Simulator
This section introduces ffs.py
, a simple FFS simulator you can use to understand better how FFS-based file and directory allocation work. See the previous lesson for details on how to run the simulator.
#! /usr/bin/env python import math import sys from optparse import OptionParser import random class file_system: def __init__(self, num_groups, blocks_per_group, inodes_per_group, large_file_exception, spread_inodes, contig_allocation_policy, spread_data_blocks, allocate_faraway, show_block_addresses, do_per_file_stats, show_file_ops, show_symbol_map, compute): self.num_groups = num_groups self.blocks_per_group = blocks_per_group self.inodes_per_group = inodes_per_group self.large_file_exception = large_file_exception self.spread_inodes = spread_inodes self.contig_allocation_policy = contig_allocation_policy self.spread_data_blocks = spread_data_blocks self.allocate_faraway = allocate_faraway self.show_block_addresses = show_block_addresses self.do_per_file_stats = do_per_file_stats self.show_file_ops = show_file_ops self.show_symbol_map = show_symbol_map self.compute = compute self.group_size = self.inodes_per_group + self.blocks_per_group self.BITMAP_FREE = '__FREE__' self.data_bitmap = [] self.inode_bitmap = [] for i in range(self.num_groups): self.inode_bitmap.append([]) self.data_bitmap.append([]) for j in range(self.blocks_per_group): self.data_bitmap[i].append(self.BITMAP_FREE) for j in range(self.inodes_per_group): self.inode_bitmap[i].append(self.BITMAP_FREE) self.init_symbols() self.total_data_free = blocks_per_group * num_groups self.total_inodes_free = inodes_per_group * num_groups # make root directory self.inode_bitmap[0][0] = 0 self.data_bitmap[0][0] = 0 # use one data block for ALL DIRS self.allocate_symbol(0, '/') self.total_data_free -= 1 self.total_inodes_free -= 1 # data for each directory, indexed by inode number self.dir_data = {} self.dir_data[0] = [('.', 0), ('..', 0)] # inode data: for each inode, type info self.inode_type = {} self.inode_type[0] = 'directory' # and which blocks comprise the file self.inode_blocks = {} self.inode_blocks[0] = [0] # map names to inode numbers self.name_to_inode_map = {} self.name_to_inode_map['/'] = 0 return def vprint(self, msg): if self.show_file_ops: print msg, return def get_parent(self, path): index_of_last_slash = path.rfind('/') if index_of_last_slash == 0: return '/', path[index_of_last_slash+1:len(path)] return path[0:index_of_last_slash], path[index_of_last_slash+1:len(path)] def name_to_inode(self, path): if path not in self.name_to_inode_map: return -1 else: return self.name_to_inode_map[path] def set_name_to_inode(self, path, inode_num): if path in self.name_to_inode_map: print 'abort: path already in mapping (internal error)' exit(1) self.name_to_inode_map[path] = inode_num return def get_free_count(self, group, bitmap): cnt = 0 for b in bitmap: if b == self.BITMAP_FREE: cnt += 1 return cnt def get_free_inode_count(self, group): return self.get_free_count(group, self.inode_bitmap[group]) def get_free_data_count(self, group): return self.get_free_count(group, self.data_bitmap[group]) def find_most_free_inodes(self, starting_point): free_inodes_max = 0 target_group = -1 for g in range(starting_point, self.num_groups): free_inodes_in_group = self.get_free_inode_count(g) if free_inodes_in_group > free_inodes_max: free_inodes_max = free_inodes_in_group target_group = g for g in range(0, starting_point): free_inodes_in_group = self.get_free_inode_count(g) if free_inodes_in_group > free_inodes_max: free_inodes_max = free_inodes_in_group target_group = g return target_group def find_free_inodes_in_range(self, group, how_many): sum_free = 0 for g in range(group, group + how_many): g_curr = g % self.num_groups free_inodes_in_group = self.get_free_inode_count(g_curr) sum_free += free_inodes_in_group return sum_free def find_most_free_inodes_multiple(self, starting_point, how_many): free_inodes_max = 0 target_group = -1 for g in range(starting_point, self.num_groups, how_many): sum_free = self.find_free_inodes_in_range(g, how_many) if sum_free > free_inodes_max: free_inodes_max = sum_free target_group = g for g in range(0, starting_point, how_many): sum_free = self.find_free_inodes_in_range(g, how_many) if sum_free > free_inodes_max: free_inodes_max = sum_free target_group = g