Re: [PATCH v2 5/5] md/md-llbitmap: optimize initial sync with write_zeroes_unmap support

[Xiao Ni]

On Mon, Feb 23, 2026 at 10:41 AM Yu Kuai <yukuai@xxxxxxxxx> wrote:
>
> For RAID-456 arrays with llbitmap, if all underlying disks support
> write_zeroes with unmap, issue write_zeroes to zero all disk data
> regions and initialize the bitmap to BitCleanUnwritten instead of
> BitUnwritten.
>
> This optimization skips the initial XOR parity building because:
> 1. write_zeroes with unmap guarantees zeroed reads after the operation
> 2. For RAID-456, when all data is zero, parity is automatically
>    consistent (0 XOR 0 XOR ... = 0)
> 3. BitCleanUnwritten indicates parity is valid but no user data
>    has been written
>
> The implementation adds two helper functions:
> - llbitmap_all_disks_support_wzeroes_unmap(): Checks if all active
>   disks support write_zeroes with unmap
> - llbitmap_zero_all_disks(): Issues blkdev_issue_zeroout() to each
>   rdev's data region to zero all disks
>
> The zeroing and bitmap state setting happens in llbitmap_init_state()
> during bitmap initialization. If any disk fails to zero, we fall back
> to BitUnwritten and normal lazy recovery.
>
> This significantly reduces array initialization time for RAID-456
> arrays built on modern NVMe SSDs or other devices that support
> write_zeroes with unmap.
>
> Signed-off-by: Yu Kuai <yukuai@xxxxxxxxx>
> ---
>  drivers/md/md-llbitmap.c | 62 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 61 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/md-llbitmap.c b/drivers/md/md-llbitmap.c
> index 461050b2771b..48bc6a639edd 100644
> --- a/drivers/md/md-llbitmap.c
> +++ b/drivers/md/md-llbitmap.c
> @@ -654,13 +654,73 @@ static int llbitmap_cache_pages(struct llbitmap *llbitmap)
>         return 0;
>  }
>
> +/*
> + * Check if all underlying disks support write_zeroes with unmap.
> + */
> +static bool llbitmap_all_disks_support_wzeroes_unmap(struct llbitmap *llbitmap)
> +{
> +       struct mddev *mddev = llbitmap->mddev;
> +       struct md_rdev *rdev;
> +
> +       rdev_for_each(rdev, mddev) {
> +               if (rdev->raid_disk < 0 || test_bit(Faulty, &rdev->flags))
> +                       continue;
> +
> +               if (bdev_write_zeroes_unmap_sectors(rdev->bdev) == 0)
> +                       return false;
> +       }
> +
> +       return true;
> +}
> +
> +/*
> + * Issue write_zeroes to all underlying disks to zero their data regions.
> + * This ensures parity consistency for RAID-456 (0 XOR 0 = 0).
> + * Returns true if all disks were successfully zeroed.
> + */
> +static bool llbitmap_zero_all_disks(struct llbitmap *llbitmap)
> +{
> +       struct mddev *mddev = llbitmap->mddev;
> +       struct md_rdev *rdev;
> +       sector_t dev_sectors = mddev->dev_sectors;
> +       int ret;
> +
> +       rdev_for_each(rdev, mddev) {
> +               if (rdev->raid_disk < 0 || test_bit(Faulty, &rdev->flags))
> +                       continue;
> +
> +               ret = blkdev_issue_zeroout(rdev->bdev,
> +                                          rdev->data_offset,
> +                                          dev_sectors,
> +                                          GFP_KERNEL, 0);
> +               if (ret) {
> +                       pr_warn("md/llbitmap: failed to zero disk %pg: %d\n",
> +                               rdev->bdev, ret);
> +                       return false;
> +               }
> +       }
> +
> +       return true;
> +}
> +
>  static void llbitmap_init_state(struct llbitmap *llbitmap)
>  {
> +       struct mddev *mddev = llbitmap->mddev;
>         enum llbitmap_state state = BitUnwritten;
>         unsigned long i;
>
> -       if (test_and_clear_bit(BITMAP_CLEAN, &llbitmap->flags))
> +       if (test_and_clear_bit(BITMAP_CLEAN, &llbitmap->flags)) {
>                 state = BitClean;
> +       } else if (raid_is_456(mddev) &&
> +                  llbitmap_all_disks_support_wzeroes_unmap(llbitmap)) {
> +               /*
> +                * All disks support write_zeroes with unmap. Zero all disks
> +                * to ensure parity consistency, then set BitCleanUnwritten
> +                * to skip initial sync.
> +                */
> +               if (llbitmap_zero_all_disks(llbitmap))
> +                       state = BitCleanUnwritten;
> +       }
>
>         for (i = 0; i < llbitmap->chunks; i++)
>                 llbitmap_write(llbitmap, state, i);
> --
> 2.51.0
>
>

Hi Kuai

This patch looks good to me.

Reviewed-by: Xiao Ni <xni@xxxxxxxxxx>