RAID-Z

	raidz_row_t *rr;
	/* The starting RAIDZ (parent) vdev sector of the block. */
	uint64_t b = zio->io_offset >> ashift;
	/* The zio's size in units of the vdev's minimum sector size. */
	uint64_t s = zio->io_size >> ashift;
	/* The first column for this stripe. */
	uint64_t f = b % dcols;
	/* The starting byte offset on each child vdev. */
	uint64_t o = (b / dcols) << ashift;
	uint64_t acols, scols;

	raidz_map_t *rm =
	    kmem_zalloc(offsetof(raidz_map_t, rm_row[1]), KM_SLEEP);
	rm->rm_nrows = 1;

	/*
	 * "Quotient": The number of data sectors for this stripe on all but
	 * the "big column" child vdevs that also contain "remainder" data.
	 */
	uint64_t q = s / (dcols - nparity);

	/*
	 * "Remainder": The number of partial stripe data sectors in this I/O.
	 * This will add a sector to some, but not all, child vdevs.
	 */
	uint64_t r = s - q * (dcols - nparity);

	/* The number of "big columns" - those which contain remainder data. */
	uint64_t bc = (r == 0 ? 0 : r + nparity);

	/*
	 * The total number of data and parity sectors associated with
	 * this I/O.
	 */
	uint64_t tot = s + nparity * (q + (r == 0 ? 0 : 1));

	for (uint64_t c = 0; c < scols; c++) {
		raidz_col_t *rc = &rr->rr_col[c];
		uint64_t col = f + c;
		uint64_t coff = o;
		if (col >= dcols) {
			col -= dcols;
			coff += 1ULL << ashift;
		}
		rc->rc_devidx = col;
		rc->rc_offset = coff;

		if (c >= acols)
			rc->rc_size = 0;
		else if (c < bc)
			rc->rc_size = (q + 1) << ashift;
		else
			rc->rc_size = q << ashift;

		asize += rc->rc_size;
	}

	ASSERT3U(asize, ==, tot << ashift);
	rm->rm_nskip = roundup(tot, nparity + 1) - tot;
	rm->rm_skipstart = bc;

static uint64_t
vdev_raidz_psize_to_asize(vdev_t *vd, uint64_t psize, uint64_t txg)
{
	vdev_raidz_t *vdrz = vd->vdev_tsd;
	uint64_t asize;
	uint64_t ashift = vd->vdev_top->vdev_ashift;
	uint64_t nparity = vdrz->vd_nparity;

	uint64_t cols = vdev_raidz_get_logical_width(vdrz, txg);

	asize = ((psize - 1) >> ashift) + 1;
	asize += nparity * ((asize + cols - nparity - 1) / (cols - nparity));
	asize = roundup(asize, nparity + 1) << ashift;

	for (int num_failures = 1; num_failures <= nparity; num_failures++) {
		int tstore[VDEV_RAIDZ_MAXPARITY + 2];
		int *ltgts = &tstore[1]; /* value is logical child ID */


		/*
		 * Determine number of logical children, n.  See comment
		 * above raidz_simulate_failure().
		 */
		int n = 0;
		for (int w = physical_width;
		    w >= original_width; w--) {
			n += w;
		}

		ASSERT3U(num_failures, <=, nparity);
		ASSERT3U(num_failures, <=, VDEV_RAIDZ_MAXPARITY);

		/* Handle corner cases in combrec logic */
		ltgts[-1] = -1;
		for (int i = 0; i < num_failures; i++) {
			ltgts[i] = i;
		}
		ltgts[num_failures] = n;

		for (;;) {
			int err = raidz_reconstruct(zio, ltgts, num_failures,
			    nparity);
			if (err == EINVAL) {
				/*
				 * Reconstruction not possible with this #
				 * failures; try more failures.
				 */
				break;
			} else if (err == 0)
				return (0);

	uint64_t s = ((psize - 1) >> ashift) + 1;
	/* The first column for this stripe. */
	uint64_t f = b % dcols;

	/* Unreachable by sequential resilver. */
	ASSERT3U(phys_birth, !=, TXG_UNKNOWN);

	if (!vdev_dtl_contains(vd, DTL_PARTIAL, phys_birth, 1))
		return (B_FALSE);

	if (s + nparity >= dcols)
		return (B_TRUE);

	for (uint64_t c = 0; c < s + nparity; c++) {
		uint64_t devidx = (f + c) % dcols;
		vdev_t *cvd = vd->vdev_child[devidx];

		/*
		 * dsl_scan_need_resilver() already checked vd with
		 * vdev_dtl_contains(). So here just check cvd with
		 * vdev_dtl_empty(), cheaper and a good approximation.
		 */
		if (!vdev_dtl_empty(cvd, DTL_PARTIAL))
			return (B_TRUE);
	}

	return (B_FALSE);