JAK Inhibitor Before Donor Stem Cell Transplant in Treating Patients With Primary or Secondary Myelofibrosis

Inclusion Criteria: PART 1: * PART 1: Disease criteria * Diagnosis of primary MF (PMF) as defined by the 2008 World Health Organization classification system or diagnosis of secondary MF as defined by the International Working Group (IWG) for Myeloproliferative Neoplasms Research and Treatment criteria * Patients meeting the criteria for intermediate-1, intermediate-2 or high-risk disease by the Dynamic International Prognostic Scoring System (DIPSS) or DIPSS-plus scoring system * PART 1: Ability to understand and the willingness to sign a written informed consent document * PART 1: Patient must be a potential hematopoietic stem cell transplant candidate PART 2: * PART 2: Meeting criteria for 1st phase as above, at time of initiation of JAK inhibitor, including ability to understand and willingness to sign a written informed consent; patients arriving to our institution for transplant and not enrolled in Part 1 may still be enrolled in Part 2 if Part 1 criteria met; these patients will have Part 1 endpoints transcribed from medical records * PART 2: Received ruxolitinib for at least 8 weeks immediately prior to conditioning and be able to continue until Day -4 pre-transplant * PART 2: Performance status score * Karnofsky \>= 70 * PART 2: Calculated creatinine clearance using the Cockcroft-Gault formula or 24 hr urine creatinine clearance must be \> 60 ml/min * PART 2: Total serum bilirubin must be \< 3 mg/dL unless the elevation is thought to be due to Gilbert's d…