source: mainline/uspace/srv/bd/hr/raid5.c

/*
 * Copyright (c) 2025 Miroslav Cimerman <mc@doas.su>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/** @addtogroup hr
 * @{
 */
/**
 * @file
 */

#include <abi/ipc/ipc.h>
#include <bd_srv.h>
#include <block.h>
#include <errno.h>
#include <hr.h>
#include <inttypes.h>
#include <io/log.h>
#include <ipc/hr.h>
#include <ipc/services.h>
#include <loc.h>
#include <mem.h>
#include <task.h>
#include <stdio.h>
#include <stdlib.h>
#include <str_error.h>

#include "io.h"
#include "parity_stripe.h"
#include "superblock.h"
#include "util.h"
#include "var.h"

static void hr_raid5_vol_state_eval_forced(hr_volume_t *);
static size_t hr_raid5_parity_extent(hr_level_t, hr_layout_t, size_t,
    uint64_t);
static size_t hr_raid5_data_extent(hr_level_t, hr_layout_t, size_t, uint64_t,
    size_t);
static errno_t hr_raid5_rebuild(void *);

/* bdops */
static errno_t hr_raid5_bd_open(bd_srvs_t *, bd_srv_t *);
static errno_t hr_raid5_bd_close(bd_srv_t *);
static errno_t hr_raid5_bd_read_blocks(bd_srv_t *, aoff64_t, size_t, void *,
    size_t);
static errno_t hr_raid5_bd_sync_cache(bd_srv_t *, aoff64_t, size_t);
static errno_t hr_raid5_bd_write_blocks(bd_srv_t *, aoff64_t, size_t,
    const void *, size_t);
static errno_t hr_raid5_bd_get_block_size(bd_srv_t *, size_t *);
static errno_t hr_raid5_bd_get_num_blocks(bd_srv_t *, aoff64_t *);

static bd_ops_t hr_raid5_bd_ops = {
        .open = hr_raid5_bd_open,
        .close = hr_raid5_bd_close,
        .sync_cache = hr_raid5_bd_sync_cache,
        .read_blocks = hr_raid5_bd_read_blocks,
        .write_blocks = hr_raid5_bd_write_blocks,
        .get_block_size = hr_raid5_bd_get_block_size,
        .get_num_blocks = hr_raid5_bd_get_num_blocks
};

extern loc_srv_t *hr_srv;

errno_t hr_raid5_create(hr_volume_t *new_volume)
{
        HR_DEBUG("%s()", __func__);

        if (new_volume->level != HR_LVL_5 && new_volume->level != HR_LVL_4)
                return EINVAL;

        if (new_volume->extent_no < 3) {
                HR_ERROR("RAID 5 volume needs at least 3 devices\n");
                return EINVAL;
        }

        hr_raid5_vol_state_eval_forced(new_volume);

        fibril_rwlock_read_lock(&new_volume->states_lock);
        hr_vol_state_t state = new_volume->state;
        fibril_rwlock_read_unlock(&new_volume->states_lock);
        if (state == HR_VOL_FAULTY || state == HR_VOL_NONE) {
                HR_NOTE("\"%s\": unusable state, not creating\n",
                    new_volume->devname);
                return EINVAL;
        }

        bd_srvs_init(&new_volume->hr_bds);
        new_volume->hr_bds.ops = &hr_raid5_bd_ops;
        new_volume->hr_bds.sarg = new_volume;

        return EOK;
}

/*
 * Called only once in volume's lifetime.
 */
errno_t hr_raid5_init(hr_volume_t *vol)
{
        HR_DEBUG("%s()", __func__);

        if (vol->level != HR_LVL_5 && vol->level != HR_LVL_4)
                return EINVAL;

        vol->data_offset = vol->meta_ops->get_data_offset();

        uint64_t single_sz = vol->truncated_blkno - vol->meta_ops->get_size();
        vol->data_blkno = single_sz * (vol->extent_no - 1);

        vol->strip_size = hr_closest_pow2(HR_STRIP_SIZE / (vol->extent_no - 1));

