SCALABILITY OF EXT2

Yancan Huang, Guoliang Jin

May 13, 2008

MOTIVATION Graph for create

MOTIVATION Graph for open

MOTIVATION

Same method, different graphs: Code for create:asmlinkage long sys_creat(const char __user * pathname,

int mode){

return sys_open(pathname, O_CREAT | O_WRONLY | O_TRUNC,

mode);}

Code for open:sys_open(pathname, O_RDWR);

Why? Create: where does the time go?

OVERVIEW

Motivation What is scalability of file system Experiment environment

Setup Techniques Benchmark

Create: where does the time go The file create process What does ext2_lookup do What dose ext2_create do The file open process

Conclusion & Future work

WHAT IS SCALABILITY OF FILE SYSTEM

Large File Systems

Large, Sparse Files

Large, Contiguous Files

Large Directories

Large Numbers of Files

EXPERIMENT ENVIRONMENT

Setup: UML Mount our own ext2 file system called ext2k With 1GB empty virtual disk

Measuring techniques gettimeofday long long c; __asm__ __volatile__ (“rdtsc” : “=A” (c));

Output techniques printk write to log on host

Benchmark Sequential create and open on a 1GB disk

OVERVIEW

Motivation What is scalability of file system Experiment environment

Setup Techniques Benchmark

Create: where does the time go The file create process What does ext2_lookup do What dose ext2_create do The file open process

Conclusion & Future work

CREATE: WHERE DOES THE TIME GO

asmlinkage long sys_creat(const char __user * pathname, int mode)

{return sys_open(pathname,

O_CREAT | O_WRONLY | O_TRUNC, mode);

}

THE FILE CREATE PROCESS

The process of sys_opensys_open { // fs/open.c

do_sys_open { // fs/open.cgetnameget_unused_fd_flagsdo_filp_open { // fs/open.c

open_namei // fs/namei.cinameidata_to_filp

}fsnotify_openfd_installputname

}prevent_tail_call

}

THE FILE CREATE PROCESS When open_namei meets file create

open_namei {……path_lookup_createlookup_hashopen_namei_create if (! error)

return 0;……

}

45%54%

THE FILE CREATE PROCESS The process of lookup_hash

lookup_hash {permission__lookup_hash {cached_lookup // always fail for create

struct dentry *new = d_allocdentry = inode->i_op->lookup if (!dentry) // always true for

createdentry = new

return dentry

}

}

THE FILE CREATE PROCESS The process of open_namei_create

open_namei_create {vfs_create {

may_createsecurity_inode_createdir->i_op->create // dir is an

inodefsnotify_create

}may_open

}

WE ARE NOW IN EXT2

Thanks to inode->i_op->lookup Thanks to inode->i_op->create

Going to ext2_lookup Going to ext2_create

WHAT DOES EXT2_LOOKUP DO

The process of ext2_lookupext2_lookup {

ext2_inode_by_name {ext2_find_entry {

……

}}igetd_splice_alias

}

WHAT DOES EXT2_FIND_ENTRY DO

The process of ext2_find_entryext2_find_entry {

do {ext2_get_pageext2_last_byte

while () {ext2_matchext2_next_entry

}} while ()

}

NOW WHERE WE ARE When open_namei meets file create

open_namei {……path_lookup_createlookup_hashopen_namei_create if (! error)

return 0;……

}

WHAT DOES EXT2_CREATE DO The process of ext2_create

ext2_create {ext2_new_inodemark_inode_dirtyext2_add_nondir {

ext2_add_linkd_instantiate

}

}

WHAT DOES EXT2_ADD_LINK DO The process of ext2_add_link

ext2_add_link {for () {

ext2_get_pageext2_last_bytewhile () {

ext2_matchext2_rec_len_from_disk

}} __ext2_write_beginext2_commit_chunk

}

HOW OFTEN THESE WHILE LOOP EXECUTED

REVISIT THE CREATE GRAPH

THE FILE OPEN PROCESS

The process of sys_opensys_open { // fs/open.c

do_sys_open { // fs/open.cgetnameget_unused_fd_flagsdo_filp_open { // fs/open.c

open_namei // fs/namei.cinameidata_to_filp

}fsnotify_openfd_installputname

}prevent_tail_call

}

THE FILE OPEN PROCESS When open_namei meets file open

open_namei {……if (!(flag & O_CREAT)) {

error = path_lookup_open(dfd, pathname, lookup_flags(flag), nd, flag);

if (error)return error;

goto ok;} ……

}

THE FILE OPEN PROCESS The process of path_lookup_open

path_lookup_open {__path_lookup_intent_open {

get_empty_filpdo_path_lookup {

path_walk {

link_path_walk {……

}}

}}

}

THE FILE OPEN PROCESS The process of link_path_walk

link_path_walk {__link_path_open // in the dcacheif (fail) { // force real lookup

requestsdgetmntgrt__link_path_open

}

}

REVISIT THE OPEN GRAPH

OVERVIEW Motivation What is scalability of file system Experiment environment

Setup Techniques Benchmark

Create: where does the time go The file create process What does ext2_lookup do What dose ext2_create do The file open process

Conclusion & Future work

CONCLUSION

In create, ext2_lookup makes sure there won’t be two files with the same name, and ext2_add_link performs a similar routine again

The dentry structure of ext2 is linear Using B-tree to manage this in memory structure

would show better performance

Another scalability issue in ext2 is that its inode number is determined when the disk is formatted

FUTURE WORK

Try to use B-tree to manage the dentry structure and test the performance But the B-tree itself is complex

Try more workload

QUESTIONS?