Re: Questions about mkfs -A option

[Wang Sheng-Hui <shhuiw@gmail.com>]

On 2012年10月24日 17:44, Alex Lyakas wrote:

> Hi Wang,
> if you just look at the code in find_free_dev_extent():
> u64 search_start = 0;
> ...
> search_start = max((u64)1024 * 1024, search_start);
> if (root->fs_info->alloc_start + num_bytes <= device->total_bytes)
> 	search_start = max(root->fs_info->alloc_start, search_start);
> ...
> key.objectid = device->devid;
> key.offset = search_start;
> key.type = BTRFS_DEV_EXTENT_KEY;
> 
> You see that it will search for DEV_EXTENTs (when allocating chunks)
> that begin after the value you specified. So it will not use some
> space at the beginning of the device for allocations. Same behavior in
> the kernel can be caused by using "alloc_start=X" mount option.
> What is the purpose of this I don't know, but what it does looks pretty clear.
> The (first copy of the) superblock, though, is always written to
> BTRFS_SUPER_INFO_OFFSET, as you mentioned.
> 
> Alex.
> 
> 


Thanks, Alex.

> 
> 
> On Wed, Oct 24, 2012 at 2:48 AM, Wang Sheng-Hui <shhuiw@gmail.com> wrote:
>> On 2012年10月23日 01:49, Alex Lyakas wrote:
>>
>>> Wang,
>>> I would say that zeroing is done to prevent from seeing some other superblock (besides btrfs) there.
>>
>>
>> That make sense.
>>
>>>
>>> Alex.
>>>
>>>
>>
>>
>> But I still confused by the -A option?
>>
>> In mkfs, we always write the sb info into BTRFS_SUPER_INFO_OFFSET.
>> From the -A comment, I think btrfs would be places after the -A value.
>> What would happen if we specify a -A value larger than it?
>>
>>
>> Thanks,
>> Sheng-Hui