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The Extended-2 Filesystem Overview The extende d-2 le system overview Gadi Oxman, tgud@tochnap c2.technion.ac.il v0.1, August 3 1995 Contents 1 Pref ace 2 2 Intro duction 3 3 A le system - Whydowenee d it ? 3 4 The Linux VFS layer 3 5 Aboutblo cks andblo ck group s 4 6 The view of ino de s f rom the p oint of view of a blo cks group 4 7 The group de scr iptors 4 8 Theblo ck bitmap allo cation blo ck 5 9 The ino de allo cation bitmap 6 10 On the ino deandthe ino detable s 6 10.1 The allo cate d blo cks : :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 7 10.2 Thei mo devar iable : :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 8 10.2.1 The r ightmost 4 o ctal digits :::: ::: :::: ::: :::: ::: :::: ::: :::: : 8 10.2.2 The leftmost twooctal digits ::: ::: :::: ::: :::: ::: :::: ::: :::: : 8 10.3 Timeanddate : ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 9 s ize ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 9 10.4 i 10.5 Us er and group id :: :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 9 10.6 Hard links :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 10 10.7 The Ext2fs extende d ags :: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 10 10.8 Symb olic links :: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 11 1. Pref ace 2 10.8.1 Fast symb olic links :: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 11 10.8.2 Slow symb olic links :: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 11 10.9 i vers ion : :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 11 10.10Re s erved var iable s :: :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 12 10.11Sp ecial re s erve d ino des :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 12 11 Director ie s 12 12 The sup erblo ck 13 12.1 sup erblo ckidenti cation ::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 14 12.2 File system xe d parameters : ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 14 12.3 Ext2fs error handling : :::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 15 12.4 Additional parameters us e d by e2fsck ::: ::: :::: ::: :::: ::: :::: ::: :::: : 15 12.5 Additional us er tunable parameters :::: ::: :::: ::: :::: ::: :::: ::: :::: : 16 12.6 File system currentstate ::: ::: :::: ::: :::: ::: :::: ::: :::: ::: :::: : 16 13 Copyr ight 16 14 Acknowle dgments 16 1 Pref ace Thi s do cumentattemptsto presentanoverview of theinter nal structure of the ext2 le system. It was wr itten in summer 95, while I was workingonthe ext2 filesystem editor project EXT2ED. In the pro ce ss of constructing EXT2ED, I acquire d knowle dge of thevar ious de s ign asp ectsofthethe ext2 le system. Thi s do cument i s a re sult of an e ort todocumentthi s knowle dge. Thi s i s only the initial vers ion of thi s do cument. It i s obviously neither error-prone nor complete, butat least it providesastarting p oint. In the pro ce ss of lear ningthesub ject, I have used the following source s / to ols: Exp er imenting with EXT2ED, as it was developed. The ext2 ker nel source s: fs.h { Themain ext2 include le, /usr/include/linux/ext2 { The contentsofthe directory /usr/src/linux/fs/ext2. { The VFS layer source s only a bit. The slides: The Second Extende d File System, CurrentState, Future Development, by Remy Card. 2. Intro duction 3 The slide s: Optimi sation in File Systems, by Stephen Tweedie. Thevar ious ext2 utilitie s. 2 Intro duction The Second Extended File System Ext2fs is very p opular among Linux us ers. If you us e Linux, chance s are thatyou are us ingthe ext2 le system. Ext2fs was de s igned by Remy Card and Wayne Davison.Itwas implemented by Remy Card andwas further enhance d by Stephen Tweedie and Theodore Ts'o. The ext2 le system i s still under development. I will do cumenthere vers ion 0.5a, which i s di str ibute d along with Linux 1.2.x. Atthi s timeofwriting, the most recentvers ion of Linux i s 1.3.13, andthevers ion of the ext2 ker nel source i s 0.5b. A lot of f ancy enhancements are planne d for the ext2 le system in Linux 1.3, so stay tuned. 3 A le system - Whydoweneedit? Ithoughtthat b efore we diveintothevar ious small details, I'll re s erve a few minute s for the di scuss ion of le systems f rom a general p oint of view. A filesystem cons i stsoftwoword - file and system. Everyone knows themeaningoftheword file -Abunchofdataput somewhere. where ? Thi s i s an imp ortant que stion. I, for example, usually throw almost everythingintoasingle drawer, andhave dicultie s nding somethinglater. Thi s i s where the system come s in - Instead of just throwingthedatatothedevice, we generalize and construct a system which will virtualize for us a nice and ordere d structure in whichwe could arrange our datainmuchthe sameway as b o oks are arrange d in a library.The purp os e of the le system, as I understand it, i s tomake it easy for us toupdateandmaintain our data. Normally,by mounting le systems, we just us e the nice and logical virtual structure. However, the di sk knows nothingaboutthat-Thedevice dr iver views the di sk as a large continuous pap er in whichwe can wr ite notes wherever we wi sh. It i s thetask of the le system managementcodetostore b o okkeeping information which will s ervetheker nel for showingusthe nice and ordere d virtual structure. In thi s do cument, we cons ider one particular admini strative structure - The Second Extende d File system. 