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

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Date:   Thu, 6 Jun 2019 15:32:44 +0300
From:   Artem Blagodarenko <artem.blagodarenko@...il.com>
To:     Jianchao Wang <jianchao.wan9@...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 ?

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.

> 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.