Simplified distributed block storage with strong consistency, like in Ceph
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#include "blockstore.h"
// Stabilize small write:
// 1) Copy data from the journal to the data device
// 2) Increase version on the metadata device and sync it
// 3) Advance clean_db entry's version, clear previous journal entries
//
// This makes 1 4K small write+sync look like:
// 512b+4K (journal) + sync + 512b (journal) + sync + 4K (data) [+ sync?] + 512b (metadata) + sync.
// WA = 2.375. It's not the best, SSD FTL-like redirect-write could probably be lower
// even with defragmentation. But it's fixed and it's still better than in Ceph. :)
// except for HDD-only clusters, because each write results in 3 seeks.
// Stabilize big write:
// 1) Copy metadata from the journal to the metadata device
// 2) Move dirty_db entry to clean_db and clear previous journal entries
//
// This makes 1 128K big write+sync look like:
// 128K (data) + sync + 512b (journal) + sync + 512b (journal) + sync + 512b (metadata) + sync.
// WA = 1.012. Very good :)
// Stabilize delete:
// 1) Remove metadata entry and sync it
// 2) Remove dirty_db entry and clear previous journal entries
// Note that it will lead to problems in a degraded cluster, because deleting 2 of 3 replicas
// and restarting the last replica will then result in extra "missing" objects. To solve that
// we need to store the "tombstones" of deleted objects. We can't do that with current simple
// metadata storage so we'll skip TRIM implementation for now.
// AND We must do it in batches, for the sake of reduced fsync call count
// AND We must know what we stabilize. Basic workflow is like:
// 1) primary OSD receives sync request
// 2) it submits syncs to blockstore and peers
// 3) after everyone acks sync it acks sync to the client
// 4) after a while it takes his synced object list and sends stabilize requests
// to peers and to its own blockstore, thus freeing the old version
int blockstore::dequeue_stable(blockstore_operation *op)
{
obj_ver_id* v;
int i, todo = 0;
for (i = 0, v = (obj_ver_id*)op->buf; i < op->len; i++, v++)
{
auto dirty_it = dirty_db.find(*v);
if (dirty_it == dirty_db.end())
{
auto clean_it = clean_db.find(v->oid);
if (clean_it == clean_db.end() || clean_it->second.version < v->version)
{
// No such object version
op->retval = -EINVAL;
op->callback(op);
return 1;
}
else
{
// Already stable
}
}
else if (IS_UNSYNCED(dirty_it->second.state))
{
// Object not synced yet. Caller must sync it first
op->retval = EAGAIN;
op->callback(op);
return 1;
}
else if (!IS_STABLE(dirty_it->second.state))
{
todo++;
}
}
if (!todo)
{
// Already stable
op->retval = 0;
op->callback(op);
return 1;
}
// Check journal space
blockstore_journal_check_t space_check(this);
if (!space_check.check_available(op, todo, sizeof(journal_entry_stable), 0))
{
return 0;
}
// There is sufficient space. Get SQEs
struct io_uring_sqe *sqe[space_check.sectors_required];
for (i = 0; i < space_check.sectors_required; i++)
{
BS_SUBMIT_GET_SQE_DECL(sqe[i]);
}
// Prepare and submit journal entries
auto cb = [this, op](ring_data_t *data) { handle_stable_event(data, op); };
int s = 0, cur_sector = -1;
for (i = 0, v = (obj_ver_id*)op->buf; i < op->len; i++, v++)
{
auto unstab_it = unstable_writes.find(v->oid);
if (unstab_it != unstable_writes.end() &&
unstab_it->second <= v->version)
{
unstable_writes.erase(unstab_it);
}
journal_entry_stable *je = (journal_entry_stable*)
prefill_single_journal_entry(journal, JE_STABLE, sizeof(journal_entry_stable));
je->oid = v->oid;
je->version = v->version;
je->crc32 = je_crc32((journal_entry*)je);
journal.crc32_last = je->crc32;
if (cur_sector != journal.cur_sector)
{
if (cur_sector == -1)
op->min_used_journal_sector = 1 + journal.cur_sector;
cur_sector = journal.cur_sector;
prepare_journal_sector_write(journal, sqe[s++], cb);
}
}
op->max_used_journal_sector = 1 + journal.cur_sector;
op->pending_ops = s;
return 1;
}
void blockstore::handle_stable_event(ring_data_t *data, blockstore_operation *op)
{
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"
);
}
op->pending_ops--;
if (op->pending_ops == 0)
{
// Release used journal sectors
if (op->min_used_journal_sector > 0)
{
uint64_t s = op->min_used_journal_sector;
while (1)
{
journal.sector_info[s-1].usage_count--;
if (s == op->max_used_journal_sector)
break;
s = 1 + s % journal.sector_count;
}
op->min_used_journal_sector = op->max_used_journal_sector = 0;
}
// First step: mark dirty_db entries as stable, acknowledge op completion
obj_ver_id* v;
int i;
for (i = 0, v = (obj_ver_id*)op->buf; i < op->len; i++, v++)
{
// Mark all dirty_db entries up to op->version as stable
auto dirty_it = dirty_db.find(*v);
if (dirty_it != dirty_db.end())
{
while (1)
{
if (dirty_it->second.state == ST_J_SYNCED)
{
dirty_it->second.state = ST_J_STABLE;
}
else if (dirty_it->second.state == ST_D_META_SYNCED)
{
dirty_it->second.state = ST_D_STABLE;
}
else if (dirty_it->second.state == ST_DEL_SYNCED)
{
dirty_it->second.state = ST_DEL_STABLE;
}
else if (IS_STABLE(dirty_it->second.state))
{
break;
}
if (dirty_it == dirty_db.begin())
{
break;
}
dirty_it--;
if (dirty_it->first.oid != v->oid)
{
break;
}
}
#ifdef BLOCKSTORE_DEBUG
printf("enqueue_flush %lu:%lu v%lu\n", v->oid.inode, v->oid.stripe, v->version);
#endif
flusher->enqueue_flush(*v);
}
}
// Acknowledge op
op->retval = 0;
op->callback(op);
}
}