Concepts with Symbian OS (779878), страница 38
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The root directory location is either stored directly in the MBR (as astorage medium address in a specific location) or it is stored on a specificplace on the storage medium. The former method is the most flexible andallows root directory information to be stored and duplicated for backuppurposes.Since all storage in Unix is a set of bytes and all storage can bereferenced as a file, it should not be surprising that Unix treats directoriesas file space that can be opened by applications and manipulated.Obviously, directory storage in Unix has a specific structure but, aswith all data, Unix does not organize the structure, but leaves that jobto the parts of the kernel that implement a file system.
It is possible toread the data from a directory and look at it in any way. You can evenlook at the contents of a directory by using the ‘cat’ command.File structureFiles are generally stored in pieces on the storage medium. As wediscussed previously, each piece is the size of a block on the storagemedium and these pieces are strung together to make a complete file.Depending on the method of storage, performance may be adverselyaffected by how a file is stored.The structure of a file’s FCB is an important start to efficient storage.How much or how little information is stored makes a difference to howfast file information is accessed.
A generic structure of an FCB is shownin Figure 8.3.How space is allocated for files plays a large part in the performance offile I/O. Contiguous allocation of all the pieces of a file is the best way tostore them for performance reasons. Performance is directly affected byhow hard the mechanical aspects of the storage medium must work. Storing file blocks contiguously minimizes the movement of the read head ondisk drives and thus renders the best performance.
Fragmenting a file intomany pieces flung widely across a hard disk make the hard disk’s readingmechanism move more over the disk surface and slows I/O performance.However, contiguous allocation of file blocks is rarely possible. Since,in most cases, it is impossible to accurately predict how much space afile needs, it is also impossible to allocate enough contiguous space for aIMPLEMENTATION OF A FILE SYSTEM181file nameownership informationsize informationcreation date/timemodification date/timelast read date/timeaccess permissionslocation of first file blockFigure 8.3A typical file-control-block formatfile to grow into the space allocated for it. It is, rather, more prudent toplace pieces of a file wherever they may fit and link the pieces togethersomehow so the file-system implementation can put them together whilereading data from the pieces.Working with linked file space requires having access to the linkageinformation.
In some systems, the linkage information can be kept withthe file table. Other systems store link information with each piece of afile on a storage medium. In these cases, the file looks like a linked list,and working through the contents of a file means traversing a linked listfrom front to back.Sometimes a compromise is made between contiguous allocation andlinked allocation.
File blocks can be put contiguously together in clustersand the clusters can be strung together to allow flexibility in file growthand allocation. A cluster is usually a group of disk sectors, which containthe file blocks. The size of a cluster is determined when the file system iscreated and remains constant over that file system.A slight variation on the method of keeping link information in a fileis the indexed approach. In the indexed approach, the address of thefirst pieces of a file is stored in the file table. The address on the storagemedium of each piece of a file is stored in the file table as an index, anaddress relative to the base address of the file.
Sometimes, if the file islarge, the index entries are stored as the first file block on the storagemedium and the entry in the FCB points to this block.We must deal with fragmentation issues when we split the storage into fixed-size blocks. All the above schemes suffer from internal182FILE SYSTEMS AND STORAGEfragmentation: files are seldom exactly the size that fits into a specificnumber of fixed blocks. There is usually space within a block that iswasted.
External fragmentation is the space wasted outside the collection of blocks. If a fixed-block scheme is used, external fragmentation iseliminated as the storage medium is carved up into blocks that fit exactly.From time to time, it is a good idea to defragment storage media.Defragmenting has nothing to do with internal or external fragmentation;it is focused on the wide dispersion of the blocks belonging to filecontent across a storage medium. The closer a file’s blocks are, the lessmechanical wear occurs from trying to move all over a medium to readthe blocks. Defragmentation moves the blocks of all files so that they areas close to each other as possible, hopefully adjacent to one another.Free space and bad blocksIn addition to file content, there are two other types of blocks on astorage medium. Free space is space available for allocation to files andis made available when files are deleted.
