Nachos文件系统目录管理的解读

首先在实现目录树中对目录项结构做了修改，导致所占数据增多，按照原来的解读，所创建的文件个数也该相应减少(受限于目录项个数)，但是却没有，为什么？

问题

当我对目录结构进行修改，改成多叉树时，会增加目录表项的占用空间，如果说目录表内容只有2个扇区的话，通过计算按理说创建五个目录表项就会出现问题，但是缺没有，这是为什么呢？

原因

首先 当format 初始化文件系统的时候

//----------------------------------------------------------------------// FileSystem::FileSystem// Initialize the file system. If format = TRUE, the disk has//nothing on it, and we need to initialize the disk to contain//an empty directory, and a bitmap of free sectors (with almost but//not all of the sectors marked as free). ////If format = FALSE, we just have to open the files//representing the bitmap and the directory.////"format" -- should we initialize the disk?//----------------------------------------------------------------------FileSystem::FileSystem(bool format){ DEBUG('f', "Initializing the file system.\n");if (format) {BitMap *freeMap = new BitMap(NumSectors);Directory *directory = new Directory(NumDirEntries);FileHeader *mapHdr = new FileHeader;FileHeader *dirHdr = new FileHeader;DEBUG('f', "Formatting the file system.\n");// First, allocate space for FileHeaders for the directory and bitmap// (make sure no one else grabs these!)freeMap->Mark(FreeMapSector); freeMap->Mark(DirectorySector);// Second, allocate space for the data blocks containing the contents// of the directory and bitmap files. There better be enough space!ASSERT(mapHdr->Allocate(freeMap, FreeMapFileSize));ASSERT(dirHdr->Allocate(freeMap, DirectoryFileSize));// Flush the bitmap and directory FileHeaders back to disk// We need to do this before we can "Open" the file, since open// reads the file header off of disk (and currently the disk has garbage// on it!).DEBUG('f', "Writing headers back to disk.\n");mapHdr->WriteBack(FreeMapSector); dirHdr->WriteBack(DirectorySector);// OK to open the bitmap and directory files now// The file system operations assume these two files are left open// while Nachos is running.freeMapFile = new OpenFile(FreeMapSector);directoryFile = new OpenFile(DirectorySector);// Once we have the files "open", we can write the initial version// of each file back to disk. The directory at this point is completely// empty; but the bitmap has been changed to reflect the fact that// sectors on the disk have been allocated for the file headers and// to hold the file data for the directory and bitmap.DEBUG('f', "Writing bitmap and directory back to disk.\n");freeMap->WriteBack(freeMapFile); // flush changes to diskdirectory->WriteBack(directoryFile);if (DebugIsEnabled('f')) {freeMap->Print();directory->Print();delete freeMap; delete directory; delete mapHdr; delete dirHdr;}} else {// if we are not formatting the disk, just open the files representing// the bitmap and directory; these are left open while Nachos is runningfreeMapFile = new OpenFile(FreeMapSector);directoryFile = new OpenFile(DirectorySector);}}

可以看出首先

Directory *directory = new Directory(NumDirEntries);

而

#define NumDirEntries 20

所以目录表里面新建了 20个目录表项

之后标记的是 Bitmap 和 Directory的文件头，分别是0 / 1 sector

freeMap->Mark(FreeMapSector); freeMap->Mark(DirectorySector);

再往后

ASSERT(mapHdr->Allocate(freeMap, FreeMapFileSize));ASSERT(dirHdr->Allocate(freeMap, DirectoryFileSize));

问题来了 Allocate的 DirectoryFileSize 是多少呢

#define DirectoryFileSize (sizeof(DirectoryEntry) * NumDirEntries)

所以Nachos分配的目录表的大小并不是固定的3、4扇区，而是根据具体的表项个数以及表项大小进行动态分配的

所以 我改成了 20个表项，就目录表而言，必须要能放开20个表项的，但是至于往后磁盘的其他空间，他并不关心.

在我插入五个文件之后

./nachos -D

我的目录项

class DirectoryEntry {public:bool inUse;// Is this directory entry in use?int sector;// Location on disk to find the // FileHeader for this file char name[FileNameMaxLen + 1];// Text name for file, with +1 for // the trailing '\0'int parent; // father dir 's indexint Child_Num; // dir's child numint children[MaxDirChildNUm]; // dir's children};#define MaxDirChildNUm 10 // the number of child that a dir can have

Entry Size = 1 + 4 + 10 + 8 + 10*4 bytes = 63bytes

而20个Entry 则20 * 63= 1260 bytes

1260 / 128 = 10 (向上取整)，故最后目录表要占10个扇区，3 ~ 13

所以其他文件的数据都是从0x704开始写，最高地址为0x20004即文件数据块能占 32*32 - 13 个扇区

1260 / 128 = 10 (向上取整)，故最后目录表要占10个扇区，3 ~ 13

[外链图片转存中…(img-rfOU0D9g-1651035093466)]

所以其他文件的数据都是从0x704开始写，最高地址为0x20004即文件数据块能占 32*32 - 13 个扇区

但是受限于设置的目录表个数，因此最多还是20个文件。