public WALChannelBuilder capacity(long capacity) {
            assert capacity == CAPACITY_NOT_SET || WALUtil.isAligned(capacity);
            this.capacity = capacity;
            return this;
        }
Get the capacity of the given channel.
It returns null if the channel has not been initialized before.
s3stream/src/main/java/com/automq/stream/s3/wal/util/WALChannel.java