        if (vol->level == HR_LVL_4)
                vol->layout = HR_LAYOUT_RAID4_N;
        else
                vol->layout = HR_LAYOUT_RAID5_NR;

        return EOK;
}

void hr_raid5_vol_state_eval(hr_volume_t *vol)
{
        HR_DEBUG("%s()", __func__);

        bool exp = true;
        if (!atomic_compare_exchange_strong(&vol->state_dirty, &exp, false))
                return;

        vol->meta_ops->inc_counter(vol);
        vol->meta_ops->save(vol, WITH_STATE_CALLBACK);

        hr_raid5_vol_state_eval_forced(vol);
}

void hr_raid5_ext_state_cb(hr_volume_t *vol, size_t extent, errno_t rc)
{
        HR_DEBUG("%s()", __func__);

        assert(fibril_rwlock_is_locked(&vol->extents_lock));

        if (rc == EOK)
                return;

        fibril_rwlock_write_lock(&vol->states_lock);

        switch (rc) {
        case ENOENT:
                hr_update_ext_state(vol, extent, HR_EXT_MISSING);
                break;
        default:
                hr_update_ext_state(vol, extent, HR_EXT_FAILED);
        }

        hr_mark_vol_state_dirty(vol);

        fibril_rwlock_write_unlock(&vol->states_lock);
}

static errno_t hr_raid5_bd_open(bd_srvs_t *bds, bd_srv_t *bd)
{
        HR_DEBUG("%s()\n", __func__);

        hr_volume_t *vol = bd->srvs->sarg;

        atomic_fetch_add_explicit(&vol->open_cnt, 1, memory_order_relaxed);

        return EOK;
}

static errno_t hr_raid5_bd_close(bd_srv_t *bd)
{
        HR_DEBUG("%s()\n", __func__);

        hr_volume_t *vol = bd->srvs->sarg;

        atomic_fetch_sub_explicit(&vol->open_cnt, 1, memory_order_relaxed);

        return EOK;
}

static errno_t hr_raid5_bd_sync_cache(bd_srv_t *bd, aoff64_t ba, size_t cnt)
{
        hr_volume_t *vol = bd->srvs->sarg;

        return hr_sync_extents(vol);
}

static errno_t hr_raid5_bd_read_blocks(bd_srv_t *bd, uint64_t ba, size_t cnt,
    void *data_read, size_t size)
{
        hr_volume_t *vol = bd->srvs->sarg;
        errno_t rc;

        if (size < cnt * vol->bsize)
                return EINVAL;

        fibril_rwlock_read_lock(&vol->states_lock);
        hr_vol_state_t vol_state = vol->state;
        fibril_rwlock_read_unlock(&vol->states_lock);

        if (vol_state == HR_VOL_FAULTY || vol_state == HR_VOL_NONE)
                return EIO;

        rc = hr_check_ba_range(vol, cnt, ba);
        if (rc != EOK)
                return rc;

        uint64_t strip_size = vol->strip_size / vol->bsize; /* in blocks */
        uint64_t strip_no = ba / strip_size;

        /* calculate number of stripes touched */
        uint64_t last_ba = ba + cnt - 1;
        uint64_t end_strip_no = last_ba / strip_size;
        uint64_t start_stripe = strip_no / (vol->extent_no - 1);
        uint64_t end_stripe = end_strip_no / (vol->extent_no - 1);
        size_t stripes_cnt = end_stripe - start_stripe + 1;

        hr_stripe_t *stripes = hr_create_stripes(vol, vol->strip_size,
            stripes_cnt, false);

        uint64_t phys_block, len;
        size_t left;

        hr_layout_t layout = vol->layout;
        hr_level_t level = vol->level;

