Python : 计算大文件MD5值

buffer = 8192 来自网络，但不知道是怎么实践出来的。

from hashlib import md5
import time
import os
 
def calMD5(str):
    m = md5()
    m.update(str)
     
    return m.hexdigest()
 
def calMD5ForFile(file):
    statinfo = os.stat(file)
     
    if int(statinfo.st_size)/(1024*1024) >= 1000 :
        print "File size > 1000, move to big file..."
        return calMD5ForBigFile(file)
    
     
    m = md5()
    f = open(file, 'rb')
    m.update(f.read())
    f.close()
     
    return m.hexdigest()
 
def calMD5ForFolder(dir,MD5File):
    outfile = open(MD5File,'w')
    for root, subdirs, files in os.walk(dir):
        for file in files:
            filefullpath = os.path.join(root, file)
            """print filefullpath"""
             
            filerelpath = os.path.relpath(filefullpath, dir)
            md5 = calMD5ForFile(filefullpath)
            outfile.write(filerelpath+' '+md5+"\n")
    outfile.close()
 
def calMD5ForBigFile(file):
    m = md5()
    f = open(file, 'rb')
    buffer = 8192    # why is 8192 | 8192 is fast than 2048
     
    while 1:
        chunk = f.read(buffer)
        if not chunk : break
        m.update(chunk)
         
    f.close()
    return m.hexdigest()
     
     
 
 
if __name__ == "__main__":
    #print calMD5("Hello World!")
     
    t = time.time()
    print calMD5ForFile("E:\\OS\\ubuntu-11.04-desktop-i386.iso")
    print time.time() - t    
    t = time.time()
    print calMD5ForBigFile("E:\\OS\\ubuntu-11.04-desktop-i386.iso")
    print time.time() - t,"\n"
     
    t = time.time()
    print calMD5ForFile("E:\\OS\\ubuntu-12.04-desktop-amd64.iso")
    print time.time() - t   
    t = time.time()
    print calMD5ForBigFile("E:\\OS\\ubuntu-12.04-desktop-amd64.iso")
    print time.time() - t,"\n"
     
    t = time.time()
    print calMD5ForFile("D:\\Virtual Machines\\Ubuntu 64-bit\\Ubuntu 64-bit-s001.vmdk")
    print time.time() - t   
    t = time.time()
    print calMD5ForBigFile("D:\\Virtual Machines\\Ubuntu 64-bit\\Ubuntu 64-bit-s001.vmdk")
    print time.time() - t,"\n"
     
 
     
#output   
#8b1085bed498b82ef1485ef19074c281
#2.57500004768
#8b1085bed498b82ef1485ef19074c281
#3.34100008011
#
#128f0c16f4734c420b0185a492d92e52
#2.632999897
#128f0c16f4734c420b0185a492d92e52
#3.39100003242
#
#File size > 1000, move to big file...
#ec1fa4dc1b32569e9da7b4744548a9ef
#5.40100002289
#ec1fa4dc1b32569e9da7b4744548a9ef
#5.42100000381