Trend clean

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This example will change all valid trend values by x2. Recommended way of testing is to create a new trend log with the same parameters as the source.

Code:

infile = '/tmp/trends/t1.trend'
outfile = '/tmp/trends/t7.trend'

function adjust(value)
  value = value * 2
  return value
end

ffi = require('ffi')

function stringtodouble(chunk)
  local dbl = ffi.new('double[1]')
  ffi.copy(dbl, chunk, 8)
  return dbl[ 0 ]
end

function doubletostring(val)
  local dbl = ffi.new('double[1]')
  dbl[ 0 ] = val
  return ffi.string(dbl, 8)
end

info = require('rrd').info(infile)

data = io.readfile(infile)
offset = info.header_size

buf = { data:sub(1, offset) }

while true do
  chunk = data:sub(offset + 1, offset + 8)

  if #chunk == 8 then
    value = stringtodouble(chunk)

    -- skip NaN (empty/undefined value)
    if value == value then
      value = adjust(value)
    end

    buf[ #buf + 1 ] = doubletostring(value)
  else
    break
  end

  offset = offset + 8
end

data = table.concat(buf)
io.writefile(outfile, data)

For counter type the editing is trickier. The stored value is not the actual counter value but a per-second change rate. This example will log the maximum of all stored values without writing to a different trend log file. You can use this value for replacement in the previous script.

Code:

infile = '/tmp/trends/t1.trend'

max = 0

function check(value)
  max = math.max(max, value)
end

ffi = require('ffi')

function stringtodouble(chunk)
  local dbl = ffi.new('double[1]')
  ffi.copy(dbl, chunk, 8)
  return dbl[ 0 ]
end

info = require('rrd').info(infile)

data = io.readfile(infile)
offset = info.header_size

while true do
  chunk = data:sub(offset + 1, offset + 8)

  if #chunk == 8 then
    value = stringtodouble(chunk)

    -- skip NaN (empty/undefined value)
    if value == value then
      check(value)
    end
  else
    break
  end

  offset = offset + 8
end

log(max)

Note that when replacing values you should not use direct comparison (==) for floating point values due to possible rounding errors.