        /* parity extent */
        size_t p_extent = hr_raid5_parity_extent(level, layout,
            vol->extent_no, strip_no);

        uint64_t strip_off = ba % strip_size;

        left = cnt;

        while (left != 0) {
                if (level == HR_LVL_5) {
                        p_extent = hr_raid5_parity_extent(level, layout,
                            vol->extent_no, strip_no);
                }

                size_t extent = hr_raid5_data_extent(level, layout,
                    vol->extent_no, strip_no, p_extent);

                uint64_t stripe_no = strip_no / (vol->extent_no - 1);
                size_t relative_si = stripe_no - start_stripe; /* relative stripe index */
                hr_stripe_t *stripe = &stripes[relative_si];
                stripe->p_extent = p_extent;

                stripe->strips_touched++;

                phys_block = stripe_no * strip_size + strip_off;
                cnt = min(left, strip_size - strip_off);
                len = vol->bsize * cnt;
                hr_add_data_offset(vol, &phys_block);

                stripe->extent_span[extent].range.start = phys_block;
                stripe->extent_span[extent].range.end = phys_block + cnt - 1;
                stripe->extent_span[extent].cnt = cnt;
                stripe->extent_span[extent].data_read = data_read;
                stripe->extent_span[extent].strip_off = strip_off;

                data_read += len;
                left -= cnt;
                strip_off = 0;
                strip_no++;
        }

        hr_range_lock_t **rlps = hr_malloc_waitok(stripes_cnt * sizeof(*rlps));

        /*
         * extent order has to be locked for the whole IO duration,
         * so that workers have consistent targets
         */
        fibril_rwlock_read_lock(&vol->extents_lock);

        for (uint64_t s = start_stripe; s <= end_stripe; s++) {
                uint64_t relative = s - start_stripe;
                rlps[relative] = hr_range_lock_acquire(vol, s, 1);
        }

retry:
        size_t bad_extent = vol->extent_no;

        uint64_t rebuild_pos = atomic_load_explicit(&vol->rebuild_blk,
            memory_order_relaxed);

        fibril_rwlock_read_lock(&vol->states_lock);

        for (size_t e = 0; e < vol->extent_no; e++) {
                hr_ext_state_t s = vol->extents[e].state;
                if ((vol->state == HR_VOL_DEGRADED && s != HR_EXT_ONLINE) ||
                    (s == HR_EXT_REBUILD && end_stripe >= rebuild_pos)) {
                        bad_extent = e;
                        break;
                }
        }

        fibril_rwlock_read_unlock(&vol->states_lock);

        for (size_t s = 0; s < stripes_cnt; s++) {
                if (stripes[s].done)
                        continue;
                hr_execute_stripe(&stripes[s], bad_extent);
        }

        for (size_t s = 0; s < stripes_cnt; s++) {
                if (stripes[s].done)
                        continue;
                hr_wait_for_stripe(&stripes[s]);
        }

        hr_raid5_vol_state_eval(vol);

        rc = EOK;

        fibril_rwlock_read_lock(&vol->states_lock);

        if (vol->state == HR_VOL_FAULTY) {
                fibril_rwlock_read_unlock(&vol->states_lock);
                rc = EIO;
                goto end;
        }

        fibril_rwlock_read_unlock(&vol->states_lock);

        for (size_t s = 0; s < stripes_cnt; s++)
                if (stripes[s].rc == EAGAIN)
                        goto retry;

        /* all stripes are done */
end:
        fibril_rwlock_read_unlock(&vol->extents_lock);

        for (size_t i = 0; i < stripes_cnt; i++)
                hr_range_lock_release(rlps[i]);

        free(rlps);

        hr_destroy_stripes(stripes, stripes_cnt);

        return rc;
}

static errno_t hr_raid5_bd_write_blocks(bd_srv_t *bd, aoff64_t ba, size_t cnt,
    const void *data_write, size_t size)
{
        hr_volume_t *vol = bd->srvs->sarg;
        errno_t rc;

        if (size < cnt * vol->bsize)
                return EINVAL;

        if (vol->vflags & HR_VOL_FLAG_READ_ONLY)
                return ENOTSUP;

        fibril_rwlock_read_lock(&vol->states_lock);
        hr_vol_state_t vol_state = vol->state;
        fibril_rwlock_read_unlock(&vol->states_lock);

        if (vol_state == HR_VOL_FAULTY || vol_state == HR_VOL_NONE)
                return EIO;

