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Single IO issue for raidz writes with skip sector
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In order to reduce contention on the vq_lock, optional skip sectors
for Raidz writes can be placed into a single IO request. This is done by
padding out the linear ABD for a parity column to contain the skip
sector and by creating gang ABD to contain the data and skip sector for
data columns.

The vdev_raidz_map_alloc() function now contains specific functions for
both reads and write to allocate the ABD's that will be issued down to
the VDEV chldren.

Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
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@bwatkinson
bwatkinson committed Nov 5, 2021
1 parent 4b87c19 commit 1b0091f
Showing 1 changed file with 135 additions and 37 deletions.
172 changes: 135 additions & 37 deletions module/zfs/vdev_raidz.c
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Original file line number Diff line number Diff line change
Expand Up @@ -174,6 +174,114 @@ const zio_vsd_ops_t vdev_raidz_vsd_ops = {
.vsd_free = vdev_raidz_map_free_vsd,
};

static void
vdev_raidz_map_alloc_write(zio_t *zio, raidz_map_t *rm, uint64_t ashift)
{
int c;
int nwrapped = 0;
uint64_t off = 0;
raidz_row_t *rr = rm->rm_row[0];

ASSERT3U(zio->io_type, ==, ZIO_TYPE_WRITE);
ASSERT3U(rm->rm_nrows, ==, 1);

/*
* We will pad any parity columns with additional space to account for
* skip sectors.
*/
if (rm->rm_skipstart < rr->rr_firstdatacol) {
ASSERT0(rm->rm_skipstart);
nwrapped = rm->rm_nskip;
} else if (rr->rr_scols < (rm->rm_skipstart + rm->rm_nskip)) {
nwrapped =
(rm->rm_skipstart + rm->rm_nskip) % rr->rr_scols;
}

/*
* Optional single skip sectors (rc_size == 0) will be handled in
* vdev_raidz_io_start_write().
*/
int skipped = rr->rr_scols - rr->rr_cols;

/* Allocate buffers for the parity columns */
for (c = 0; c < rr->rr_firstdatacol; c++) {
raidz_col_t *rc = &rr->rr_col[c];

/*
* Parity columns will pad out a linear ABD to account for
* the skip sector. A linear ABD is used here because
* parity calculations use the ABD buffer directly to calculate
* parity. This avoids doing a memcpy back to the ABD after the
* parity has been calculated. By issuing the parity column
* with the skip sector we can reduce contention on the child
* VDEV queue locks (vq_lock).
*/
if (c < nwrapped) {
rc->rc_abd = abd_alloc_linear(
rc->rc_size + (1ULL << ashift), B_FALSE);
abd_zero_off(rc->rc_abd, rc->rc_size, 1ULL << ashift);
skipped++;
} else {
rc->rc_abd = abd_alloc_linear(rc->rc_size, B_FALSE);
}
}

for (off = 0; c < rr->rr_cols; c++) {
raidz_col_t *rc = &rr->rr_col[c];
abd_t *abd = abd_get_offset_struct(&rc->rc_abdstruct,
zio->io_abd, off, rc->rc_size);

/*
* Generate I/O for skip sectors to improve aggregation
* continuity. We will use gang ABD's to reduce contention
* on the child VDEV queue locks (vq_lock) by issuing
* a single I/O that contains the data and skip sector.
*
* It is important to make sure that rc_size is not updated
* even though we are adding a skip sector to the ABD. With
* calculating the parity in vdev_raidz_generate_parity_row()
* the rc_size is used for iterating through the ABD's. We
* can not have zero'd out skip sectors used for calculating
* parity for raidz, because those same sectors are not used
* during reconstruction.
*/
if (c >= rm->rm_skipstart && skipped < rm->rm_nskip) {
rc->rc_abd = abd_alloc_gang();
abd_gang_add(rc->rc_abd, abd, B_TRUE);
abd_gang_add(rc->rc_abd,
abd_get_zeros(1ULL << ashift), B_TRUE);
skipped++;
} else {
rc->rc_abd = abd;
}
off += rc->rc_size;
}

ASSERT3U(off, ==, zio->io_size);
ASSERT3S(skipped, ==, rm->rm_nskip);
}

static void
vdev_raidz_map_alloc_read(zio_t *zio, raidz_map_t *rm)
{
int c;
raidz_row_t *rr = rm->rm_row[0];

ASSERT3U(rm->rm_nrows, ==, 1);

