Re: Required help for understanding ext4 block allocation

Akshay Nehe
2012-03-22T15:22:33Z

On 3/22/12, Jan Kara <jack@suse.cz> 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@suse.cz>
> SUSE Labs, CR
>
Thanks for help, I will go through both mballoc file and your presentation file.

--
Regards,
Akshay Nehe.