assert(target_group != -1) return target_group def find_free_inodes_near(self, target_group): grower = (target_group + 1) % self.num_groups shrinker = (target_group - 1) % self.num_groups for i in range(self.num_groups - 1): if i % 2 == 0: current_group = grower grower = (grower + 1) % self.num_groups else: current_group = shrinker shrinker = (shrinker - 1) % self.num_groups if self.get_free_inode_count(current_group) > 0: return current_group return 1 def pick_group(self, parent, filename, type): if type == 'regular' and self.spread_inodes == False: # FFS policy: pick based on parent parent_inode_number = self.name_to_inode(parent) target_group = parent_inode_number / self.inodes_per_group # ensure group has free inode... (and free data blocks?) num_free_inodes = self.get_free_inode_count(target_group) if num_free_inodes == 0: target_group = self.find_free_inodes_near(target_group) return target_group elif type == 'directory' or self.spread_inodes == True: # find group with most free inodes return self.find_most_free_inodes_multiple(0, self.allocate_faraway) else: print 'abort: bad file type [%s] (internal error)' % type exit(1) return 0 def range_free(self, group, index, needed, chunks_free): if needed < chunks_free: chunks_free = needed # make list of blocks to check for freedom index_begin = index index_end = index + chunks_free - 1 if index_end >= self.blocks_per_group: return False for i in range(index_begin, index_end+1): if self.data_bitmap[group][i] != self.BITMAP_FREE: return False return True if self.data_bitmap[group][index] == self.BITMAP_FREE: return True return False # group is just the group where the inode is # size is how many are needed total def allocate_blocks(self, target_group, size, inode_number): assert(size <= self.total_data_free) allocated = [] index = 0 allocated_in_group = 0 current_group = target_group chunks_free = self.contig_allocation_policy while True: if self.range_free(current_group, index, size-len(allocated), chunks_free): # print ' local alloc', current_group, index assert(self.data_bitmap[current_group][index] == self.BITMAP_FREE) self.data_bitmap[current_group][index] = inode_number allocated_in_group += 1 allocated.append((current_group, index)) index += 1 if len(allocated) == size: # print 'done', allocated break # this moves allocation interest to next group when needed # i.e., when you've searched this entire group or # when you've exhausted the large file exception if index == self.blocks_per_group or \ (self.large_file_exception > 0 and \ allocated_in_group == self.large_file_exception): allocated_in_group = 0 index = 0 current_group = (current_group + 1) % self.num_groups if current_group == target_group: chunks_free = 1 return allocated def find_free_inode(self, group): inode_number = -1 for i in range(self.inodes_per_group): if self.inode_bitmap[group][i] == self.BITMAP_FREE: inode_number = i break # don't think this can ever happen but ... if inode_number == -1: self.vprint('[cannot find free inode]') return -1 return inode_number def find_min_data_usage(self): min_group = -1 min_usage = 0 for g in range(self.num_groups): data_free = self.get_free_data_count(g) if data_free > min_usage: min_usage = data_free min_group = g return min_group def mark_inode_used(self, group, inode_index): self.inode_bitmap[group][inode_index] = inode_index + \ (group * self.inodes_per_group) return def do_delete(self, path): parent, filename = self.get_parent(path) # now, find the file in parent directory parent_inode_number = self.name_to_inode(parent) if parent_inode_number == -1: self.vprint('[cannot find parent inode %s]' % parent) return -1 del_index = -1 for i in range(len(self.dir_data[parent_inode_number])): name, inode_number = self.dir_data[parent_inode_number][i] if name == filename: del_index = i break if del_index == -1: self.vprint('[cannot find %s in dir %s]' % (filename, parent)) return -1 if self.inode_type[inode_number] == 'directory': self.vprint('[cannot delete directories]') return -1 self.dir_data[parent_inode_number].remove((name, inode_number)) for b in self.inode_blocks[inode_number]: data_group = b / self.num_groups data_index = b % self.num_groups self.data_bitmap[data_group][data_index] = self.BITMAP_FREE inode_group = inode_number / self.num_groups inode_index = inode_number % self.num_groups