/* Allocate buffers for the parity columns */
for (c = 0; c < rr->rr_firstdatacol; c++)
rr->rr_col[c].rc_abd =
abd_alloc_linear(rr->rr_col[c].rc_size, B_FALSE);

for (uint64_t off = 0; c < rr->rr_cols; c++) {
raidz_col_t *rc = &rr->rr_col[c];
rc->rc_abd = abd_get_offset_struct(&rc->rc_abdstruct,
zio->io_abd, off, rc->rc_size);
off += rc->rc_size;
}
}

/*
* Divides the IO evenly across all child vdevs; usually, dcols is
* the number of children in the target vdev.
Expand Down Expand Up @@ -287,17 +395,6 @@ vdev_raidz_map_alloc(zio_t *zio, uint64_t ashift, uint64_t dcols,
rm->rm_nskip = roundup(tot, nparity + 1) - tot;
rm->rm_skipstart = bc;

for (c = 0; c < rr->rr_firstdatacol; c++)
rr->rr_col[c].rc_abd =
abd_alloc_linear(rr->rr_col[c].rc_size, B_FALSE);

for (uint64_t off = 0; c < acols; c++) {
raidz_col_t *rc = &rr->rr_col[c];
rc->rc_abd = abd_get_offset_struct(&rc->rc_abdstruct,
zio->io_abd, off, rc->rc_size);
off += rc->rc_size;
}

/*
* If all data stored spans all columns, there's a danger that parity
* will always be on the same device and, since parity isn't read
Expand Down Expand Up @@ -333,6 +430,12 @@ vdev_raidz_map_alloc(zio_t *zio, uint64_t ashift, uint64_t dcols,
rm->rm_skipstart = 1;
}

if (zio->io_type == ZIO_TYPE_WRITE) {
vdev_raidz_map_alloc_write(zio, rm, ashift);
} else {
vdev_raidz_map_alloc_read(zio, rm);
}

/* init RAIDZ parity ops */
rm->rm_ops = vdev_raidz_math_get_ops();

Expand Down Expand Up @@ -1482,6 +1585,7 @@ vdev_raidz_child_done(zio_t *zio)
{
raidz_col_t *rc = zio->io_private;

ASSERT3P(rc->rc_abd, !=, NULL);
rc->rc_error = zio->io_error;
rc->rc_tried = 1;
rc->rc_skipped = 0;
Expand Down Expand Up @@ -1525,40 +1629,34 @@ vdev_raidz_io_start_write(zio_t *zio, raidz_row_t *rr, uint64_t ashift)
{
vdev_t *vd = zio->io_vd;
raidz_map_t *rm = zio->io_vsd;
int c, i;

vdev_raidz_generate_parity_row(rm, rr);

for (int c = 0; c < rr->rr_cols; c++) {
for (int c = 0; c < rr->rr_scols; c++) {
raidz_col_t *rc = &rr->rr_col[c];
if (rc->rc_size == 0)
continue;
vdev_t *cvd = vd->vdev_child[rc->rc_devidx];

/* Verify physical to logical translation */
vdev_raidz_io_verify(vd, rr, c);

zio_nowait(zio_vdev_child_io(zio, NULL,
vd->vdev_child[rc->rc_devidx], rc->rc_offset,
rc->rc_abd, rc->rc_size, zio->io_type, zio->io_priority,
0, vdev_raidz_child_done, rc));
}

/*
* Generate optional I/Os for skip sectors to improve aggregation
* contiguity.
*/
for (c = rm->rm_skipstart, i = 0; i < rm->rm_nskip; c++, i++) {
ASSERT(c <= rr->rr_scols);
if (c == rr->rr_scols)
c = 0;

raidz_col_t *rc = &rr->rr_col[c];
vdev_t *cvd = vd->vdev_child[rc->rc_devidx];

zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
rc->rc_offset + rc->rc_size, NULL, 1ULL << ashift,
zio->io_type, zio->io_priority,
ZIO_FLAG_NODATA | ZIO_FLAG_OPTIONAL, NULL, NULL));
/*
* Generate optional I/O for single skip sector to improve
* aggregation contiguity.
*/
if (rc->rc_size == 0) {
ASSERT3P(rc->rc_abd, ==, NULL);
zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
rc->rc_offset, NULL, 1ULL << ashift,
zio->io_type, zio->io_priority,
ZIO_FLAG_NODATA | ZIO_FLAG_OPTIONAL, NULL,
NULL));
} else {
ASSERT3P(rc->rc_abd, !=, NULL);
zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
rc->rc_offset, rc->rc_abd,
abd_get_size(rc->rc_abd), zio->io_type,
zio->io_priority, 0, vdev_raidz_child_done, rc));
}
}
}

Expand Down

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