/*
 * This is the size of the cache, minus the amount used for
 * flash-only devices
 */
uint64_t cache_sectors = c->nbuckets * c->sb.bucket_size - atomic_long_read(&c->flash_dev_dirty_sectors);

/*
 * Unfortunately there is no control of global dirty data.  If the
 * user states that they want 10% dirty data in the cache, and has,
 * e.g., 5 backing volumes of equal size, we try and ensure each
 * backing volume uses about 2% of the cache for dirty data.
 */
uint32_t bdev_share = div64_u64(bdev_sectors(dc->bdev) << WRITEBACK_SHARE_SHIFT, c->cached_dev_sectors);

uint64_t cache_dirty_target = div_u64(cache_sectors * dc->writeback_percent, 100);

/* Ensure each backing dev gets at least one dirty share */
if (bdev_share < 1)
    bdev_share = 1;

return (cache_dirty_target * bdev_share) >> WRITEBACK_SHARE_SHIFT;