Mass Storage Systems: File System Implementation and Free-Space Management, Slides of Operating Systems

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Lecture 19 Ch 11: File System Implementation Ch. 12: Mass Storage Structure Docsity.com Free-Space Management • Bit vector (n blocks) … 0 1 2 n-1 bit[i] =    0 ⇒ block[i] free 1 ⇒ block[i] occupied Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit Docsity.com Free-Space Management (Cont.) • Need to protect: – Pointer to free list – Bit map • Must be kept on disk • Copy in memory and disk may differ • Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk – Solution: • Set bit[i] = 1 in disk • Allocate block[i] • Set bit[i] = 1 in memory Docsity.com Efficiency and Performance • Efficiency dependent on: – disk allocation and directory algorithms – types of data kept in file’s directory entry • Performance – disk cache • separate section of main memory for frequently used blocks – free-behind and read-ahead • techniques to optimize sequential access – improve PC performance by dedicating section of memory as virtual disk, or RAM disk Docsity.com Page Cache • A page cache caches pages rather than disk blocks using virtual memory techniques • Memory-mapped I/O uses a page cache • Routine I/O through the file system uses the buffer (disk) cache Docsity.com Log Structured File Systems • Log structured (or journaling) file systems record each update to the file system as a transaction • All transactions are written to a log – A transaction is considered committed once it is written to the log – However, the file system may not yet be updated • The transactions in the log are asynchronously written to the file system – When the file system is modified, the transaction is removed from the log • If the file system crashes, all remaining transactions in the log must still be performed Docsity.com Network File System (NFS) • An implementation and a specification of a software system for accessing remote files across LANs (or WANs) • The implementation is part of the Solaris and SunOS operating systems – running on Sun workstations – using an unreliable datagram protocol • UDP/IP protocol and Ethernet Docsity.com NFS (Cont.) • Interconnected workstations viewed as a set of independent machines with independent file systems, which allows sharing among these file systems in a transparent manner – A remote directory is mounted over a local file system directory • The mounted directory looks like an integral subtree of the local file system, – replacing the subtree descending from the local directory – Specification of the remote directory for the mount operation is nontransparent; • the host name of the remote directory has to be provided • files in the remote directory can then be accessed in a transparent manner – Subject to access-rights accreditation, potentially any file system (or directory within a file system), can be mounted remotely on top of any local directory Docsity.com Mounting | U: usr local dirt (a) Mounts n NFS usr local dirt (b) Cascading mounts Docsity.com NFS Mount Protocol • Establishes initial logical connection between server and client • Mount operation includes name of remote directory to be mounted and name of server machine storing it – Mount request is mapped to corresponding RPC and forwarded to mount server running on server machine – Export list: specifies local file systems that server exports for mounting, along with names of machines that are permitted to mount them Flli t t tht f Docsity.com NFS Protocol • Provides a set of remote procedure calls for remote file operations. • The procedures support the following operations – searching for a file within a directory – reading a set of directory entries – manipulating links and directories – accessing file attributes – reading and writing files • NFS servers are stateless; each request has to provide a full set of arguments Docsity.com NFS Remote Operations • Nearly one-to-one correspondence between regular UNIX system calls and the NFS protocol RPCs – except opening and closing files • NFS adheres to the remote-service paradigm, – but employs buffering and caching techniques for the sake of performance • File-blocks cache: when a file is opened, the kernel checks with the remote server whether to fetch or revalidate the cached attributes – Cached file blocks are used only if the Docsity.com NFS Path-Name Translation • Performed by breaking the path into component names and performing a separate NFS lookup call for every pair of component name and directory vnode • To make lookup faster, – a directory name lookup cache on the client’s side holds the vnodes for remote directory names Docsity.com Docsity.com Overview of Mass Storage Structure • Magnetic disks – provide bulk of secondary storage of modern computers – Drives rotate at 60 to 200 times per second – Transfer rate is rate at which data flow between drive and computer – Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency) – Head crash results from disk head making contact with the disk surface • That’s bad • Disks can be removable • Drive attached to computer via I/O bus – Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI – Host controller in computer uses bus to talk to disk controller built into drive or storage array Docsity.com Moving-head Disk Mechanism track t sector s | | | | . | cylinder c > | | platter — rotation << Se es l<— spindle Uw 2a read-write head ali arm i— arm assembly Docsity.com Overview of Mass Storage Structure (Cont.) • Magnetic tape – Was early secondary-storage medium – Relatively permanent and holds large quantities of data – Access time slow – Random access ~1000 times slower than disk – Mainly used for backup, storage of infrequently- used data, transfer medium between systems – Kept in spool and wound or rewound past read- write head Docsity.com Network-Attached Storage • Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus) • NFS and CIFS are common protocols • Implemented via remote procedure calls (RPCs) between host and storage • New iSCSI protocol uses IP network to carry the SCSI protocol Docsity.com Storage Area Network • Common in large storage environments (and becoming more common) • Multiple hosts attached to multiple storage arrays - flexible Docsity.com Disk Scheduling • The operating system is responsible for using hardware efficiently – for the disk drives, this means having a fast access time and disk bandwidth. • Access time has two major components – Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector. – Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. Docsity.com SSTF • Selects the request with the minimum seek time from the current head position. • SSTF scheduling is a form of SJF scheduling; – may cause starvation of some requests. • Illustration shows total head movement of 236 cylinders. Docsity.com SCAN • The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. • Sometimes called the elevator algorithm. • Illustration shows total head movement of 208 cylinders. Docsity.com C-SCAN • Provides a more uniform wait time than SCAN. • The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. • Treats the cylinders as a circular list that wraps around from the last cylinder to the first one. Docsity.com