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Re: Location of backup superblocks

To: Felipe Monteiro de Carvalho <felipemonteiro.carvalho@xxxxxxxxx>, xfs@xxxxxxxxxxx
Subject: Re: Location of backup superblocks
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Tue, 12 Aug 2014 09:52:10 -0700
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <53EA43E6.5010100@xxxxxxxxxxx>
References: <CACyNnZNfTxxy0PsGKHgthoiGc74O_28uumGeG0CjLprxtiDv-Q@xxxxxxxxxxxxxx> <53EA43E6.5010100@xxxxxxxxxxx>
On 8/12/14, 9:42 AM, Eric Sandeen wrote:
> On 8/12/14, 3:49 AM, Felipe Monteiro de Carvalho wrote:
>> Hello,
>>
>> I am trying to recover a XFS partition (or series of partitions) where
>> the main superblock was corrupted, and I see that there are multiple
>> superblock copies. I'd like to guess the partition start and size
>> also.
> 
> so I guess you mean that the partition table was corrupted as well?
> 
>> I see that the partition size can be calculated with sb_blocksize* sb_dblocks
>>
>> But what about the partition start? I think I can guess it based in
>> the position of the found superblocks.
>>
>> Any ideas where is the code that writes to disk those backup
>> superblocks? So far to me it looks like that their position is
>> calculated as disk_size / 4 rounded (nearest rounding?) to a multiple
>> of sb_blocksize, but having the exact code part would be better of
>> course =) Or even better, a list of backup superblock positions...
> 
> How hard have you tried looking?  ;)
> 
> In xfsprogs, you can find this if you search for "backup":
> 
> " set allocation group superblock\n"
> "\n"
> " Example:\n"
> "\n"
> " 'sb 7' - set location to 7th allocation group superblock, set type to 
> 'sb'\n"
> "\n"
> " Located in the first sector of each allocation group, the superblock\n"
> " contains the base information for the filesystem.\n"
> " The superblock in allocation group 0 is the primary.  The copies in the\n"
> " remaining allocation groups only serve as backup for filesystem recovery.\n"
> " The icount/ifree/fdblocks/frextents are only updated in superblock 0.\n"
> 
> The on-disk structure document describes the same thing:
> 
> http://xfs.org/docs/xfsdocs-xml-dev/XFS_Filesystem_Structure//tmp/en-US/html/Allocation_Groups.html#Superblocks

or the xfs(5) manpage :)

-eric

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