Re: [PATCH 1/9] btrfs: Add support for reading a filesystem with a RAID 5 or RAID 6 profile.

[Goffredo Baroncelli]

On 06/14/2018 01:17 PM, Daniel Kiper wrote:
> On Sun, Jun 03, 2018 at 08:53:40PM +0200, Goffredo Baroncelli wrote:
>> Signed-off-by: Goffredo Baroncelli <address@hidden>
>> ---
>>  grub-core/fs/btrfs.c | 70 ++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 70 insertions(+)
>>
>> diff --git a/grub-core/fs/btrfs.c b/grub-core/fs/btrfs.c
>> index be195448d..4d418859b 100644
>> --- a/grub-core/fs/btrfs.c
>> +++ b/grub-core/fs/btrfs.c
>> @@ -119,6 +119,8 @@ struct grub_btrfs_chunk_item
>>  #define GRUB_BTRFS_CHUNK_TYPE_RAID1         0x10
>>  #define GRUB_BTRFS_CHUNK_TYPE_DUPLICATED    0x20
>>  #define GRUB_BTRFS_CHUNK_TYPE_RAID10        0x40
>> +#define GRUB_BTRFS_CHUNK_TYPE_RAID5         0x80
>> +#define GRUB_BTRFS_CHUNK_TYPE_RAID6         0x100
>>    grub_uint8_t dummy2[0xc];
>>    grub_uint16_t nstripes;
>>    grub_uint16_t nsubstripes;
>> @@ -764,6 +766,74 @@ grub_btrfs_read_logical (struct grub_btrfs_data *data, 
>> grub_disk_addr_t addr,
>>            stripe_offset = low + chunk_stripe_length
>>              * high;
>>            csize = chunk_stripe_length - low;
>> +          break;
>> +        }
>> +      case GRUB_BTRFS_CHUNK_TYPE_RAID5:
>> +      case GRUB_BTRFS_CHUNK_TYPE_RAID6:
>> +        {
>> +          grub_uint64_t nparities, stripe_nr, high, low;
>> +
>> +          redundancy = 1;   /* no redundancy for now */
>> +
>> +          if (grub_le_to_cpu64 (chunk->type) & GRUB_BTRFS_CHUNK_TYPE_RAID5)
>> +            {
>> +              grub_dprintf ("btrfs", "RAID5\n");
>> +              nparities = 1;
>> +            }
>> +          else
>> +            {
>> +              grub_dprintf ("btrfs", "RAID6\n");
>> +              nparities = 2;
>> +            }
>> +
>> +          /*
>> +           * Below is an example of a RAID 6 layout and the meaning of the
>> +           * variables. The same applies to RAID 5. The only differences is
>> +           * that there is only one parity disk instead of two.
>> +           *
>> +           * A RAID 6 layout consists of several stripes spread
>> +           * on the disks, following a layout like the one below
>> +           *
>> +           *   Disk1  Disk2  Disk3  Ddisk4
> 
> Numbering seems confusing to me. I think that it should be
>                    Disk0  Disk1  Disk2  Disk3
> 
>> +           *
>> +           *    A1     B1     P1     Q1
>> +           *    Q2     A2     B2     P2
>> +           *    P3     Q3     A3     B3
>> +           *  [...]
>> +           *
>> +           *  Note that the placement of the parities depends on row index.
>> +           *  In the code below:
>> +           *  - stripe_nr is the stripe number not considering the parities
>> +           *    (A1=0, B1=1, A2 = 2, B2 = 3, ...),
> 
> Please be consistent. A1 = 0, B1 = 1, A2 = 2, B2 = 3, ...
> 
>> +           *  - high is the row number (0 for A1...Q1, 1 for Q2..P2, ...),
> 
> Ditto. Please always use "..." not "..".
> 
>> +           *  - stripen is the column number (or disk number),
> 
> AIUI starting from 0. Right? If yes then I think that
> drawing above requires disks/columns renumbering.
right
> 
>> +           *  - off is the logical address to read (from the beginning of
>> +           *    the chunk space),
> 
> s/chunk space/chunk/?
> 
>> +           *  - chunk_stripe_length is the size of a stripe (typically 64k),
>> +           *  - nstripes is the number of disks,
>> +           *  - low is the offset of the data inside a stripe,
>> +           *  - stripe_offset is the offset from the beginning of the chunk
>> +           *    disks physical address,
> 
> I am not sure that I understand. Could clarify this?
- stripe_offset is the offset (in bytes) from the beginning of the chunk 
portion 
  stored on disk.

You can think "stripe_offset" as the "row" in the drawing, but measured in 
bytes.

> 
>> +           *  - csize is the "potential" data to read. It will be reduced to
>> +           *    size if the latter is smaller.
>> +           */
>> +          stripe_nr = grub_divmod64 (off, chunk_stripe_length, &low);
> 
> OK.
> 
>> +          /*
>> +           * stripen is evaluated without considering
>> +           * the parities (0 for A1, A2, A3... 1 for B1, B2...).
>> +           */
>> +          high = grub_divmod64 (stripe_nr, nstripes - nparities, &stripen);
> 
> OK.
> 
>> +          /*
>> +           * stripen now considers also the parities (0 for A1, 1 for A2,
>> +           * 2 for A3....). The math is performed modulo number of disks.
>> +           */
>> +          grub_divmod64 (high + stripen, nstripes, &stripen);
> 
> OK.
> 
>> +          stripe_offset = low + chunk_stripe_length * high;
> 
> Hmmm... I am confused. What does it mean?
> 
> Daniel
> 
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> 


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