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

[Daniel Kiper]

On Wed, May 16, 2018 at 08:48:11PM +0200, Goffredo Baroncelli wrote:
> Signed-off-by: Goffredo Baroncelli <address@hidden>
> ---
>  grub-core/fs/btrfs.c | 68 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
>
> diff --git a/grub-core/fs/btrfs.c b/grub-core/fs/btrfs.c
> index be195448d..394611cbb 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 +766,72 @@ 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 an example of a RAID6 layout and the meaning of the

s/an example of a RAID6/is an example of a RAID 6/

> +            * variables. The same apply to RAID5. The only differences is 
> that

s/The same apply to RAID5/The same applies to RAID 5/

> +            * there is only one parity instead of two.

s/parity/parity disk/

> +            *
> +            * A RAID6 6 layout consists in several stripes spread

s/RAID6 6/RAID 6/
s/consists in/consists of/

> +            * on the disks, following a layout like the one below

s/on the disks/over the disks/

> +            *
> +            *   Disk1  Disk2  Disk3  Ddisk4
> +            *
> +            *    A1     B1     P1     Q1
> +            *    Q2     A2     B2     P2
> +            *    P3     Q3     A3     B3
> +            *  [...]
> +            *
> +            *  Note that the placement of the parities depends on row index;

Please, please, please, do not abuse ";". Full stop is preffered at the
end of sentence. Please fix this in all patches.

> +            *  In the code below:
> +            *  stripe_nr -> is the stripe number not considering the parities
> +            *               (A1=0, B1=1, A2 = 2, B2 = 3, ...)

May I ask you to do this like that.

The variables below have following meaning:
  - stripe_nr is the stripe number not considering the parities
    (A1 = 0, B1 = 1, A2 = 2, B2 = 3, ...),
  - high is the row number (0 for A1...Q1, 1 for Q2...P2, ...),
    ...
  - low is the offset of the data inside a stripe.

> +            *  high -> is the row number (0 for A1...Q1, 1 for Q2..P2, ...)
> +            *  stripen -> is the column number (or disk number)
> +            *  off -> logical address to read (from the beginning of the
> +            *         chunk space)

What do you mean by "chunk"? Is it e.g. A1 + B1 region? Please make it
clear what do you mean by "chunk".

> +            *  chunk_stripe_length -> size of a stripe (typically 64k)
> +            *  nstripes -> number of disks
> +            *  low -> offset of the data inside a stripe

It looks that stripe_offset and csize explanation is missing here.

> +            *
> +            */
> +           stripe_nr = grub_divmod64 (off, chunk_stripe_length, &low);
> +
> +           /*
> +            * In the line below stripen is evaluated without considering

s/In the line below //

> +            * the parities (0 for A1, A2, A3... 1 for B1, B2...);

s/;/./

> +            */
> +           high = grub_divmod64 (stripe_nr, nstripes - nparities, &stripen);
> +
> +           /*
> +            * In the line below stripen, now consider also the parities (0

s/In the line below stripen, now/Now/

> +            * for A1, 1 for A2, 2 for A3....); the math is done "modulo"

s/; the/. The/
s/"modulo"/modulo/

> +            * number of disks

Full stop at the end of sentence please.

Daniel