4 The Linux VFS layer When Linux was rst developed, it supp orte d only one le system-TheMinix le system. Today, Linux has theabilityto supp ort s everal le systems concurrently. Thi s was donebytheintro duction of another layer between theker nel andthe le system co de-The Virtual File System VFS. 5. Aboutblo cks andblo ck group s 4 Theker nel "sp eaks" withthe VFS layer. The VFS layer pass e s theker nel's reque st tothe prop er le system managementcode. I haven't lear ned muchofthe VFS layer as I didn't nee d it for the construction of EXT2ED so that I can't elab orate on it. Just b e aware that it exi sts. 5 Aboutblo cks andblo ck group s In order to eas e management, the ext2 le system logically divides thediskintosmall units calle d blocks.A 1 blo ckisthesmalle st unit which can b e allo cate d. Each blo ckinthe le system can b e allocated or free. The blo ck s ize can b e s elected to b e 1024, 2048 or 4096 bytes when creatingthe le system. Ext2fs groups together a xe d numb er of s equential blo cks intoa group block.The re sultingsituation i s thatthe le system i s manage d as a s er ie s of group blo cks. Thi s i s done in order tokeep relate d information phys ically clos e on thediskandto eas e themanagementtask. As a re sult, muchofthe le system management re duce s tomanagementofa single blo cks group. 6 The view of ino de s f rom the p oint of view of a blo cks group Each le in the le system i s re s erve d a sp ecial inode. I don't wantto explain ino des now. Rather, I would liketo treat it as another re source, much likeablock - Each blo cks group contains a limited number of ino de, while any sp eci c ino de can b e allocated or unallocated. 7 The group de scr iptors Each blo cks group i s accompanie d byagroup descriptor.The group de scr iptor summar ize s somenece ssary information aboutthe sp eci c group blo ck. Follows thede nition of the group de scr iptor, as de ned in /usr/include/linux/ext2 fs.h: struct ext2_group_desc { __u32 bg_block_bitmap; /* Blocks bitmap block */ __u32 bg_inode_bitmap; /* Inodes bitmap block */ __u32 bg_inode_table; /* Inodes table block */ __u16 bg_free_blocks_count; /* Free blocks count */ __u16 bg_free_inodes_count; /* Free inodes count */ __u16 bg_used_dirs_count; /* Directories count */ __u16 bg_pad; __u32 bg_reserved[3]; }; 1 The Ext2fs source co de refers tothe concept of fragments, which I b elieve are supposed tobesub-blo ck allo cations. As f ar as I know, f ragments are currently unsupp orte d in Ext2fs. 8. Theblo ck bitmap allo cation blo ck 5 The last three var iable s: bg free blocks count, bg free inodes count and bg used dirs count provide statistics aboutthe us e of thethree re source s in a blo cks group - The blocks,the inodes andthe directories. I b elievethatthey are us e d bytheker nel for balancingthe load b etween thevar ious blo cks groups. bg block bitmap contains the blo cknumberofthe block allocation bitmap block. Thi s i s us e d to allo cate/ deallo cate each blo ckinthe sp eci c blo cks group. inode bitmap i s fully analogous tothe previous var iable - It contains the blo cknumberofthe inode bg allocation bitmap block, whichisusedto allo cate/ deallo cate each sp eci c ino deinthe le system. inode table contains the blo cknumberofthestart of the inode table of the current blocks group. bg The inode table i s just the actual ino de s which are re s erve d for the current blo ck. The blo ck bitmap blo ck, ino de bitmap blo ckandthe ino detable are created when the le system i s created. The group de scr iptors are place d one after theother. Together they makethe group descriptors table. Each blo cks group contains theentire table of group de scr iptors in its s econd blo ck, r ight after the sup erblo ck. However, only the rst copy in group 0 i s actually us e d bytheker nel. Theother copie s are there for backup purp os e s and can b e of us e if themain copy gets corrupted. 8 Theblo ck bitmap allo cation blo ck Each blo cks group contains one sp ecial blo ck whichisactually a map of theentire blo cks in the group, with re sp ect tothe ir allo cation status.
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