        /* increment metadata counter only on first write */
        bool exp = false;
        if (atomic_compare_exchange_strong(&vol->first_write, &exp, true)) {
                vol->meta_ops->inc_counter(vol);
                vol->meta_ops->save(vol, WITH_STATE_CALLBACK);
        }

        rc = hr_check_ba_range(vol, cnt, ba);
        if (rc != EOK)
                return rc;

        uint64_t strip_size = vol->strip_size / vol->bsize; /* in blocks */
        uint64_t strip_no = ba / strip_size;

        /* calculate number of stripes touched */
        uint64_t last_ba = ba + cnt - 1;
        uint64_t end_strip_no = last_ba / strip_size;
        uint64_t start_stripe = strip_no / (vol->extent_no - 1);
        uint64_t end_stripe = end_strip_no / (vol->extent_no - 1);
        size_t stripes_cnt = end_stripe - start_stripe + 1;

        hr_stripe_t *stripes = hr_create_stripes(vol, vol->strip_size,
            stripes_cnt, true);

        uint64_t stripe_size = strip_size * (vol->extent_no - 1);

        for (uint64_t stripe = start_stripe; stripe <= end_stripe; stripe++) {
                uint64_t relative_stripe = stripe - start_stripe;

                uint64_t s_start = stripe * stripe_size;
                uint64_t s_end = s_start + stripe_size - 1;

                uint64_t overlap_start;
                if (ba > s_start)
                        overlap_start = ba;
                else
                        overlap_start = s_start;

                uint64_t overlap_end;
                if (last_ba < s_end)
                        overlap_end = last_ba;
                else
                        overlap_end = s_end;

                uint64_t start_strip_index =
                    (overlap_start - s_start) / strip_size;
                uint64_t end_strip_index = (overlap_end - s_start) / strip_size;
                size_t strips_touched = end_strip_index - start_strip_index + 1;

                stripes[relative_stripe].strips_touched = strips_touched;

                uint64_t first_offset = (overlap_start - s_start) % strip_size;
                uint64_t last_offset = (overlap_end - s_start) % strip_size;

                size_t partials = 0;
                if (first_offset != 0)
                        partials++;
                if (last_offset != strip_size - 1)
                        partials++;
                if (start_strip_index == end_strip_index && partials == 2)
                        partials = 1;

                stripes[relative_stripe].strips_touched = strips_touched;
                stripes[relative_stripe].partial_strips_touched = partials;

                if (strips_touched < (vol->extent_no - 1) / 2)
                        stripes[relative_stripe].subtract = true;
        }

        uint64_t phys_block, len;
        size_t left;

        hr_layout_t layout = vol->layout;
        hr_level_t level = vol->level;

        /* parity extent */
        size_t p_extent = hr_raid5_parity_extent(level, layout,
            vol->extent_no, strip_no);

        uint64_t strip_off = ba % strip_size;

        left = cnt;

        while (left != 0) {
                if (level == HR_LVL_5) {
                        p_extent = hr_raid5_parity_extent(level, layout,
                            vol->extent_no, strip_no);
                }

                size_t extent = hr_raid5_data_extent(level, layout,
                    vol->extent_no, strip_no, p_extent);

                uint64_t stripe_no = strip_no / (vol->extent_no - 1);
                size_t relative_si = stripe_no - start_stripe; /* relative stripe index */
                hr_stripe_t *stripe = &stripes[relative_si];
                stripe->p_extent = p_extent;

                phys_block = stripe_no * strip_size + strip_off;
                cnt = min(left, strip_size - strip_off);
                len = vol->bsize * cnt;
                hr_add_data_offset(vol, &phys_block);

                stripe->extent_span[extent].range.start = phys_block;
                stripe->extent_span[extent].range.end = phys_block + cnt - 1;
                stripe->extent_span[extent].cnt = cnt;
                stripe->extent_span[extent].data_write = data_write;
                stripe->extent_span[extent].strip_off = strip_off;

                data_write += len;
                left -= cnt;
                strip_off = 0;
                strip_no++;
        }

        hr_range_lock_t **rlps = hr_malloc_waitok(stripes_cnt * sizeof(*rlps));

