Xv6 File System - pages.cs.wisc.edupages.cs.wisc.edu › ~gerald › cs537 › Fall17 › handouts › p5.pdfNov 28, 2017  · Bitmap in xv6 •Each bit in the bitmap is associated

Xv6FileSystemZhewenSong11/28/2017

FileSystemLayoutinxv6

Unused | Superblock | Inodes ... | Unused | Bitmap | Data ...

Inode ofaregularfileinxv6

type = 2majorminornlinksizeaddr 1...

addr 12indirect addr 1

...addr 128

data

data

data

data

...

64 Bytes512 Bytes

512 Bytes

What’sthemaximumpossiblesizeofaregularfile?

...

Inode ofadirectoryinxv6

type = 1majorminornlinksizeaddr 1...

addr 12indirect addr 1

...addr 128

dirent 1...

dirent 32

dirent 1...

dirent 32

dirent 1...

dirent 32

dirent 1...

dirent 32

...

...

64 Bytes512 Bytes

512 Bytes

Howmanyfiles/subdirectoriescanadirectoryhaveatmost?

Bitmapinxv6

• Eachbitinthebitmapisassociatedwithablock,NOTaninode.• Althoughtheveryfirstblockisunused,itisalwaysmarkedas1inthefirstbitofthebitmap,andsoarealltheblockswheretheinodes andbitmapitselfreside.• Bitmapisgroupedinbyte.• Intelx86processorsuselittle-endian.• Example:• ff c2 => 1111 1111 1100 0010

7 6 5 4 3 2 1 0 | 15 14 13 12 11 10 9 8

Demos

• Howtobuildyourownfilesystemimageandreflectinxv6• Closerlookatmkfs.c

• Howxv6fileschangetheimage• Closerlookatfs.img with xxd• Sometrickseditingfileimageswithvim:

• :%!xxd toopentheimage;:%!xxd –r tosavechanges

• Howtoreadimage?– mmap()!

Demos• UsingGDB,gothroughtheentirewritesystemcall.• Keymethodstounderstand:

• writei() in kernel/sysfile.c• bmap() inkernel/fs.c• bread(), bwrite() in kernel/bio.c

• Howdoesxv6normallyhandlelargefiles?• Whatdowedotohandlesmallfileshere?