linux-ext4 - Re: No data blocks at all in my ext4 journal

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Message-Id: <F8A93D7A-71FE-4D52-9D7B-029178FB97A8@dilger.ca>
Date:   Mon, 19 Mar 2018 17:46:23 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Jidong Xiao <0121167@...an.edu.cn>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: No data blocks at all in my ext4 journal

On Mar 19, 2018, at 5:31 PM, Jidong Xiao <0121167@...an.edu.cn> wrote:
> 
> Hi,
> 
> When I use the dd command to examine the journal, it turns out every block starts with the jbd2 magic number, meaning they are either a superblock, a descriptor block, or a commit block, or a revoke block, but there is no data block.
> 
> I manually and randomly tried like 50 blocks after that superblock, but I haven't seen any data blocks.
> 
> The kernel version I am using is, 4.14. Is this a serious bug or am I doing something wrong? Anyone has any suggestions, or tell me how to find the data blocks in my journal, or are they actually missing from the journal? By data blocks, I mean metadata, as my system is using the default mode, i.e., the ordered journal mode, only metadata will be logged.
> 
> Thanks. Let me know if more information is needed.

You can decode the journal structure using debugfs, and it will show you the
blocks more easily:

# debugfs -c -R "logdump -a" /dev/vg_mookie/lv_root
debugfs 1.42.13.wc5 (15-Apr-2016)
Journal starts at block 13970, transaction 1103250
Found expected sequence 1103250, type 1 (descriptor block) at block 13970
Dumping descriptor block, sequence 1103250, at block 13970:
  FS block 12058640 logged at journal block 13971 (flags 0x0)
  FS block 3 logged at journal block 13972 (flags 0x2)
  FS block 12059015 logged at journal block 13973 (flags 0x2)
  FS block 12058703 logged at journal block 13974 (flags 0x2)
  FS block 12066941 logged at journal block 13975 (flags 0x2)
  FS block 12058641 logged at journal block 13976 (flags 0x2)
  FS block 12059178 logged at journal block 13977 (flags 0x2)
  FS block 0 logged at journal block 13978 (flags 0xa)
Found expected sequence 1103250, type 2 (commit block) at block 13979

Cheers, Andreas






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