        /*
         * extent order has to be locked for the whole IO duration,
         * so that workers have consistent targets
         */
        fibril_rwlock_read_lock(&vol->extents_lock);

        for (uint64_t s = start_stripe; s <= end_stripe; s++) {
                uint64_t relative = s - start_stripe;
                rlps[relative] = hr_range_lock_acquire(vol, s, 1);
        }

retry:
        size_t bad_extent = vol->extent_no;

        uint64_t rebuild_pos = atomic_load_explicit(&vol->rebuild_blk,
            memory_order_relaxed);

        fibril_rwlock_read_lock(&vol->states_lock);

        for (size_t e = 0; e < vol->extent_no; e++) {
                hr_ext_state_t s = vol->extents[e].state;
                if ((vol->state == HR_VOL_DEGRADED && s != HR_EXT_ONLINE) ||
                    (s == HR_EXT_REBUILD && start_stripe > rebuild_pos)) {
                        bad_extent = e;
                        break;
                }
        }

        fibril_rwlock_read_unlock(&vol->states_lock);

        for (size_t s = 0; s < stripes_cnt; s++) {
                if (stripes[s].done)
                        continue;
                hr_execute_stripe(&stripes[s], bad_extent);
        }

        for (size_t s = 0; s < stripes_cnt; s++) {
                if (stripes[s].done)
                        continue;
                hr_wait_for_stripe(&stripes[s]);
        }

        hr_raid5_vol_state_eval(vol);

        rc = EOK;

        fibril_rwlock_read_lock(&vol->states_lock);

        if (vol->state == HR_VOL_FAULTY) {
                fibril_rwlock_read_unlock(&vol->states_lock);
                rc = EIO;
                goto end;
        }

        fibril_rwlock_read_unlock(&vol->states_lock);

        for (size_t s = 0; s < stripes_cnt; s++)
                if (stripes[s].rc == EAGAIN)
                        goto retry;

        /* all stripes are done */
end:
        fibril_rwlock_read_unlock(&vol->extents_lock);

        for (size_t i = 0; i < stripes_cnt; i++)
                hr_range_lock_release(rlps[i]);

        free(rlps);

        hr_destroy_stripes(stripes, stripes_cnt);

        return rc;
}

static errno_t hr_raid5_bd_get_block_size(bd_srv_t *bd, size_t *rsize)
{
        hr_volume_t *vol = bd->srvs->sarg;

        *rsize = vol->bsize;
        return EOK;
}

static errno_t hr_raid5_bd_get_num_blocks(bd_srv_t *bd, aoff64_t *rnb)
{
        hr_volume_t *vol = bd->srvs->sarg;

        *rnb = vol->data_blkno;
        return EOK;
}

static void hr_raid5_vol_state_eval_forced(hr_volume_t *vol)
{
        fibril_rwlock_read_lock(&vol->extents_lock);
        fibril_rwlock_write_lock(&vol->states_lock);

        hr_vol_state_t state = vol->state;

        size_t bad = 0;
        for (size_t i = 0; i < vol->extent_no; i++)
                if (vol->extents[i].state != HR_EXT_ONLINE)
                        bad++;

        size_t invalid_no = hr_count_extents(vol, HR_EXT_INVALID);

        size_t rebuild_no = hr_count_extents(vol, HR_EXT_REBUILD);

        fibril_mutex_lock(&vol->hotspare_lock);
        size_t hs_no = vol->hotspare_no;
        fibril_mutex_unlock(&vol->hotspare_lock);

        switch (bad) {
        case 0:
                if (state != HR_VOL_OPTIMAL)
                        hr_update_vol_state(vol, HR_VOL_OPTIMAL);
                break;
        case 1:
                if (state != HR_VOL_DEGRADED && state != HR_VOL_REBUILD)
                        hr_update_vol_state(vol, HR_VOL_DEGRADED);

