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

[Daniel Kiper]

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.

> +            *  - 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?

> +            *  - 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