linux-ext4 - Re: block allocation for ext2

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Message-ID: <2d08ef090812191049p1da42e74jc9cce8f076a56ab7@mail.gmail.com>
Date:	Sat, 20 Dec 2008 00:19:54 +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

Is there a difference between how ext2 allocates block between
inode's indirect blocks and data blocks.


On Fri, Dec 19, 2008 at 8:59 PM, Peter Teoh <htmldeveloper@...il.com> wrote:
> On Fri, Dec 19, 2008 at 7:18 PM, Rohit Sharma <imreckless@...il.com> wrote:
>> 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.
>>
>>
>
> http://lwn.net/Articles/81357/ ===> from here, we deduced that it
> should be done at every new block request level, and not at the file
> level.   This is because when file level request is initiated, the
> first thing is see if any existing block can be reused or not, before
> allocating new block (which then comes with reservation features - to
> ensure contiguity of datablocks).
>
>
> --
> Regards,
> Peter Teoh
>
> Ernest Hemingway - "Never mistake motion for action."
>
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