BalancerPolicy::ChunkInfo* BalancerPolicy::balance( const string& ns,
const ShardToLimitsMap& shardToLimitsMap,
const ShardToChunksMap& shardToChunksMap,
int balancedLastTime ) {
pair<string,unsigned> min("",numeric_limits<unsigned>::max());
pair<string,unsigned> max("",0);
vector<string> drainingShards;
bool maxOpsQueued = false;
for (ShardToChunksIter i = shardToChunksMap.begin(); i!=shardToChunksMap.end(); ++i ) {
// Find whether this shard's capacity or availability are exhausted
const string& shard = i->first;
BSONObj shardLimits;
ShardToLimitsIter it = shardToLimitsMap.find( shard );
if ( it != shardToLimitsMap.end() ) shardLimits = it->second;
const bool maxedOut = isSizeMaxed( shardLimits );
const bool draining = isDraining( shardLimits );
const bool opsQueued = hasOpsQueued( shardLimits );
// Is this shard a better chunk receiver then the current one?
// Shards that would be bad receiver candidates:
// + maxed out shards
// + draining shards
// + shards with operations queued for writeback
const unsigned size = i->second.size();
if ( ! maxedOut && ! draining && ! opsQueued ) {
if ( size < min.second ) {
min = make_pair( shard , size );
}
}
else if ( opsQueued ) {
MONGO_LOG(1) << "won't send a chunk to: " << shard << " because it has ops queued" << endl;
}
else if ( maxedOut ) {
MONGO_LOG(1) << "won't send a chunk to: " << shard << " because it is maxedOut" << endl;
}
// Check whether this shard is a better chunk donor then the current one.
// Draining shards take a lower priority than overloaded shards.
if ( size > max.second ) {
max = make_pair( shard , size );
maxOpsQueued = opsQueued;
}
if ( draining && (size > 0)) {
drainingShards.push_back( shard );
}
}
// If there is no candidate chunk receiver -- they may have all been maxed out,
// draining, ... -- there's not much that the policy can do.
if ( min.second == numeric_limits<unsigned>::max() ) {
log() << "no available shards to take chunks" << endl;
return NULL;
}
if ( maxOpsQueued ) {
log() << "biggest shard " << max.first << " has unprocessed writebacks, waiting for completion of migrate" << endl;
return NULL;
}
MONGO_LOG(1) << "collection : " << ns << endl;
MONGO_LOG(1) << "donor : " << max.second << " chunks on " << max.first << endl;
MONGO_LOG(1) << "receiver : " << min.second << " chunks on " << min.first << endl;
if ( ! drainingShards.empty() ) {
string drainingStr;
joinStringDelim( drainingShards, &drainingStr, ',' );
MONGO_LOG(1) << "draining : " << ! drainingShards.empty() << "(" << drainingShards.size() << ")" << endl;
}
// Solving imbalances takes a higher priority than draining shards. Many shards can
// be draining at once but we choose only one of them to cater to per round.
// Important to start balanced, so when there are few chunks any imbalance must be fixed.
const int imbalance = max.second - min.second;
int threshold = 8;
if (balancedLastTime || max.second < 20) threshold = 2;
else if (max.second < 80) threshold = 4;
string from, to;
if ( imbalance >= threshold ) {
from = max.first;
to = min.first;
}
else if ( ! drainingShards.empty() ) {
from = drainingShards[ rand() % drainingShards.size() ];
to = min.first;
}
else {
// Everything is balanced here!
return NULL;
}
const vector<BSONObj>& chunksFrom = shardToChunksMap.find( from )->second;
const vector<BSONObj>& chunksTo = shardToChunksMap.find( to )->second;
BSONObj chunkToMove = pickChunk( chunksFrom , chunksTo );
log() << "chose [" << from << "] to [" << to << "] " << chunkToMove << endl;
return new ChunkInfo( ns, to, from, chunkToMove );
}
BalancerPolicy::ChunkInfo* BalancerPolicy::balance( const string& ns,
const ShardToLimitsMap& shardToLimitsMap,
const ShardToChunksMap& shardToChunksMap,
int balancedLastTime ){
pair<string,unsigned> min("",numeric_limits<unsigned>::max());
pair<string,unsigned> max("",0);
vector<string> drainingShards;
for (ShardToChunksIter i = shardToChunksMap.begin(); i!=shardToChunksMap.end(); ++i ){
// Find whether this shard has reached its size cap or whether it is being removed.
const string& shard = i->first;
BSONObj shardLimits;
ShardToLimitsIter it = shardToLimitsMap.find( shard );
if ( it != shardToLimitsMap.end() ) shardLimits = it->second;
const bool maxedOut = isSizeMaxed( shardLimits );
const bool draining = isDraining( shardLimits );
// Check whether this shard is a better chunk receiver then the current one.
// Maxed out shards or draining shards cannot be considered receivers.
const unsigned size = i->second.size();
if ( ! maxedOut && ! draining ){
if ( size < min.second ){
min = make_pair( shard , size );
}
}
// Check whether this shard is a better chunk donor then the current one.
// Draining shards take a lower priority than overloaded shards.
if ( size > max.second ){
max = make_pair( shard , size );
}
if ( draining && (size > 0)){
drainingShards.push_back( shard );
}
}
// If there is no candidate chunk receiver -- they may have all been maxed out,
// draining, ... -- there's not much that the policy can do.
if ( min.second == numeric_limits<unsigned>::max() ){
log() << "no availalable shards to take chunks" << endl;
return NULL;
}
log(1) << "collection : " << ns << endl;
log(1) << "donor : " << max.second << " chunks on " << max.first << endl;
log(1) << "receiver : " << min.second << " chunks on " << min.first << endl;
if ( ! drainingShards.empty() ){
string drainingStr;
joinStringDelim( drainingShards, &drainingStr, ',' );
log(1) << "draining : " << ! drainingShards.empty() << "(" << drainingShards.size() << ")" << endl;
}
// Solving imbalances takes a higher priority than draining shards. Many shards can
// be draining at once but we choose only one of them to cater to per round.
const int imbalance = max.second - min.second;
const int threshold = balancedLastTime ? 2 : 8;
string from, to;
if ( imbalance >= threshold ){
from = max.first;
to = min.first;
} else if ( ! drainingShards.empty() ){
from = drainingShards[ rand() % drainingShards.size() ];
to = min.first;
} else {
// Everything is balanced here!
return NULL;
}
const vector<BSONObj>& chunksFrom = shardToChunksMap.find( from )->second;
const vector<BSONObj>& chunksTo = shardToChunksMap.find( to )->second;
BSONObj chunkToMove = pickChunk( chunksFrom , chunksTo );
log() << "chose [" << from << "] to [" << to << "] " << chunkToMove << endl;
return new ChunkInfo( ns, to, from, chunkToMove );
}