self.inode_bitmap[inode_group][inode_index] = self.BITMAP_FREE self.free_symbol(inode_number) del self.name_to_inode_map[path] self.total_inodes_free += 1 self.total_data_free += len(self.inode_blocks[inode_number]) self.inode_type[inode_number] = '' self.inode_blocks[inode_number] = [] return 0 def do_create(self, path, size, type): parent, filename = self.get_parent(path) # do global checks here if self.total_inodes_free == 0: self.vprint('[out of inodes]') return -1 if self.total_data_free < size: self.vprint('[out of disk space]') return -1 # check if foo already exists parent_inode_number = self.name_to_inode(parent) if parent_inode_number == -1: self.vprint('[failed to find parent %s]' % parent) return -1 for (name, __inode_number) in self.dir_data[parent_inode_number]: if name == filename: self.vprint('[file %s already exists in dir %s]' % (filename, parent)) return -1 # allocate new inode: which group? group = self.pick_group(parent, filename, type) # pick inode now: it is guaranteed here that group will have a free inode # thanks to pick_group()... inode_index = self.find_free_inode(group) # calc global inode number inode_number = inode_index + (group * self.inodes_per_group) # file allocation policy: could fail # allocated = self.allocate_blocks(group, size, inode_number) if self.spread_data_blocks: dest_block_group = self.find_min_data_usage() print 'target alloc', dest_block_group allocated = self.allocate_blocks(dest_block_group, size, inode_number) else: allocated = self.allocate_blocks(group, size, inode_number) if len(allocated) == 0: # allocation failed: no room in file system return -1 # now do all the allocation work: won't fail from here on down self.mark_inode_used(group, inode_index) self.dir_data[parent_inode_number].append((filename, inode_number)) # now, allocate some data blocks self.inode_type[inode_number] = type self.inode_blocks[inode_number] = [] for (selected_group, index) in allocated: global_block_number = index + (selected_group * self.blocks_per_group) # print 'global allocated', global_block_number self.inode_blocks[inode_number].append(global_block_number) # record name to inode number mapping for future lookups self.set_name_to_inode(path, inode_number) self.allocate_symbol(inode_number, filename) # have to fill in contents of empty directory if type == 'directory': self.dir_data[inode_number] = [('.', 0), ('..', 0)] # global accounting self.total_inodes_free -= 1 self.total_data_free -= size return 0 def init_symbols(self): self.symbol_map = {} self.used_symbols = [] self.available_symbols = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','K','L','M','O','P','Q','R','S','T','U','V','W','X','Y','Z','0','1','2','3','4','5','6','7','8','9','!','@','#','%','^','&','*','(',')','[',']','{','}','/','.','<','>','|'] return def allocate_symbol(self, inode_number, suggested_name): if suggested_name not in self.used_symbols \ and suggested_name in self.available_symbols: choice = suggested_name else: assert(len(self.available_symbols) > 0) choice = self.available_symbols[0] # print 'sym', suggested_name, '->', choice self.used_symbols.append(choice) self.available_symbols.remove(choice) self.symbol_map[inode_number] = choice return def free_symbol(self, inode_number): symbol = self.symbol_map[inode_number] del self.symbol_map[inode_number] self.used_symbols.remove(symbol) self.available_symbols.append(symbol) return def get_symbol(self, inode_number): return self.symbol_map[inode_number] def print_success_or_fail(self, rc): if self.show_file_ops: if rc == 0: print 'success' else: print 'failed' return def do_verify(self): data_used = 0 inodes_used = 0 for g in range(self.num_groups): for i in range(self.blocks_per_group): if self.data_bitmap[g][i] != self.BITMAP_FREE: data_used += 1 for i in range(self.inodes_per_group): if self.inode_bitmap[g][i] != self.BITMAP_FREE: inodes_used += 1 assert(data_used + self.total_data_free == (self.num_groups * self.blocks_per_group)) assert(inodes_used + self.total_inodes_free == (self.num_groups * self.inodes_per_group)) for f in self.name_to_inode_map: inode_number = self.name_to_inode_map[f] blocks = self.inode_blocks[inode_number] for b in blocks: data_group = b / self.blocks_per_group data_index = b % self.blocks_per_group assert(self.data_bitmap[data_group][data_index] == inode_number) return