Simplified distributed block storage with strong consistency, like in Ceph
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#include "blockstore.h"
journal_flusher_t::journal_flusher_t(int flusher_count, blockstore *bs)
{
this->bs = bs;
this->flusher_count = flusher_count;
active_flushers = 0;
active_until_sync = 0;
sync_required = true;
sync_threshold = flusher_count == 1 ? 1 : flusher_count/2;
journal_trim_interval = 1;//sync_threshold; //FIXME
journal_trim_counter = 0;
journal_superblock = (uint8_t*)memalign(512, 512);
co = new journal_flusher_co[flusher_count];
for (int i = 0; i < flusher_count; i++)
{
co[i].bs = bs;
co[i].flusher = this;
}
}
journal_flusher_co::journal_flusher_co()
{
wait_state = 0;
simple_callback = [this](ring_data_t* data)
{
if (data->res != data->iov.iov_len)
{
throw std::runtime_error(
"write operation failed ("+std::to_string(data->res)+" != "+std::to_string(data->iov.iov_len)+
"). in-memory state is corrupted. AAAAAAAaaaaaaaaa!!!111"
);
}
wait_count--;
};
}
journal_flusher_t::~journal_flusher_t()
{
free(journal_superblock);
delete[] co;
}
void journal_flusher_t::loop()
{
for (int i = 0; i < flusher_count; i++)
{
if (!active_flushers && !flush_queue.size())
{
return;
}
co[i].loop();
}
}
void journal_flusher_t::queue_flush(obj_ver_id ov)
{
auto it = flush_versions.find(ov.oid);
if (it != flush_versions.end())
{
it->second = ov.version;
}
else
{
flush_versions[ov.oid] = ov.version;
flush_queue.push_back(ov.oid);
}
}
void journal_flusher_t::unshift_flush(obj_ver_id ov)
{
auto it = flush_versions.find(ov.oid);
if (it != flush_versions.end())
{
it->second = ov.version;
}
else
{
flush_versions[ov.oid] = ov.version;
flush_queue.push_front(ov.oid);
}
}
#define await_sqe(label) \
resume_##label:\
sqe = bs->get_sqe();\
if (!sqe)\
{\
wait_state = label;\
return;\
}\
data = ((ring_data_t*)sqe->user_data);
void journal_flusher_co::loop()
{
// This is much better than implementing the whole function as an FSM
// Maybe I should consider a coroutine library like https://github.com/hnes/libaco ...
if (wait_state == 1)
goto resume_1;
else if (wait_state == 2)
goto resume_2;
else if (wait_state == 3)
goto resume_3;
else if (wait_state == 4)
goto resume_4;
else if (wait_state == 5)
goto resume_5;
else if (wait_state == 6)
goto resume_6;
else if (wait_state == 7)
goto resume_7;
else if (wait_state == 8)
goto resume_8;
else if (wait_state == 9)
goto resume_9;
else if (wait_state == 10)
goto resume_10;
else if (wait_state == 11)
goto resume_11;
else if (wait_state == 12)
goto resume_12;
else if (wait_state == 13)
goto resume_13;
resume_0:
if (!flusher->flush_queue.size())
return;
cur.oid = flusher->flush_queue.front();
cur.version = flusher->flush_versions[cur.oid];
flusher->flush_queue.pop_front();
flusher->flush_versions.erase(cur.oid);
dirty_end = bs->dirty_db.find(cur);
if (dirty_end != bs->dirty_db.end())
{
repeat_it = flusher->sync_to_repeat.find(cur.oid);
if (repeat_it != flusher->sync_to_repeat.end())
{
// We don't flush different parts of history of the same object in parallel
// So we check if someone is already flushing this object
// In that case we set sync_to_repeat to 2 and pick another object
// Another coroutine will see this "2" and re-queue the object after it finishes
repeat_it->second = cur.version;
wait_state = 0;
goto resume_0;
}
else
repeat_it->second = 0;
dirty_it = dirty_end;
flusher->active_flushers++;
flusher->active_until_sync++;
v.clear();
wait_count = 0;
clean_loc = UINT64_MAX;
skip_copy = false;
while (1)
{
if (dirty_it->second.state == ST_J_STABLE && !skip_copy)
{
// First we submit all reads
offset = dirty_it->second.offset;
len = dirty_it->second.len;
it = v.begin();
while (1)
{
for (; it != v.end(); it++)
if (it->offset >= offset)
break;
if (it == v.end() || it->offset > offset)
{
submit_len = it == v.end() || it->offset >= offset+len ? len : it->offset-offset;
await_sqe(1);
it = v.insert(it, (copy_buffer_t){ .offset = offset, .len = submit_len, .buf = memalign(512, submit_len) });
data->iov = (struct iovec){ v.back().buf, (size_t)submit_len };
data->callback = simple_callback;
my_uring_prep_readv(
