There is a script that for merge chunks on mongodb
use config
// dbName: Set this to the database name
// collection: Set this to the collection you want to merge chunks in
// keyPattern: Set this to the key pattern, ie { “_id” : “hashed” }
// sleepValue: How long to sleep between each chunk (for backoff)
mergeChunks = function(dbName, collection, keyPattern, sleepValue) {
ns = dbName + “.” + collection
checkChunkForMerge = function(chunk) {
hasDocs = chunkHasDocs(chunk)
if(hasDocs) {
// Not an empty chunk
print(“Count is non-zero!”)
return
}
print(“Running datasize”)
dataSize = db.getSiblingDB(dbName).runCommand({
“dataSize”: chunk.ns,
“keyPattern”: keyPattern,
“min”: chunk.min,
“max”: chunk.max
})
print(“DataSize of ” + dataSize.size + ” and took ” + dataSize.millis + “ms”)
// Found an empty chunk
if(dataSize.size != 0) {
// Not an empty chunk
return
}
// Lets try and find a colocated chunk to merge with
mergableRange = null
prevChunk = db.chunks.findOne({ns: ns, “max”: chunk.min})
nextChunk = null
if(prevChunk && prevChunk.shard == chunk.shard) {
mergableRange = [prevChunk.min, chunk.max]
} else {
nextChunk = db.chunks.findOne({ns: ns, “min”: chunk.max})
if(nextChunk && nextChunk.shard == chunk.shard) {
mergableRange = [chunk.min, nextChunk.max]
}
}
// If we cant we have to move the empty chunk
if(mergableRange == null) {
dest = null
if(prevChunk) {
dest = prevChunk.shard;
mergableRange = [prevChunk.min, chunk.max]
} else if(nextChunk) {
dest = nextChunk.shard;
mergableRange = [chunk.min, nextChunk.max]
} else {
print(“Unable to move empty chunk to mergeable range”)
return
}
print(“Moving chunk to ” + dest)
printjson(chunk)
db.getSiblingDB(‘admin’).runCommand({
moveChunk : chunk.ns,
bounds : [chunk.min, chunk.max],
to : dest,
_secondaryThrottle : true,
_waitForDelete : true
})
chunk.shard = dest
}
// Lets try the merge
if(mergableRange) {
print(“Merging a chunk on ” + chunk.shard)
db.getSiblingDB(‘admin’).runCommand({
mergeChunks: chunk.ns,
bounds: mergableRange
})
} else {
print(“Unable to merge with another chunk!”)
}
}
chunkHasDocs = function(chunk) {
query = {}
shardKeys = Object.keys(keyPattern)
if (shardKeys.length > 1) {
firstKey = shardKeys[0]
secondKey = shardKeys[1]
if (chunk.min[firstKey].toString() == chunk.max[firstKey].toString()) {
query[firstKey] = chunk.min[firstKey]
query[secondKey] = {$gte: chunk.min[secondKey], $lt: chunk.max[secondKey]}
} else {
lowerRange = {}
lowerRange[firstKey] = chunk.min[firstKey]
lowerRange[secondKey] = {$gte: chunk.min[secondKey]}
midRange = {}
midRange[firstKey] = {$gt: chunk.min[firstKey] , $lt: chunk.max[firstKey]}
upperRange = {}
upperRange[firstKey] = chunk.max[firstKey]
upperRange[secondKey] = {$lt: chunk.max[secondKey]}
query.$or = [
lowerRange,
midRange,
upperRange
]
}
} else {
shardKey = shardKeys[0]
// If its hashed then we cant do the count
if (keyPattern[shardKey] == “hashed”) return 0
query[shardKey] = {$gte: chunk.min[shardKey], $lt: chunk.max[shardKey]}
}
print(“Running count”)
printjson(query)
return db.getSiblingDB(dbName)[collection].find(query, {“_id”: 1}).readPref(“secondary”).limit(1).maxTimeMS(10000).hasNext()
}
print(“Loading cursor to iterate through chunks…”)
cursor = db.chunks.find({ns: ns}).sort({“max”: 1}).addOption(DBQuery.Option.noTimeout)
print(“Done!”)
docsProcessed = 0
cursor.forEach(function(chunk){
checkChunkForMerge(chunk)
docsProcessed = docsProcessed + 1
print(“Processed ” + docsProcessed + ” documents!”)
if(sleepValue) {
print(“Sleeping…”)
sleep(sleepValue)
}
})
}
