Re: Using O_DIRECT in ext4

[Mingming Cao <cmm@us.ibm.com>]

Frank Mayhar wrote:
> On Tue, 2009-07-21 at 11:38 -0500, Eric Sandeen wrote:
>   
>> Curt Wohlgemuth wrote:
>>     
>>> On Mon, Jul 20, 2009 at 8:41 PM, Eric Sandeen<sandeen@redhat.com> wrote:
>>>       
>>>> Xiang Wang wrote:
>>>>         
>>>>> For comparison, I did the same experiment on an ext2 partition,
>>>>> resulting in each file having only 1 extent.
>>>>>           
>>>> Interestinng, not sure I would have expected that.
>>>>         
>>> Same with us; we're looking into more variables to understand it.
>>>       
>> To be more clear, I would not have expected ext2 to deal well with it
>> either, is more what I meant ;)  I'm not terribly surprised that ext4
>> gets fragmented.
>>     
>
> Ext2 deals with it via the block reservation code added some time ago.
> It turns out it works pretty well for this case.  Ext4, of course,
> doesn't use the block reservation code.
>   
ext4 mballoc code use per cpu preallocation, so all threads running on 
the same cpu which needs new blocks will be assign blocks next to each 
other. This will makes files created by those threads interleave each 
other as a result, causing fragmentation. Preallocation will help, but 
that a persistant preallocation.

Mingming