                if (state != HR_VOL_REBUILD) {
                        if (hs_no > 0 || invalid_no > 0 || rebuild_no > 0) {
                                fid_t fib = fibril_create(hr_raid5_rebuild,
                                    vol);
                                if (fib == 0)
                                        break;
                                fibril_start(fib);
                                fibril_detach(fib);
                        }
                }
                break;
        default:
                if (state != HR_VOL_FAULTY)
                        hr_update_vol_state(vol, HR_VOL_FAULTY);
                break;
        }

        fibril_rwlock_write_unlock(&vol->states_lock);
        fibril_rwlock_read_unlock(&vol->extents_lock);
}

static size_t hr_raid5_parity_extent(hr_level_t level,
    hr_layout_t layout, size_t extent_no, uint64_t strip_no)
{
        switch (level) {
        case HR_LVL_4:
                switch (layout) {
                case HR_LAYOUT_RAID4_0:
                        return (0);
                case HR_LAYOUT_RAID4_N:
                        return (extent_no - 1);
                default:
                        assert(0 && "invalid layout configuration");
                }
        case HR_LVL_5:
                switch (layout) {
                case HR_LAYOUT_RAID5_0R:
                        return ((strip_no / (extent_no - 1)) % extent_no);
                case HR_LAYOUT_RAID5_NR:
                case HR_LAYOUT_RAID5_NC:
                        return ((extent_no - 1) -
                            (strip_no / (extent_no - 1)) % extent_no);
                default:
                        assert(0 && "invalid layout configuration");
                }
        default:
                assert(0 && "invalid layout configuration");
        }
}

static size_t hr_raid5_data_extent(hr_level_t level,
    hr_layout_t layout, size_t extent_no, uint64_t strip_no, size_t p_extent)
{
        switch (level) {
        case HR_LVL_4:
                switch (layout) {
                case HR_LAYOUT_RAID4_0:
                        return ((strip_no % (extent_no - 1)) + 1);
                case HR_LAYOUT_RAID4_N:
                        return (strip_no % (extent_no - 1));
                default:
                        assert(0 && "invalid layout configuration");
                }
        case HR_LVL_5:
                switch (layout) {
                case HR_LAYOUT_RAID5_0R:
                case HR_LAYOUT_RAID5_NR:
                        if ((strip_no % (extent_no - 1)) < p_extent)
                                return (strip_no % (extent_no - 1));
                        else
                                return ((strip_no % (extent_no - 1)) + 1);
                case HR_LAYOUT_RAID5_NC:
                        return (((strip_no % (extent_no - 1)) + p_extent + 1) %
                            extent_no);
                default:
                        assert(0 && "invalid layout configuration");
                }
        default:
                assert(0 && "invalid layout configuration");
        }
}

static errno_t hr_raid5_rebuild(void *arg)
{
        HR_DEBUG("%s()", __func__);

        hr_volume_t *vol = arg;
        errno_t rc = EOK;
        size_t rebuild_idx;

        if (vol->vflags & HR_VOL_FLAG_READ_ONLY)
                return ENOTSUP;
        if (!(vol->meta_ops->get_flags() & HR_METADATA_ALLOW_REBUILD))
                return ENOTSUP;

        rc = hr_init_rebuild(vol, &rebuild_idx);
        if (rc != EOK)
                return rc;

        uint64_t max_blks = DATA_XFER_LIMIT / vol->bsize;
        uint64_t left =
            vol->data_blkno / (vol->extent_no - 1) - vol->rebuild_blk;

        uint64_t strip_size = vol->strip_size / vol->bsize; /* in blocks */

        size_t cnt;
        uint64_t ba = vol->rebuild_blk;
        hr_add_data_offset(vol, &ba);

        /*
         * this is not necessary because a rebuild is
         * protected by itself, i.e. there can be only
         * one REBUILD at a time
         */
        fibril_rwlock_read_lock(&vol->extents_lock);

