backup volume disk usage

[zemul]

System Setup

• List the command line to start "weed master", "weed volume", "weed filer", "weed s3", "weed mount".
  master : weed master -mdir=/export/data/seaweedfs/master/mdir1 -peers=10.17.100.28:9333,10.17.100.29:9333,10.17.100.30:9333 -port=9333 -volumeSizeLimitMB=5000 -defaultReplication=001

• OS version. [centos7]

• output of weed version [version 30GB 2.88 linux amd64]

Describe the bug
command : weed backup -dir=. -collection=qqqq -volumeId=3273
Master Set volumeSizeLimitMB =5000, if voulme CompactionRevision is greater than 0，Backup file size =30G，I don't actually use that much

/vol/status result:
{
"Id": 3273,
"Size": 4168,
"ReplicaPlacement": {
"node": 1
},
"Ttl": {
"Count": 0,
"Unit": 0
},
"DiskType": "",
"Collection": "qqqq",
"Version": 3,
"FileCount": 3,
"DeleteCount": 0,
"DeletedByteCount": 0,
"ReadOnly": false,
"CompactRevision": 2,
"ModifiedAtSecond": 1650448176,
"RemoteStorageName": "",
"RemoteStorageKey": ""
}

I suspect that's the problem, but I don't understand

[chrislusf]

Thanks for the good details! Added a fix to remove the pre-allocation for backup.