On 3/22/12, Jan Kara <jack@xxxxxxx> wrote: > Hello, > > On Thu 22-03-12 18:54:26, Akshay Nehe wrote: >> Yes, i got extents as you have suggested, but one thing i found that >> each time modification occurs on any file from ext4 file system then >> kernel deallocates all current blocks and allocate new blocks and >> write modified data on newly allocated blocks. > That is not true. Why do you think it is? Maybe you are modifying the > file with something like a text editor? That certainly truncates the old > file and writes a new one. But generally ext4 supports overwriting. > Yes, i m using vim editor to modify files. Actually due to this inode number of file is changing each time after modification. >> So by tracing kernel code i got some useful functions, for >> deallocation: ext4_free_blocks, for allocation: ext4_map_blocks, but >> found difficult to analyse each of them. Can anyone suggest correct >> functions which actually dose allocation and deallocation? > Ext4 uses multiblock allocator which is in fs/ext4/mballoc.c. In > particular function allocating blocks in ext4_mb_new_blocks(), function > freeing blocks is ext4_free_blocks(). But you will need to understand most > of mballoc.c to understand how mballoc works. You can read comments in the > beginning of the file to understand some basics about mballoc. You can > also read some basics from by presentation about ext4 at Linux Kongress in > 2009 (http://www.linux-kongress.org/2009/slides/ext4+btrfs_jan_kara.pdf). > > Honza > -- > Jan Kara <jack@xxxxxxx> > SUSE Labs, CR > Thanks for help, I will go through both mballoc file and your presentation file. -- Regards, Akshay Nehe. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
