Simplified distributed block storage with strong consistency, like in Ceph
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// Copyright (c) Vitaliy Filippov, 2019+
// License: VNPL-1.1 (see README.md for details)
#include "blockstore_impl.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
// We have 2 problems here:
// - In the cluster environment, we must store the "tombstones" of deleted objects until
// all replicas (not just quorum) agrees about their deletion. That is, "stabilize" is
// not possible for deletes in degraded placement groups
// - With simple "fixed" metadata tables we can't just clear the metadata entry of the latest
// object version. We must clear all previous entries, too.
// FIXME Fix both problems - probably, by switching from "fixed" metadata tables to "dynamic"
// 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_impl_t::dequeue_stable(blockstore_op_t *op)
{
if (PRIV(op)->op_state)
{
return continue_stable(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 = -ENOENT;
FINISH_OP(op);
return 2;
}
else
{
// Already stable
}
}
else if (IS_IN_FLIGHT(dirty_it->second.state))
{
// Object write is still in progress. Wait until the write request completes
return 0;
}
else if (!IS_SYNCED(dirty_it->second.state))
{
// Object not synced yet. Caller must sync it first
op->retval = -EBUSY;
FINISH_OP(op);
return 2;
}
else if (!IS_STABLE(dirty_it->second.state))
{
todo++;
}
}
if (!todo)
{
// Already stable
op->retval = 0;
FINISH_OP(op);
return 2;
}
// 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_to_write];
for (i = 0; i < space_check.sectors_to_write; 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++)
{
// FIXME: Only stabilize versions that aren't stable yet
if (!journal.entry_fits(sizeof(journal_entry_stable)) &&
journal.sector_info[journal.cur_sector].dirty)
{
if (cur_sector == -1)
PRIV(op)->min_flushed_journal_sector = 1 + journal.cur_sector;
prepare_journal_sector_write(journal, journal.cur_sector, sqe[s++], cb);
cur_sector = journal.cur_sector;
}
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;
}
prepare_journal_sector_write(journal, journal.cur_sector, sqe[s++], cb);
assert(s == space_check.sectors_to_write);
if (cur_sector == -1)
PRIV(op)->min_flushed_journal_sector = 1 + journal.cur_sector;
PRIV(op)->max_flushed_journal_sector = 1 + journal.cur_sector;
PRIV(op)->pending_ops = s;
PRIV(op)->op_state = 1;
return 1;
}
int blockstore_impl_t::continue_stable(blockstore_op_t *op)
{
if (PRIV(op)->op_state == 2)
goto resume_2;
else if (PRIV(op)->op_state == 3)
goto resume_3;
else if (PRIV(op)->op_state == 5)
goto resume_5;
else
return 1;
resume_2:
// Release used journal sectors
release_journal_sectors(op);
resume_3:
if (!disable_journal_fsync)
{
io_uring_sqe *sqe;
BS_SUBMIT_GET_SQE_DECL(sqe);
ring_data_t *data = ((ring_data_t*)sqe->user_data);
my_uring_prep_fsync(sqe, journal.fd, IORING_FSYNC_DATASYNC);
data->iov = { 0 };
data->callback = [this, op](ring_data_t *data) { handle_stable_event(data, op); };
PRIV(op)->min_flushed_journal_sector = PRIV(op)->max_flushed_journal_sector = 0;
PRIV(op)->pending_ops = 1;
PRIV(op)->op_state = 4;
return 1;
}
resume_5:
// 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
mark_stable(*v);
}
// Acknowledge op
op->retval = 0;
FINISH_OP(op);
return 2;
}
void blockstore_impl_t::mark_stable(const obj_ver_id & v, bool forget_dirty)
{
auto dirty_it = dirty_db.find(v);
if (dirty_it != dirty_db.end())
{
while (1)
{
if ((dirty_it->second.state & BS_ST_WORKFLOW_MASK) == BS_ST_SYNCED)
{
dirty_it->second.state = (dirty_it->second.state & ~BS_ST_WORKFLOW_MASK) | BS_ST_STABLE;
}
if (forget_dirty && (IS_BIG_WRITE(dirty_it->second.state) ||
IS_DELETE(dirty_it->second.state)))
{
// Big write overrides all previous dirty entries
auto erase_end = dirty_it;
while (dirty_it != dirty_db.begin())
{
dirty_it--;
if (dirty_it->first.oid != v.oid)
{
dirty_it++;
break;
}
}
auto clean_it = clean_db.find(v.oid);
uint64_t clean_loc = clean_it != clean_db.end()
? clean_it->second.location : UINT64_MAX;
erase_dirty(dirty_it, erase_end, clean_loc);
break;
}
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;
}
}
flusher->enqueue_flush(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);
}
}
void blockstore_impl_t::handle_stable_event(ring_data_t *data, blockstore_op_t *op)
{
live = true;
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"
);
}
PRIV(op)->pending_ops--;
if (PRIV(op)->pending_ops == 0)
{
PRIV(op)->op_state++;
ringloop->wakeup();
}
}