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// balancer_policy.cpp
/**
* Copyright (C) 2010 10gen Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include "pch.h"
#include "config.h"
#include "../client/dbclient.h"
#include "../util/stringutils.h"
#include "../util/unittest.h"
#include "balancer_policy.h"
namespace mongo {
// limits map fields
BSONField<long long> LimitsFields::currSize( "currSize" );
BSONField<bool> LimitsFields::hasOpsQueued( "hasOpsQueued" );
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 ) {
LOG(1) << "won't send a chunk to: " << shard << " because it has ops queued" << endl;
}
else if ( maxedOut ) {
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;
}
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 );
}
BSONObj BalancerPolicy::pickChunk( const vector<BSONObj>& from, const vector<BSONObj>& to ) {
// It is possible for a donor ('from') shard to have less chunks than a receiver one ('to')
// if the donor is in draining mode.
if ( to.size() == 0 )
return from[0];
if ( from[0]["min"].Obj().woCompare( to[to.size()-1]["max"].Obj() , BSONObj() , false ) == 0 )
return from[0];
if ( from[from.size()-1]["max"].Obj().woCompare( to[0]["min"].Obj() , BSONObj() , false ) == 0 )
return from[from.size()-1];
return from[0];
}
bool BalancerPolicy::isSizeMaxed( BSONObj limits ) {
// If there's no limit information for the shard, assume it can be a chunk receiver
// (i.e., there's not bound on space utilization)
if ( limits.isEmpty() ) {
return false;
}
long long maxUsage = limits[ ShardFields::maxSize.name() ].Long();
if ( maxUsage == 0 ) {
return false;
}
long long currUsage = limits[ LimitsFields::currSize.name() ].Long();
if ( currUsage < maxUsage ) {
return false;
}
return true;
}
bool BalancerPolicy::isDraining( BSONObj limits ) {
BSONElement draining = limits[ ShardFields::draining.name() ];
if ( draining.eoo() || ! draining.trueValue() ) {
return false;
}
return true;
}
bool BalancerPolicy::hasOpsQueued( BSONObj limits ) {
BSONElement opsQueued = limits[ LimitsFields::hasOpsQueued.name() ];
if ( opsQueued.eoo() || ! opsQueued.trueValue() ) {
return false;
}
return true;
}
} // namespace mongo
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