linux-ext4 - Re: [HELP] What are the allocated blocks on a newly created ext4 fs ?

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Message-ID: <22ccd72a-8926-0d12-0fbe-8ad5604d1584@gmail.com>
Date:   Mon, 10 Jun 2019 11:25:47 +0800
From:   Jianchao Wang <jianchao.wan9@...il.com>
To:     Artem Blagodarenko <artem.blagodarenko@...il.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca
Subject: Re: [HELP] What are the allocated blocks on a newly created ext4 fs ?

Hi Artem 

Thanks so much for your help.

On 2019/6/6 20:32, Artem Blagodarenko wrote:
> Hello Jianchao,
> 
> Not enought input data to give an answer. It depends on mkfs options. For example, if flex_bg option is enabled, then several block groups are tied together as one logical block group; the bitmap spaces and the inode table space in the first block group, so some groups are not totally free just after FS creating.

In my environment, there are 16 bgs per flex_bg.
The bitmaps and inode table .etc should lay on the first bg of every flex_bg.
So I can see there are about 8223 blocks allocated in the 1st bg of every flex_bg.

But as you can see in the output of mb_groups, there are some bgs which get allocated about 1024 blocks.

I have out figured out what are they for.

Thanks
Jianchao

> 
>> On 6 Jun 2019, at 13:41, Jianchao Wang <jianchao.wan9@...il.com> wrote:
>>
>> Dear all
>>
>> After I newly created a ext4 fs and check the mb_group,
>>
>>       #group: free  frags first [ 2^0   2^1   2^2   2^3   2^4   2^5   2^6   2^7   2^8   2^9   2^10  2^11  2^12  2^13  ]
>>       #0    : 23513 1     9255  [ 1     0     0     1     1     0     1     1     1     1     0     1     1     2     ]
>>       #1    : 31743 1     1025  [ 1     1     1     1     1     1     1     1     1     1     0     1     1     3     ]
>>                           ^^^^
>>       #2    : 32768 1     0     [ 0     0     0     0     0     0     0     0     0     0     0     0     0     4     ]
>>       #3    : 31743 1     1025  [ 1     1     1     1     1     1     1     1     1     1     0     1     1     3     ]
>>       #4    : 32768 1     0     [ 0     0     0     0     0     0     0     0     0     0     0     0     0     4     ]
>>       #5    : 31743 1     1025  [ 1     1     1     1     1     1     1     1     1     1     0     1     1     3     ]
>>       #6    : 32768 1     0     [ 0     0     0     0     0     0     0     0     0     0     0     0     0     4     ]
>>       #7    : 31743 1     1025  [ 1     1     1     1     1     1     1     1     1     1     0     1     1     3     ]
>>       #8    : 32768 1     0     [ 0     0     0     0     0     0     0     0     0     0     0     0     0     4     ]
>>       #9    : 31743 1     1025  [ 1     1     1     1     1     1     1     1     1     1     0     1     1     3     ]
>>       #10   : 32768 1     0     [ 0     0     0     0     0     0     0     0     0     0     0     0     0     4     ]
>>       #11   : 32768 1     0     [ 0     0     0     0     0     0     0     0     0     0     0     0     0     4     ]
>>
>> There are some bgs that have 1024 blocks allocated. What are they for ?
>>
> BTW, I don’t see from mb_group output why 1024 blocks allocated in group #1
>> Many thanks in advance
>> Jianchao
> 
> Best regards,
> Artem Blagodarenko.
>