def create(self, path, size): self.vprint('op create %s [size:%d] ->' % (path, size)) rc = self.do_create(path, size, 'regular') self.print_success_or_fail(rc) return rc def mkdir(self, path): self.vprint('op mkdir %s ->' % path) rc = self.do_create(path, 1, 'directory') self.print_success_or_fail(rc) return rc def delete(self, path): self.vprint('op delete %s ->' % path) rc = self.do_delete(path) self.print_success_or_fail(rc) return def read_input(self, filename): fd = open(filename) for line in fd: in_line = line.strip('\n') line_len = len(in_line) if line_len == 0: continue tmp = in_line.split() if len(tmp) == 0: continue if tmp[0] == 'file': assert(len(tmp) == 3) name, size = tmp[1], int(tmp[2]) self.create(name, size) elif tmp[0] == 'dir': assert(len(tmp) == 2) name = tmp[1] self.mkdir(name) elif tmp[0] == 'delete': assert(len(tmp) == 2) name = tmp[1] self.delete(name) elif tmp[0] == 'dump': assert(len(tmp) == 1) self.dump() else: print 'command not recognized', tmp[0] exit(1) self.do_verify() fd.close() return def list_to_string(self, bitmap): out_str = '' for i in range(len(bitmap)): if i > 0 and i % 10 == 0: out_str += ' ' if bitmap[i] == self.BITMAP_FREE: out_str += '-' else: out_str += '%s' % self.get_symbol(bitmap[i]) return out_str def do_numeric_header(self, power_level, how_many): power = int(math.pow(10, power_level)) counter = 0 value = 0 out_str = '' for i in range(how_many): if i > 0 and i % 10 == 0: out_str += ' ' out_str += '%d' % (value % 10) counter += 1 if counter == power: value += 1 counter = 0 print out_str, return def dump(self): print '' print 'num_groups: ', self.num_groups print 'inodes_per_group:', self.inodes_per_group print 'blocks_per_group:', self.blocks_per_group print '' print 'free data blocks: %d (of %d)' % (self.total_data_free, (self.num_groups * self.blocks_per_group)) print 'free inodes: %d (of %d)' % (self.total_inodes_free, (self.num_groups * self.inodes_per_group)) print '' print 'spread inodes? ', self.spread_inodes print 'spread data? ', self.spread_data_blocks print 'contig alloc: ', self.contig_allocation_policy print '' inode_power = len('%s' % self.inodes_per_group) - 1 data_power = len('%s' % self.blocks_per_group) - 1 if inode_power > data_power: max_power = inode_power else: max_power = data_power while max_power >= 0: print ' ', # spacing before inode print out if inode_power >= max_power: self.do_numeric_header(max_power, self.inodes_per_group) else: out_str = ' ' * self.inodes_per_group print out_str, if data_power >= max_power: self.do_numeric_header(max_power, self.blocks_per_group) else: out_str = ' ' * self.inodes_per_group print out_str, print '' max_power -= 1 print '\ngroup %s' % ('inodes'[0:self.inodes_per_group]), out_str = '' for i in range(self.inodes_per_group - len('inodes')): out_str += ' ' print '%sdata' % out_str count = 0 for i in range(self.num_groups): # print ' %3d inodes %s' % (i, self.list_to_string(self.inode_bitmap[i])) # print ' data %s' % (self.list_to_string(self.data_bitmap[i])) if self.compute: print ' %3d %s %s' % (i, self.list_to_string(self.inode_bitmap[i]), self.list_to_string(self.data_bitmap[i])), if self.show_block_addresses: print ' [%4d-%4d]' % (count, count + self.group_size - 1) else: print '' else: print ' %3d %s %s' % (i, '?' * self.inodes_per_group, '?' * self.blocks_per_group) count += self.group_size if self.show_symbol_map == False: print '' return print '\nsymbol inode# filename filetype ', if self.do_per_file_stats: print 'block_addresses' else: print '' # sorted(student_tuples, key=lambda student: student[2]) for name in sorted(self.name_to_inode_map): inode_number = self.name_to_inode_map[name] file_type = self.inode_type[inode_number] print '%-6s %6d %-11s %9s' % \ (self.get_symbol(inode_number), inode_number, name, file_type), if self.do_per_file_stats: for i in self.inode_blocks[inode_number]: print i, print '' else: print '' print '' return def get_dist(self, a, b): if a > b: return a - b else: return b - a def get_spans(self, path): inode_number = self.name_to_inode(path) inode_group = inode_number / self.inodes_per_group inode_index = inode_number % self.inodes_per_group inode_address = inode_index + inode_group * self.group_size # now find all data blocks min_address = 1 + (self.num_groups * self.group_size) max_address = -1 