        /* increment metadata counter only on first write */
        bool exp = false;
        if (atomic_compare_exchange_strong(&vol->first_write, &exp, true)) {
                vol->meta_ops->inc_counter(vol);
                vol->meta_ops->save(vol, WITH_STATE_CALLBACK);
        }

        hr_range_lock_t *rl = NULL;
        hr_stripe_t *stripe = hr_create_stripes(vol, max_blks * vol->bsize, 1,
            false);

        HR_NOTE("\"%s\": REBUILD started on extent no. %zu at "
            "block %" PRIu64 ".\n",
            vol->devname, rebuild_idx, ba);

        uint64_t written = 0;
        unsigned int percent, old_percent = 100;
        while (left != 0) {
                cnt = min(left, max_blks);

                uint64_t strip_no = ba / strip_size;
                uint64_t last_ba = ba + cnt - 1;
                uint64_t end_strip_no = last_ba / strip_size;
                uint64_t start_stripe = strip_no / (vol->extent_no - 1);
                uint64_t end_stripe = end_strip_no / (vol->extent_no - 1);
                size_t stripes_cnt = end_stripe - start_stripe + 1;

                stripe->ps_to_be_added = vol->extent_no - 1;
                stripe->p_count_final = true;

                hr_fgroup_t *worker_group =
                    hr_fgroup_create(vol->fge, vol->extent_no);

                rl = hr_range_lock_acquire(vol, start_stripe, stripes_cnt);

                atomic_store_explicit(&vol->rebuild_blk, ba,
                    memory_order_relaxed);

                for (size_t e = 0; e < vol->extent_no; e++) {
                        if (e == rebuild_idx)
                                continue;

                        hr_io_raid5_t *io = hr_fgroup_alloc(worker_group);
                        io->extent = e;
                        io->ba = ba;
                        io->cnt = cnt;
                        io->strip_off = 0;
                        io->vol = vol;
                        io->stripe = stripe;

                        hr_fgroup_submit(worker_group,
                            hr_io_raid5_reconstruct_reader, io);
                }

                hr_io_raid5_t *io = hr_fgroup_alloc(worker_group);
                io->extent = rebuild_idx;
                io->ba = ba;
                io->cnt = cnt;
                io->strip_off = 0;
                io->vol = vol;
                io->stripe = stripe;

                hr_fgroup_submit(worker_group, hr_io_raid5_parity_writer, io);

                size_t failed;
                (void)hr_fgroup_wait(worker_group, NULL, &failed);
                if (failed > 0) {
                        hr_range_lock_release(rl);
                        HR_NOTE("\"%s\": REBUILD aborted.\n", vol->devname);
                        goto end;
                }

                percent = ((ba + cnt) * 100) / vol->data_blkno;
                if (percent != old_percent) {
                        if (percent % 5 == 0)
                                HR_DEBUG("\"%s\" REBUILD progress: %u%%\n",
                                    vol->devname, percent);
                }

                if (written * vol->bsize > HR_REBUILD_SAVE_BYTES) {
                        vol->meta_ops->save_ext(vol, rebuild_idx,
                            WITH_STATE_CALLBACK);
                        written = 0;
                }

                hr_range_lock_release(rl);
                hr_reset_stripe(stripe);

                written += cnt;
                ba += cnt;
                left -= cnt;
                old_percent = percent;

                /*
                 * Let other IO requests be served
                 * during rebuild.
                 */
        }

        HR_DEBUG("hr_raid5_rebuild(): rebuild finished on \"%s\" (%" PRIun "), "
            "extent number %zu\n", vol->devname, vol->svc_id, rebuild_idx);

        fibril_rwlock_write_lock(&vol->states_lock);

        hr_update_ext_state(vol, rebuild_idx, HR_EXT_ONLINE);

        atomic_store_explicit(&vol->rebuild_blk, 0, memory_order_relaxed);

        hr_mark_vol_state_dirty(vol);

        hr_update_vol_state(vol, HR_VOL_DEGRADED);

        fibril_rwlock_write_unlock(&vol->states_lock);
end:
        fibril_rwlock_read_unlock(&vol->extents_lock);

        hr_raid1_vol_state_eval(vol);

        hr_destroy_stripes(stripe, 1);

        return rc;
}

/** @}
 */