Disk Access

DISK STRUCTURE Sector: Smallest unit of data transfer from/to disk; 512B 2/4/8 adjacent sectors transferred together: Blocks Read/write heads can be positioned over 1 cylinder at a time

DISK ACCESS Seek time: Mechanically moving the R/W head to the correct track ~msec Rotation time: Rotate the required sector(s) under the R/W head ~msec Transfer time: Actual read/write of the data from/to disk surface

Interface to the rest of the System CPU connected to I/O device: I/O bus I/O bus addresses for each connected device: I/O ports I/O interface converts between I/O bus and device specific commands: SCSI, ATA/IDE I/O controller: hardware on the disk drive to do the actual data transfer. E.g. move the R/W head

Each device has I/O ports for a command/ status/ data read and data write register OS can write a command (say Write), write the data into the data write register and keep looking at the status register to see when write completes: POLLING Or OS can write command and write data and disk can INTERRUPT it when data has been written Use DMA to transfer multiple bytes between disk and memory; Interrupt CPU after transferring a block of bytes

Types of I/O Synchronous: Process which requested I/O is put in wait queue. Back to run queue when I/O completes Asynchronous: I/O system call returns immediately even without any data having been transferred

Filesystems Present an interface to the disk storage with filenames, protection, organization into directories … Major filesystem functions: 1.Map filename to areas on the disk 2.Manage free blocks on the disk

Disk Allocation Each file has some ‘header’ information, including disk block information Contiguous allocation: header has starting block and number of blocks Scattered allocation: header has to have each block number

Unix/Linux Implementation File header = inode Each partition is divided into block groups (BG), usually groups of adjacent cylinders on disk BGs reduce fragmentation; kernel tries to keep data blocks of a file in a single BG

Block Groups Data Block Bitmap: Free space management Inode Table: List of inodes in the BG Inodes provide mapping between file offset (file block number) and disk block number (logical block number)

Inodes File Blk No. 0 1 Logical Blk No. Single Indirect Double Indirect Triple Indirect Direct Disk Blocks

Speeding up things Read-Ahead: Read adjacent data ahead of time; store in cache Data block allocation: Allocate block near previously allocated blocks/ same BG as inode/ other BGs Pre-allocation: Allocate more (upto 8 adjacent) blocks than requested

RAID Redundant Array of Independent Disks Advantages 1.Reliability: Redundant copies/ Error correction information 2.Performance: Parallel/ Striped transfer

RAID levels RAID 0 Non-redundant striping + Striping improves I/O transfer rate -No reliability improvement RAID 1 Disk mirroring + High reliability + Parallel reads - High cost

RAID levels RAID 2 Memory style Error Correction Code; Bit- level striping + Reliability at a possibly lower cost -Parity calculation overhead for writes RAID 3/RAID 4 Byte/Block interleaved parity + Reliability with only 1 extra disk for parity - Parity calculation for each write; overload parity disk

RAID levels RAID 5/ RAID 6 Distributed parity + Parity load distributed + RAID 6 can recover from multi-disk failures Nested RAID RAID 0+1 RAID 1+0