data_blocks = self.inode_blocks[inode_number] for d in data_blocks: data_group = d / self.blocks_per_group data_index = d % self.blocks_per_group data_address = data_index + (data_group * self.group_size) + self.inodes_per_group if data_address > max_address: max_address = data_address if data_address < min_address: min_address = data_address return inode_number, inode_address, min_address, max_address def do_all_spans(self): current = 0 total_dist = 0 min_group = 1e6 max_group = -1 print 'span: files' span_results = {} filespan_sum = 0 filespan_cnt = 0 for f in self.name_to_inode_map: inode_number, inode_address, min_address, max_address = self.get_spans(f) if self.inode_type[inode_number] == 'directory': continue data_span = max_address - min_address abs_min, abs_max = min_address, max_address if inode_address < abs_min: abs_min = inode_address if inode_address > abs_max: abs_max = inode_address file_span = abs_max - abs_min assert(inode_number not in span_results) span_results[inode_number] = (inode_address, min_address, max_address) if options.solve: print ' file: %10s filespan: %3d' % (f, file_span) else: print ' file: %10s filespan: %s' % (f, '?') filespan_sum += file_span filespan_cnt += 1 if filespan_cnt > 0: filespan_avg = '%3.2f' % (float(filespan_sum)/float(filespan_cnt)) if options.solve: print ' avg filespan: %6s' % (filespan_avg) else: print ' avg filespan: ?' print '\nspan: directories' dirspan_sum = 0 dirspan_cnt = 0 for f in self.name_to_inode_map: all_addresses = [] inode_number, inode_address, min_address, max_address = self.get_spans(f) if self.inode_type[inode_number] != 'directory': continue for address in [inode_address, min_address, max_address]: all_addresses.append(address) for entry_name, entry_inode_number in self.dir_data[inode_number]: if entry_name == '.' or entry_name == '..': continue if self.inode_type[entry_inode_number] == 'directory': continue for i in range(3): all_addresses.append(span_results[entry_inode_number][i]) all_sorted = sorted(all_addresses) # print 'dirspan', f, all_sorted[len(all_sorted)-1], all_sorted[0] dirspan = all_sorted[len(all_sorted)-1] - all_sorted[0] dirspan_sum += dirspan dirspan_cnt += 1 if options.solve: print ' dir: %10s dirspan: %3d' % (f, dirspan) else: print ' dir: %10s dirspan: ?' % (f) dirspan_avg = '%3.2f' % (float(dirspan_sum)/float(dirspan_cnt)) if options.solve: print ' avg dirspan: %6s' % (dirspan_avg) else: print ' avg dirspan: ?' print '' return # # main program # parser = OptionParser() parser.add_option('-s', '--seed', default=0, help='the random seed', action='store', type='int', dest='seed') parser.add_option('-n', '--num_groups', default=10, help='number of block groups', action='store', type='int', dest='num_groups') parser.add_option('-d', '--datablocks_per_groups', default=30, help='data blocks per group', action='store', type='int', dest='blocks_per_group') parser.add_option('-i', '--inodes_per_group', default=10, help='inodes per group', action='store', type='int', dest='inodes_per_group') parser.add_option('-L', '--large_file_exception', default=30, help='0:off, N>0:blocks in group before spreading file to next group', action='store', type='int', dest='large_file_exception') parser.add_option('-f', '--input_file', default='/no/such/file', help='command file', action='store', type='string', dest='input_file') parser.add_option('-I', '--spread_inodes', default=False, help='Instead of putting file inodes in parent dir group, \ spread them evenly around all groups', action='store_true', dest='spread_inodes') parser.add_option('-D', '--spread_data', default=False, help='Instead of putting data near inode, \ spread them evenly around all groups', action='store_true', dest='spread_data_blocks') parser.add_option('-A', '--allocate_faraway', default=1, help='When picking a group, examine this many groups at a time', action='store', dest='allocate_faraway', type='int') parser.add_option('-C', '--contig_allocation_policy', default=1, help='number of contig free blocks needed to alloc', action='store', type='int', dest='contig_allocation_policy') parser.add_option('-T', '--show_spans', help='show file and directory spans', default=False, action='store_true', dest='show_spans') parser.add_option('-M', '--show_symbol_map', help='show symbol