sqe, bs->journal.fd, &data->iov, 1, bs->journal.offset + dirty_it->second.location + offset
);
wait_count++;
}
if (it == v.end() || it->offset+it->len >= offset+len)
{
break;
}
}
}
else if (dirty_it->second.state == ST_D_STABLE)
{
// There is an unflushed big write. Copy small writes in its position
printf("found ");
if (!skip_copy)
{
clean_loc = dirty_it->second.location;
}
skip_copy = true;
}
else if (!IS_STABLE(dirty_it->second.state))
{
throw std::runtime_error("BUG: Unexpected dirty_entry state during flush: " + std::to_string(dirty_it->second.state));
}
if (dirty_it == bs->dirty_db.begin())
{
break;
}
dirty_it--;
if (dirty_it->first.oid != cur.oid)
{
break;
}
}
if (wait_count == 0 && clean_loc == UINT64_MAX)
{
// Nothing to flush
flusher->active_flushers--;
flusher->active_until_sync--;
repeat_it = flusher->sync_to_repeat.find(cur.oid);
if (repeat_it->second != 0)
{
// Requeue version
flusher->unshift_flush({ .oid = cur.oid, .version = repeat_it->second });
}
flusher->sync_to_repeat.erase(repeat_it);
wait_state = 0;
goto resume_0;
}
if (clean_loc == UINT64_MAX)
{
// Find it in clean_db
clean_it = bs->clean_db.find(cur.oid);
if (clean_it == bs->clean_db.end())
{
// Object not present at all. This is a bug.
throw std::runtime_error("BUG: Object we are trying to flush is not allocated on the data device");
}
else
clean_loc = clean_it->second.location;
}
else
clean_it = bs->clean_db.end();
// Also we need to submit the metadata read. We do a read-modify-write for every operation.
// But we must check if the same sector is already in memory.
// Another option is to keep all raw metadata in memory all the time. Maybe I'll do it sometime...
// And yet another option is to use LSM trees for metadata, but it sophisticates everything a lot,
// so I'll avoid it as long as I can.
meta_sector = ((clean_loc >> bs->block_order) / (512 / sizeof(clean_disk_entry))) * 512;
meta_pos = ((clean_loc >> bs->block_order) % (512 / sizeof(clean_disk_entry)));
meta_it = flusher->meta_sectors.find(meta_sector);
if (meta_it == flusher->meta_sectors.end())
{
// Not in memory yet, read it
meta_it = flusher->meta_sectors.emplace(meta_sector, (meta_sector_t){
.offset = meta_sector,
.len = 512,
.state = 0, // 0 = not read yet
.buf = memalign(512, 512),
.usage_count = 1,
}).first;
await_sqe(2);
data->iov = (struct iovec){ meta_it->second.buf, 512 };
data->callback = [this](ring_data_t* data)
{
if (data->res != data->iov.iov_len)
{
throw std::runtime_error(
"write operation failed ("+std::to_string(data->res)+" != "+std::to_string(data->iov.iov_len)+
"). in-memory state is corrupted. AAAAAAAaaaaaaaaa!!!111"
);
}
meta_it->second.state = 1;
wait_count--;
};
my_uring_prep_readv(
sqe, bs->meta_fd, &data->iov, 1, bs->meta_offset + meta_sector
);
wait_count++;
}
else
meta_it->second.usage_count++;
wait_state = 3;
resume_3:
if (wait_count > 0)
return;
// Reads completed, submit writes
for (it = v.begin(); it != v.end(); it++)
{
await_sqe(4);
data->iov = (struct iovec){ it->buf, (size_t)it->len };
data->callback = simple_callback;
my_uring_prep_writev(
sqe, bs->data_fd, &data->iov, 1, bs->data_offset + clean_loc + it->offset
);
wait_count++;
}
// And a metadata write
resume_5:
if (meta_it->second.state == 0)
{
// metadata sector is still being read, wait for it
wait_state = 5;
return;
}
*((clean_disk_entry*)meta_it->second.buf + meta_pos) = {
.oid = cur.oid,
.version = cur.version,
};
// I consider unordered writes to data & metadata safe here
// BUT it requires that journal entries even older than clean_db should be replayed after restart
await_sqe(6);
data->iov = (struct iovec){ meta_it->second.buf, 512 };
data->callback = simple_callback;
my_uring_prep_writev(
sqe, bs->meta_fd, &data->iov, 1, bs->meta_offset + meta_sector
);
wait_count++;
wait_state = 7;
resume_7:
if (wait_count > 0)
return;
// Done, free all buffers
meta_it->second.usage_count--;
if (meta_it->second.usage_count == 0)
{
free(meta_it->second.buf);
flusher->meta_sectors.erase(meta_it);
}
for (it = v.begin(); it != v.end(); it++)
{
free(it->buf);
}
v.clear();
flusher->active_until_sync--;
if (flusher->sync_required)
{
// And sync everything (in batches - not per each operation!)
