relation Logical volumes Member disks Physical disks Duplex disks Boot disks

Logical Volumes A logical volume A multimember logical volume The advantage of using multimember logical volumes The rules for assigning logical volume names: –The name of each volume within a system must be unique –A logical volume name cannot contain a period

Member Disks Could be a duplex disk or a nonduplex disk; Each member disk is given a member number, between 0 and 9 inclusive; For boot disks, member number is assigned automatically as 0; For nonboot disk, member numbers should be assigned sequentially starting from 0; Each member disk in a logical volume can be any disk model; but both partners of a duplex disk must always be the same model;

Physical Disk Physical disk name consists of the member disk name with a suffix. The suffix.pri refers to the primary partner of duplex disk The suffix.sec refers to the secondary partner of a duplex disk.

Duplex Disk Two physical disks that operate as a single member disk; It is known to the system and to users by the same member disk name; Physical disk are called partners of a duplex disk; The partner disk serves as logical copies of each other. This means that if one of the partners fails, the other partner will continue to process I/O for the member disk, and the system will not be affected.

Duplex Disk (Continued) The operating system writes the same information to each partner, but reads only from one partner. When reading, the operating system chooses the partner that is not busy or whose access arm is in the best position to read information. The partner disk must be the same model.

Nonduplex Disk The nonduplex disk refer to a duplex disk with only one partner; Operating system look every physical disk as partner of a duplex disk; Nonduplex disk can be either a primary or a secondary partner of a duplex disk.

Boot Disk Each module has at least one boot disk whose member number is always 0; The boot disk contains a copy of the operating system that is needed to boot the module. User initialize boot disk with initialise boot disk command. The command automatically initialises the disk as the primary partner of number 0 of the specified logical volume.

Disks Components Disk labels Several types of disk partitions Bad disk block

Disk Partitions Every physical disk is divided into partitions, which are logical regions of a disk. Table 1 lists seven types of partitions and the types of disks.

Table1. The Types of Partitions and the Types of Disks

Boot Partition Boot partition contains a copy of the operating system. Boot partition exists only on boot disk. Each disk has eight partitions, usually only one or two of them contains copies of operating system. The default boot partition is the boot partition that contains the copy of operating system for automatic start-up. This partition is identified by a special field in the disk label.

Dump Partition Dump partition is available for holding dump image in the event of the failure of a module. As show in Table 1, Dump partitions also exist only on boot disk; each boot disk has only one dump partition. A dump image consists of all modified disk blocks of the operating system. Every dump partition should have sufficient space pre- allocated to hold dump image. Another factor influencing the size of dump partition is the size of the disk cache on the module.

Paging Partition A paging partition is a partition the operation system uses for temporary storage; it does not contain any permanent information and is not saved or restored. Boot disks must have paging partition. For nonboot disks, user could establish paging partition by changing the default page part value. Nonduplex disks should not have paging partition. It is risking a module failure when a nonduplex disk with a paging partition fail. The total paging partition space on a module must be sufficient to support the virtual memory demands of all users on the module.

Other Partitions File partition provides permanent storage for files, directories, and index. Every disk has a file partition. Diagnostic partition is a partition that the operating system uses to test the disk. Same, every disk has a diagnostic partition. A backup partition is reserved for future use. A spare partition is a partition that is created when a disk initialised and used to dynamically remap bad blocks. It usually one percent of the disk’s capacity. The operation system uses the blocks in this partition to replace physical bad block on the disk.

Bad Blocks A bad block is the block that the operating system has identified as unusable. The operating system uses dynamic bad block remapping to handle bad blocks. When a bad block is detected on a disk, the operating system adds it to its list of bad blocks and remaps it to a block from the spare partition. Once the operating system uses a block from the spare partition, it sees the disk with no bad blocks. The duplex disks that use dynamic bad block remapping are logical mirrors of each other rather than physical mirrors.

Conclusions Stratus define disks name like a tree structure. On the top is the logical volume. Other disks beneath the logical volume. When the operatiog system working on disks, disks managing operation always duplicate to prevent any failure from corrupting data or interrupting system operation. When writing data requested, the operating system writes the data to both disks. When reading data required, it comes from the disk whose read-write heads are closest to the data, this minimise the access time and offer performance benefits in read-heavy environments.

Conclusions(continued) If a disk failure occurs, all disk I/O operations continue working on the good drive until the malfunction is repaired. When the failure is repaired, the system automatically restores the disk. Here again, the application software is unaware of the failure or the redundant hardware architecture.

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