map', default=False, action='store_true', dest='show_symbol_map') parser.add_option('-B', '--show_block_addresses', help='show block addresses alongside groups', action='store_true', default=False, dest='show_block_addresses') parser.add_option('-S', '--do_per_file_stats', help='print out detailed inode stats', action='store_true', default=False, dest='do_per_file_stats') parser.add_option('-v', '--show_file_ops', help='print out detailed per-op success/failure', action='store_true', default=False, dest='show_file_ops') parser.add_option('-c', '--compute', help='compute answers for me', action='store_true', default=False, dest='solve') (options, args) = parser.parse_args() random.seed(options.seed) fs = file_system(num_groups=options.num_groups, blocks_per_group=options.blocks_per_group, inodes_per_group=options.inodes_per_group, large_file_exception=options.large_file_exception, spread_inodes=options.spread_inodes, contig_allocation_policy=options.contig_allocation_policy, spread_data_blocks=options.spread_data_blocks, allocate_faraway=options.allocate_faraway, show_block_addresses=options.show_block_addresses, do_per_file_stats=options.do_per_file_stats, show_file_ops=options.show_file_ops, show_symbol_map=options.show_symbol_map, compute=options.solve) fs.read_input(options.input_file) fs.dump() if options.show_spans: fs.do_all_spans()
Questions
-
Examine the file
in.largefile
, and then run the simulator with flag-f in.largefile
and-L 4
. The latter sets the large-file exception to 4 blocks. What will the resulting allocation look like? Run with-c
to check. -
Now run with
-L 30
. What do you expect to see? Once again, turn on-c
to see if you were right. You can also use-S
to see exactly which blocks were allocated to the file/a
. -
Now we will compute some statistics about the file. The first is something we call filespan, which is the max distance between any two data blocks of the file or between the inode and any data block. Calculate the filespan of
/a
. Runffs.py -f in.largefile -L 4 -T -c
to see what it is. Do the same with-L 100
. What difference do you expect in filespan as the large-file exception parameter changes from low values to high values? -
Now let’s look at a new input file,
in.manyfiles
. How do you think the FFS policy will lay these files out across groups? (you can run with-v
to see what files and directories are created, or justcat in.manyfiles
). Run the simulator with-c
to see if you were right. -
A metric to evaluate FFS is called dirspan. This metric calculates the spread of files within a particular directory, specifically the max distance between the inodes and data blocks of all files in the directory and the inode and data block of the directory itself. Run with
in.manyfiles
and the-T
flag, and calculate the dirspan of the three directories. Run with-c
to check. How good of a job does FFS do in minimizing dirspan? -
Now change the size of the inode table per group to 5 (
-I 5
). How do you think this will change the layout of the files? Run with-c
to see if you were right. How does it affect the dirspan? -
Which group should FFS place inode of a new directory in? The default (simulator) policy looks for the group with the most free inodes. A different policy looks for a set of groups with the most free inodes. For example, if you run with
-A 2
, when allocating a new directory, the simulator will look at groups in pairs and pick the best pair for the allocation. Run./ffs.py -f in.manyfiles -I 5 -A 2 -c
to see how allocation changes with this strategy. How does it affect dirspan? Why might this policy be good? -
One last policy change we will explore relates to file fragmentation. Run .
/ffs.py -f in.fragmented -v
and see if you can predict how the files that remain are allocated. Run with-c
to confirm your answer. What is interesting about the data layout of file/i
? Why is it problematic? -
A new policy, which we call contiguous allocation (
-C
), tries to ensure that each file is allocated contiguously. Specifically, with-C n
, the file system tries to ensure thatn
contiguous blocks are free within a group before allocating a block. Run./ffs.py -f in.fragmented -v -C 2 -c
to see the difference. How does layout change as the parameter passed to-C
increases? Finally, how does-C
affect filespan and dirspan?
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