CSE451 Introduction to Operating Systems Spring 2008

Tour of the Windows NT File Systems

Gary Kimura & Mark Zbikowski

2

History

• Portable System Group formed November 1988– Started with five software engineers– Later became Windows NT

• FAT for NT was written in early 1989– Supported DOS disk format– Entirely new code base and programming model– FASTFAT started in late 1989 (two person effort)

• Other file systems done by the Core NT group were– HPFS, NTFS, CDFS, NPFS

• FS development was done concurrently with kernel development– A lot of cooperative design between the file systems, I/O system, Memory

Management, and the Cache manager

3

Fat on-disk Structure

• Boot sector, followed by reserved sector Area

• File Allocation Table (FAT)– 12, 16, and 32 bit FAT

– Clusters size

– Typically two copies of FAT

• Root Directory– Fixed size

– 32 byte Directory Entries (dirent)

– 8.3 names

• Data area– Once only for file data, later expanded to include subdirectories

1.44 MB Floppy format

• Number of sectors = 0xB40 (2,880)• Sector size = 0x200 (512)• Number of Fats = 0x2, Sectors per Fat = 0x9• Root directory Size = 0xE (14) sectors, 0xE0 (224) entries• Layout

– Sector 0x0: boot sector

– Sector 0x1: start of first FAT

– Sector 0xA: start of second FAT

– Sector 0x13: start of root directory

– Sector 0x21: start of data area

• Disk Edit utility to help in your project– Added to the project4 directory (copy everything to your vhd)

4

5

I/O – FS Interface

• Layering of drivers (file systems, disks, etc.)– All orchestrated by the I/O System– Driver Objects (one per file system). Each driver “registers” with the I/O

system, see DriverEntry(…) in FatInit.c– Device Objects (one per volume). Created when a new volume is mounted, see

FatMountVolume(…) in Fsctrl.c– File Objects (one per CreateFile call). See FatCommonCreate(…) in create.c

• I/O Call Driver model using I/O Request Packets (IRP)– IRP Stack locations (see “inc\ddk\wdm.h”)– Major and minor functions– IoCallDriver(…) and IoCompleteRequest(…)

• File Object– FsContext and FsContext2

6

Windows FS Architecture

• Data Structure Layout

– Mimics as much as practical the on-disk directory/file hierarchy

– Keeps in memory the opened files and directories

– Volume Control Block (VCB)

– File Control Block (FCB) and Directory Control Block (DCB)

– Context Control Block (CCB)

– All rooted in FatData (See FatStruc.h)

Windows FS Architecture (Continued)

• FSD/FSP Model– User Thread versus Worker Thread– Driven off of IRP Major Function

• See DriverEntry(…) in FatInit.c– Fat Common Routines

• Everything looks like a file– FatReadVolumeFile(…)

• Some things to watch out for– Kernel Stack Space Limitation– Being able to wait for I/O– Recursive routines

7

8

Cache/MM Interaction

• Virtual caching

• Memory Mapped Files

• Fast read and fast write

• Ancillary file system data also cached– Borrowing a pin and unpin logic from databases

• Noncached I/O– Where the actual read/write to disk occur

– Synchronous and Asynchronous I/O

– See read.c, write,c and deviosup.c

9

CreateFile Operation

• Some of the main files and functions to look at

• Create.c and dirsup.c

• FatFsdCreate

• FatCommonCreate

• FatCreateNewFile

• FatCreateNewDirent

• FatConstructDirent

Programming Style

• Engineering practices

• Comments

• Indentation

• Variable names

• Project 4 description has a synopsis of each source file

10