On 2017-02-03 11:44, Lakshmipathi.G wrote:
> Hi.
>
> Came across this thread
> https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg55161.html
> Exploring possibility of adding test-scripts around these area using
> dump-tree & corrupt-block.But
> unable to figure-out how to get parity of file or find its location.
> dump-tree output gave,
>
> item 5 key (FIRST_CHUNK_TREE CHUNK_ITEM 145096704) itemoff 15557 itemsize
> 144
> length 134217728 owner 2 stripe_len 65536 type DATA|RAID5
> io_align 65536 io_width 65536 sector_size 4096
> num_stripes 3 sub_stripes 0
> stripe 0 devid 3 offset 63111168 # Is this
> parity?Seems empty?
> dev_uuid f62df114-186c-4e48-8152-9ed15aa078b4
> stripe 1 devid 2 offset 63111168 # Contains file
> data-stripe-1
> dev_uuid c0aeaab0-e57e-4f7a-9356-db1878876d9f
> stripe 2 devid 1 offset 83034112 # Contains file
> data-stripe-2
> dev_uuid 637b3666-9d8f-4ec4-9969-53b0b933b9b1
> thanks.
IIRC, the parity is spread across the disk stripes of the chunk.
So first you have to find the logical-offset [LO] where the the file begins.
Then you have to map this offset to the chunk which holds the data. The chunk
has the following info:
- chunk start [CS], chunk length [CL]
- for each stripe:
where the stripe starts
If you subtract the chunk-start from the logical-offset [ CO == LO-CS], you
will find the offset where the data belongs in the chunk.
As stated above, the PARITY is spread across the chunk stripes. So (supposing
that the stripe size is 64K, the raid level is 5, the disks are three),
- the first 64k of stripe 0, is data [0..64K)
- the first 64k of stripe 1, is data [64..128K)
- the first 64k of stripe 2 is parity,
- the 2nd 64k of stripe 0 is parity,
- the 2nd 64k of stripe 1, is data [128..196K)
- the 2nd 64k of stripe 2, is data [192..256K)
- the 3rd 64k of stripe 0, is data [256..320K)
- the 3rd 64k of stripe 1 is parity,
- the 3rd 64k of stripe 2, is data [320..384K)
and so on,
To find the data, You have to compare the CO to the data [...) range.
If you look to an my old patch (unfinished :-( ), you can find some example to
dump the different stripe
[BTRFS-PROGS][PATCH][V2] Add two new commands: 'btrfs insp physical-find' and
'btrfs insp physical-dump'
> Cheers.
> Lakshmipathi.G
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