Because of block-allocationpatterns, free space comes available in blocks that are scattered all overthe medium. These blocks must be accessible when space is needed andtherefore are usually linked together using the same methods used to linkfile blocks together. Contiguous or linked methods apply to free blocksas well.Occasionally, a block ‘goes bad’ – becomes damaged in some wayso that it is unusable. A bad block can simply be avoided on a storagemedium. As with file blocks and free space, bad blocks are usually linkedtogether so they can be found and avoided.FAT and VFAT File SystemsWhen Microsoft developed the first operating system to run on IBMhardware, it needed to invent a file system for the computer’s hard drives.In 1977, the FAT file system debuted on IBM PCs using the MicrosoftDisk Basic system. This first file system – called the FAT file system forits use of a file-allocation table – is still in use, in more evolved forms, inmodern versions of Microsoft Windows.
The FAT file system is also usefor most mobile media storage (for example, compact flash or multimediacards).The initial version of the FAT file system is called FAT12. This firstversion was very simple and restricted: no support for hierarchical directories, disk addresses were 12 bits long and the disk size was stored asIMPLEMENTATION OF A FILE SYSTEM183a 16-bit count of disk sectors, which limited the size to 32 MB. Themaximum size of a partition was 32 MB.The release of MS-DOS 2.0 occurred at the beginning of 1983.This version of FAT12 introduced hierarchical directories. The use ofdirectories allowed FAT12 to store many more files on the hard disk,as the maximum number of files was no longer constrained by the rootdirectory size.
This number could now be equal to the number of clusters(or even greater, using zero-sized files). The format of the FAT itself didnot change. The 10 MB hard disk on the PC XT had 4 KB clusters. If a20 MB hard disk was later installed and formatted with MS-DOS 2.0, theresultant cluster size would be 8 KB, the boundary at 15.9 MB.In 1988, with the release of MS-DOS 4.0, the FAT16 file system wasfinalized.
In this file system, the disk address was now 16 bits and themaximum partition size jumped to 2 GB. The maximum cluster size in aFAT16 file system is 32 KB.FAT12 and FAT16 file systems had what is known as the 8.3 limitation.Filenames on the system were limited to eight characters with a threecharacter suffix. A variant of the FAT16 file system allowed longerfilenames to be used. This variant was known as the VFAT file systemafter the Microsoft Windows 95 VxD device driver.The FAT32 file system was introduced in 1996 and is still in usetoday. The FAT32 file system uses 32 bits for disk addressing and forclusters.
Only 28 of the 32 bits are used to address clusters, but even thisallows for 228 clusters, which allows FAT32 to support media sizes up to2 TB. Unfortunately, limitations in other Microsoft utilities mean that thefile-allocation table is not allowed to grow beyond 222 clusters, whichsupports media sizes up to 124 GB. Because of the 32-bit disk address,files can grow to a maximum size of 4 GB. Also the long filenames fromVFAT were implemented.Each of the FAT file-system variants shares common characteristics.They are implemented using a file-allocation table with file pieces puttogether using a linked list of clusters.
There is one file-allocationtable per disk volume. The FAT has a common structure, illustratedby Figure 8.4.Boot sectorOptional extraboot sectorspaceFigure 8.4FileallocationtableDuplicatefileallocationtableRoot filedirectoryThe generic format of a FAT file systemFile blockstorage184FILE SYSTEMS AND STORAGEThe first section of the FAT is known as the boot sector. This section ofthe file system contains the operating system’s boot-loader implementation. This is the place that the computer goes to retrieve code that bootsthe operating system. In some implementations, this is code that initializesthe system for operating system execution. In others, the operating systemcode is stored elsewhere on the disk and the boot sector only contains anaddress where this code can be found.There is a section of this format that is made up of optional reservedsectors.
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