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

xlog-diff.lisp

#! /usr/bin/env sbcl --script

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

(defparameter *xlog-seg-size* (* 16 1024 1024)
  "Xlog segment size, as per PostgreSQL pg_config.h")

(defparameter *xlog-segments-per-xlog-id* 254
  "How many Xlog segments we find per xlog ID (00 and FF are skipped)")

(defun xlog-from-filename (filename)
  "Return the current XLOG position from given filename"
  ;; see PostgreSQL src/include/access/xlog_internal.h XLogFromFileName()
  (let* ((position (parse-integer filename :radix 16))
   (seg      (ldb (byte 32 0) position))	 ; first 8 hex digits
	 (log      (ldb (byte 32 32) position))	 ; next 8 hex digits
	 (tli      (ldb (byte 32 64) position))) ; next 8 hex digits
    (+ seg (* log *xlog-segments-per-xlog-id*))))

(defun pretty-print-bytes (bytes)
  "Return a string to reprensent bytes in human readable format, with units"
  (loop
     for unit in '("TB" "GB" "MB" "kB")
     for power in '(40 30 20 10 1)
     for limit = (expt 2 power)
     until (<= limit bytes)
     finally (return (format nil "~4,1f ~a" (/ bytes limit) unit))))

(let* ((args (rest sb-ext:*posix-argv*))

       (wal1 (first args))
       (wal2 (second args))

       (pos1 (xlog-from-filename wal1))
       (pos2 (xlog-from-filename wal2))

       (diff (if (< pos1 pos2) (- pos2 pos1) (- pos1 pos2)))
       (bytes (* diff *xlog-seg-size*)))

  (format t "XLog diff in WAL segments: ~d~%" diff)
  (format t "Xlog diff in bytes:        ~a~%" (pretty-print-bytes bytes)))