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
// Sync it before writing metadata if we want to keep metadata consistent
// Overall it's optional because it can be replayed from the journal until
// it's cleared, and reads are also fulfilled from the journal
// 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 with defragmentation
// could probably be lower even with defragmentation. But it's fixed and it's still
// better than in Ceph. :)
// 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 :)
// AND We must do it in batches, for the sake of reduced fsync call count
int blockstore::dequeue_stable(blockstore_operation *op)
{
auto dirty_it = dirty_db.find((obj_ver_id){
.oid = op->oid,
.version = op->version,
});
if (dirty_it == dirty_db.end())
{
auto clean_it = clean_db.find(op->oid);
if (clean_it == clean_db.end() || clean_it->second.version < op->version)
{
// No such object version
op->retval = EINVAL;
}
else
{
// Already stable
op->retval = 0;
}
op->callback(op);
return 1;
}
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))
{
// 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, 1, sizeof(journal_entry_stable), 0))
{
return 0;
}
// There is sufficient space. Get SQE
BS_SUBMIT_GET_SQE(sqe, data);
// Got SQE. Prepare journal sector write
journal_entry_stable *je = (journal_entry_stable*)prefill_single_journal_entry(journal, JE_STABLE, sizeof(struct journal_entry_stable));
je->oid = op->oid;
je->version = op->version;
je->crc32 = je_crc32((journal_entry*)je);
journal.crc32_last = je->crc32;
data->iov = (struct iovec){ journal.sector_buf + 512*journal.cur_sector, 512 };
data->op = op;
io_uring_prep_writev(
sqe, journal.fd, &data->iov, 1, journal.offset + journal.sector_info[journal.cur_sector].offset
);
journal.sector_info[journal.cur_sector].usage_count++;
op->pending_ops = 1;
op->min_used_journal_sector = op->max_used_journal_sector = 1 + journal.cur_sector;
return 1;
}
int blockstore::continue_stable(blockstore_operation *op)
{
return 0;
}
void blockstore::handle_stable_event(ring_data_t *data, blockstore_operation *op)
{
if (data->res < 0)
{
// sync error
// FIXME: our state becomes corrupted after a write error. maybe do something better than just die
throw new std::runtime_error("write operation failed. in-memory state is corrupted. AAAAAAAaaaaaaaaa!!!111");
}
op->pending_ops--;
if (op->pending_ops == 0)
{
// Mark all dirty_db entries up to op->version as stable
auto dirty_it = dirty_db.find((obj_ver_id){
.oid = op->oid,
.version = op->version,
});
if (dirty_it->second.state == ST_J_SYNCED)
{
dirty_it->second.state = ST_J_STABLE;
// Acknowledge op
op->retval = 0;
op->callback(op);
}
else if (dirty_it->second.state == ST_D_META_SYNCED)
{
dirty_it->second.state = ST_D_STABLE;
// Acknowledge op
op->retval = 0;
op->callback(op);
}
else if (dirty_it->second.state == ST_J_STABLE)
{
}
else if (dirty_it->second.state == ST_D_STABLE)
{
}
}
}