Re: Problems with mballoc and uninit_groups option

[Valerie Clement <valerie.clement@bull.net>]

Aneesh Kumar K.V wrote:
> On Fri, Jan 11, 2008 at 03:04:08PM +0100, Valerie Clement wrote:
>> Hi,
>>
>> I've got problems with mballoc when I create the ext4 filesystem with  
>> the "uninit_groups" option enabled.
>>
>> First, I do a single test on a filesystem created without the  
>> "uninit_groups" option and mounted with the defaults option:
>>   dd if=/dev/zero of=/mnt/test/foo bs=1M count=1024
>>
>> In this case, the file blocks are allocated in the groups 4, 5, 6, 7, 8,  
>> 9, 10.
>>
>> When the filesystem is created with the "uninit_groups" option enabled  
>> and mounted with the defaults option, I do the same dd command.
>>
>> In this case, the file blocks are allocated in the groups 5, 7, 9, 25,  
>> 27, 49, 81. It seems that the blocks could be allocated only in the  
>> already initialized groups.
>>
> 
> That is because we skip the uninitialized group in ext4_mb_good_group.
> I guess we are trying criteria 0 allocation and we skip uninit group for
> criteria 0 . You can tune by setting higher value for
> /proc/fs/ext4/partition/orders2_req. Setting it to a high value would
> skip criteria 0 allocation for small requests.

Hi Aneesh,
thanks for your answer but I'm not sure your solution is the right one 
to fix the problem.

I added traces in the mballoc code and found perhaps something 
incorrect: while running the dd command, the function 
ext4_mb_init_cache() is called, which calls ext4_init_block_bitmap() for 
an uninitialized group. But the flag EXT4_BG_BLOCK_UNINIT is not reset 
after. Shouldn't it be reset here?

> I guess you are using delayed allocation ?
Yes

   Valérie