compute lag between two XLOG file names in terms of files and bytes

xlog-diff.py

#! /usr/bin/env python

import os, sys

# compute lag between two XLOG file names in terms of files and bytes
# given 000000010000DAD7000000C4 and 000000010000DB1E00000032 we have:
# (int('DB1E',16) - int('DAD7', 16)) * 254 - int('C4',16) + int('132', 16)
#
# in pgsql:src/include/access/xlog_internal.h we have
#
#  XLogFromFileName(fname, tli, logSegNo)
#   uint32 log;
# 	uint32 seg;
# 	sscanf(fname, "%08X%08X%08X", tli, &log, &seg);
# 	*logSegNo = (uint64) log * XLogSegmentsPerXLogId + seg;
#
# and the files 00 and FF are skipped
#
# and in pg_config.h we have XLOG_SEG_SIZE (16 * 1024 * 1024)

XLOG_SEG_SIZE = 16 * 1024 * 1024
XLogSegmentsPerXLogId = 254

KB = 1024.0
MB = 1024 * KB
GB = 1024 * MB
TB = 1024 * GB

def xlog_from_filename(filename):
    "Return the current XLOG position from given filename"
    tli = int(filename[0:8], 16)
    log = int(filename[8:16], 16)
    seg = int(filename[16:24], 16)

    return log * XLogSegmentsPerXLogId + seg

def pprint_bytes(n):
    "Return a string to reprensent bytes in human readable format, with units"
    if n >= TB:
        return "%4.1f TB" % (n / TB)
    elif n >= GB:
        return "%4.1f GB" % (n / GB)
    elif n >= MB:
        return "%4.1f MB" % (n / MB)
    elif n >= GB:
        return "%4.1f KB" % (n / KB)
    else:
        return "%d B" % n

if __name__ == '__main__':
    wal1 = sys.argv[1]
    wal2 = sys.argv[2]

    pos1 = xlog_from_filename(wal1)
    pos2 = xlog_from_filename(wal2)

    diff = pos2 - pos1
    if diff < 0:
        diff = pos1 - pos2

    print "XLog diff in WAL segments: %d" % diff
    print "Xlog diff in bytes:        %s" % pprint_bytes(diff * XLOG_SEG_SIZE)