btrfs-smr-balance.py

btrfs-smr-balance.py

#!/usr/bin/env python3

# The goal of this is to gradually balance a btrfs filesystem which contains DM-SMR drives.
# Such drive are described in detail at https://www.usenix.org/node/188434

# A normal drive should be able to balance a single 1GB chunk in under 30s.
# Such a stripe would normally be written directly to the shingled blocks, but in the case
# it was cached, it would take roughly 100s to clean.

# So our heuristic here is:
# * balance two chunks from the drive with the amount of unallocated space.
# * if it took longer than 60s, increase the per-chunk sleep interval
# * otherwise, decrease the per-chunk sleep interval
# * keep going until the stdev of the drive unallocated space is below 10GB

from functools import lru_cache
import logging
import statistics
import subprocess
import sys
import time

logging.basicConfig(
    format='%(asctime)s %(levelname)-8s %(message)s',
    level=logging.INFO,
    datefmt='%Y-%m-%d %H:%M:%S')

FILESYSTEM = '/media/btrfs'
CHUNK_TIMEOUT = 60 # seconds
MAX_SLEEP = 7200 # seconds
STDEV_LIMIT = 10*1024*1024*1024 # bytes

@lru_cache
def fib(n):
    if n < 2:
        return 1
    return fib(n-2) + fib(n-1)

def sizeof_fmt(num, suffix='B'):
    for unit in ['','Ki','Mi','Gi','Ti','Pi','Ei','Zi']:
        if abs(num) < 1024.0:
            return "%3.1f%s%s" % (num, unit, suffix)
        num /= 1024.0
    return "%.1f%s%s" % (num, 'Yi', suffix)

def bal_chunk():
    least_empty_dev_id = None
    least_empty_dev_path = None
    least_empty_dev_unallocated = float('inf')

    free = []
    for line in subprocess.getoutput('btrfs fi show --raw %s | grep devid' % FILESYSTEM).split('\n'):
        x = line.split()
        devid = x[1]
        size = int(x[3])
        used = int(x[5])
        path = x[7]

        unallocated = size - used
        free.append(unallocated)

        if unallocated < least_empty_dev_unallocated:
            least_empty_dev_id = devid
            least_empty_dev_path = path
            least_empty_dev_unallocated = unallocated

    stdev = statistics.stdev(free)
    if stdev < STDEV_LIMIT:
        logging.info('Unallocated space stdev %s is below %s, exiting...' % (sizeof_fmt(stdev), sizeof_fmt(STDEV_LIMIT)))
        sys.exit()
    else:
        logging.info('Unallocated space stdev %s is above %s, continuing...' % (sizeof_fmt(stdev), sizeof_fmt(STDEV_LIMIT)))

    logging.info('Balancing the least empty device: %s with %s unallocated' % (least_empty_dev_path, sizeof_fmt(least_empty_dev_unallocated)))

    cmd = 'btrfs balance start -ddevid=%s,limit=2 %s' % (least_empty_dev_id, FILESYSTEM)
    ret, out = subprocess.getstatusoutput(cmd)

    if ret != 0:
        logging.warning(out)
        time.sleep(30)
    else:
        logging.info(out)

def fib_sleep(index):
    seconds = fib(index)
    until = time.strftime("%H:%M:%S", time.localtime(time.time() + seconds))
    logging.info("Sleeping %ds until %s" % (seconds, until))
    time.sleep(seconds)

backoff = 0
while True:
    start = time.time()
    bal_chunk()
    duration = time.time() - start

    yeet = "Ouch!" if duration > CHUNK_TIMEOUT else "Nice!"
    logging.info("%s Last chunk took %ds" % (yeet, duration))

    if (duration > CHUNK_TIMEOUT):
        if fib(backoff + 1) < MAX_SLEEP: backoff += 1
        fib_sleep(backoff)
    else:
        if backoff > 1: backoff -= 2 # that's optimism
        elif backoff > 0: backoff -= 1
        fib_sleep(backoff)