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
// AND We must know what we stabilize. Basic workflow is like:
// 1) primary OSD receives sync request
// 2) it determines his own unsynced writes from blockstore's information
// just before submitting fsync
// 3) it submits syncs to blockstore and peers
// 4) after everyone acks sync it takes the object list and sends stabilize requests to everyone
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
int s = 0, cur_sector = -1;
for (i = 0, v = (obj_ver_id*)op->buf; i < op->len; i++, v++)
{
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(op, journal, sqe[s++]);
}
}
op->max_used_journal_sector = 1 + journal.cur_sector;
op->pending_ops = s;
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)
{
// First step: mark dirty_db entries as stable, acknowledge op completion
// FIXME: oops... we seem to have to copy object id/version pairs...
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((obj_ver_id){
.oid = v->oid,
.version = v->version,
});
if (dirty_it != dirty_db.end())
{
do
{
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;
}
dirty_it--;
} while (dirty_it != dirty_db.begin() && dirty_it->first.oid == v->oid);
}
// Acknowledge op
op->retval = 0;
op->callback(op);
}
}
}