cur_sync = flusher->syncs.end();
if (cur_sync == flusher->syncs.begin())
cur_sync = flusher->syncs.emplace(flusher->syncs.end(), (flusher_sync_t){ .ready_count = 0, .state = 0 });
else
cur_sync--;
cur_sync->ready_count++;
if (cur_sync->ready_count >= flusher->sync_threshold ||
!flusher->active_until_sync && !flusher->flush_queue.size())
{
// Sync batch is ready. Do it.
await_sqe(9);
data->callback = simple_callback;
data->iov = { 0 };
my_uring_prep_fsync(sqe, bs->data_fd, 0);
wait_count++;
if (bs->meta_fd != bs->data_fd)
{
await_sqe(10);
data->callback = simple_callback;
data->iov = { 0 };
my_uring_prep_fsync(sqe, bs->meta_fd, 0);
wait_count++;
}
wait_state = 11;
resume_11:
if (wait_count > 0)
return;
// Sync completed. All previous coroutines waiting for it must be resumed
cur_sync->state = 1;
}
// Wait until someone else sends and completes a sync.
resume_8:
if (!cur_sync->state)
{
wait_state = 8;
return;
}
cur_sync->ready_count--;
if (cur_sync->ready_count == 0)
{
flusher->syncs.erase(cur_sync);
}
}
// Update clean_db and dirty_db, free old data locations
if (clean_it != bs->clean_db.end() && clean_it->second.location != clean_loc)
{
allocator_set(bs->data_alloc, clean_it->second.location >> bs->block_order, false);
}
bs->clean_db[cur.oid] = {
.version = cur.version,
.location = clean_loc,
};
dirty_it = dirty_end;
do
{
if (IS_BIG_WRITE(dirty_it->second.state) && dirty_it->second.location != clean_loc)
{
allocator_set(bs->data_alloc, dirty_it->second.location >> bs->block_order, false);
}
int used = --bs->journal.used_sectors[dirty_it->second.journal_sector];
if (used == 0)
{
bs->journal.used_sectors.erase(dirty_it->second.journal_sector);
}
dirty_it--;
} while (dirty_it != bs->dirty_db.begin() && dirty_it->first.oid == cur.oid);
// Then, basically, remove everything up to the current version from dirty_db...
if (dirty_it->first.oid != cur.oid)
dirty_it++;
bs->dirty_db.erase(dirty_it, std::next(dirty_end));
// Clear unused part of the journal every <journal_trim_interval> flushes
if (!((++flusher->journal_trim_counter) % flusher->journal_trim_interval))
{
flusher->journal_trim_counter = 0;
journal_used_it = bs->journal.used_sectors.lower_bound(bs->journal.used_start);
if (journal_used_it == bs->journal.used_sectors.end())
{
// Journal is cleared to its end, restart from the beginning
journal_used_it = bs->journal.used_sectors.begin();
if (journal_used_it == bs->journal.used_sectors.end())
{
// Journal is empty
bs->journal.used_start = bs->journal.next_free;
}
else
{
bs->journal.used_start = journal_used_it->first;
}
}
else if (journal_used_it->first > bs->journal.used_start)
{
// Journal is cleared up to <journal_used_it>
bs->journal.used_start = journal_used_it->first;
}
else
{
// Can't trim journal
goto do_not_trim;
}
// Update journal "superblock"
await_sqe(12);
data->callback = simple_callback;
*((journal_entry_start*)flusher->journal_superblock) = {
.crc32 = 0,
.magic = JOURNAL_MAGIC,
.type = JE_START,
.size = sizeof(journal_entry_start),
.reserved = 0,
.journal_start = bs->journal.used_start,
};
((journal_entry_start*)flusher->journal_superblock)->crc32 = je_crc32((journal_entry*)flusher->journal_superblock);
data->iov = (struct iovec){ flusher->journal_superblock, 512 };
my_uring_prep_writev(sqe, bs->journal.fd, &data->iov, 1, bs->journal.offset);
wait_count++;
wait_state = 13;
resume_13:
if (wait_count > 0)
return;
}
do_not_trim:
// All done
wait_state = 0;
flusher->active_flushers--;
repeat_it = flusher->sync_to_repeat.find(cur.oid);
if (repeat_it->second != 0)
{
// Requeue version
flusher->unshift_flush({ .oid = cur.oid, .version = repeat_it->second });
}
flusher->sync_to_repeat.erase(repeat_it);
goto resume_0;
}
}