linux-ext4 - Re: block allocation for ext2

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Date:	Fri, 19 Dec 2008 16:48:33 +0530
From:	"Rohit Sharma" <imreckless@...il.com>
To:	"Peter Teoh" <htmldeveloper@...il.com>
Cc:	"Jan Kara" <jack@...e.cz>,
	Kernelnewbies <kernelnewbies@...linux.org>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: block allocation for ext2_mkdir

On Fri, Dec 19, 2008 at 3:05 PM, Peter Teoh <htmldeveloper@...il.com> wrote:
> On Fri, Dec 19, 2008 at 3:42 PM, Rohit Sharma <imreckless@...il.com> wrote:
>> On Fri, Dec 19, 2008 at 4:09 AM, Jan Kara <jack@...e.cz> wrote:
>>>> On Fri, Dec 19, 2008 at 12:12 AM, Rohit Sharma <imreckless@...il.com> wrote:
>>>> > How does ext2 allocate blocks for directory.
>>>> >
>>>> > ext2_mkdir               calls            ext2_make_empty
>>>> >
>>>> > ext2_make_empty     calls           __ext2_write_begin
>>>> >
>>>> > __ ext2_write_begin   calls           block_write_begin
>>>> >
>>>> > my query is that how does this function allocates blocks for directories. ??
>>>> >
>>>>
>>>> Does ext2 uses ext2_get_block to allocate blocks for both files and
>>>> directories. ??
>>>  Yes, exactly. Ext2 (unlike ext3 or ext4) treats directories the same
>>> ways as ordinary files and thus ext2_get_block is used for block
>>> allocation.
>>>
>>
>> Can i modify this function to restrict allocation of blocks
>> to a particular block group ?
>>
>> Like i want that my abc.txt file should be in block group 5 suppose, then
>> what should i do?
>>
>
> I think this can be done.   Or is done in the kernel source in several
> ways (indirectly).
>
> 1.   ext2_find_goal()-->this will look for the best new block.   when
> searching, it will try to find the block to be allocated to follow the
> one last allocated in the file, so as to maintain storage contiguity.
>
> 2.   looking into fs/ext2/balloc.c: ext2_try_to_allocate(), there is a
> argument called group_goal, whose purpose is to specify the goal block
> group, and the algorithm will start searching from the group specified
> (look for "start=grp_goal").   Of course, if search is in vain it will
> proceed to other block group, which u don't want ....so may be can
> customize from here.
>
> Not sure if I am right?
>
When we create new file, do we use reservation window to allocate blocks
or we use reservation window every time we need a new data blocks
for file.


> --
> Regards,
> Peter Teoh
>
> Ernest Hemingway - "Never mistake motion for action."
>
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