Reducing wear of SD card for datalogging

Projects General Guidance
1

Hello,

I have a question regarding the SD card library.

Given that SD cards have a limited amount of write and erase cycles, is it better for a data-logging application to concatenate all the variables into a char array, including the delimiters, and then perform a single file.print() with the array?

Is each usage of file.print() a separate write cycle?

Or does it make no difference to just file.print() each variable and delimiter separately?

Also, is there any speed advantage with using the concatenation method?

I made 2 working methods below to illustrate what I'm comparing.

Thanks!

#include <SPI.h>
#include <SD.h>

File myFile;

void setup() {
  // Open serial communications and wait for port to open:
  Serial.begin(9600);
  Serial.print("Initializing SD card...");

  if (!SD.begin(10)) {
    Serial.println("initialization failed!");
    while (1);
  }
  Serial.println("initialization done.");

SD.remove("test.txt");


int var1 = 111;
unsigned long var2 = 222;
unsigned int var3 = 333;
char var4[11] = "helloworld";
float var5 = 4.44;


//METHOD 1; concatenating variables into char array

char array_to_print[100]; 

//convert float to char
char var5_to_Char[5];
dtostrf(var5, 3, 2, var5_to_Char);

sprintf(array_to_print, "<%d,%lu,%d,%s,%s>\n",var1,var2,var3,var4,var5_to_Char);

  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  myFile = SD.open("test.txt", FILE_WRITE);

  // if the file opened okay, write to it:
  if (myFile) {
    Serial.print("Writing to test.txt...");
    myFile.print(array_to_print);
    // close the file:
    myFile.close();
    Serial.println("done.");
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }

//METHOD 2; printing each variable and delimmiter individually

  myFile = SD.open("test.txt", FILE_WRITE);

  // if the file opened okay, write to it:
  if (myFile) {
    Serial.print("Writing to test.txt...");
    myFile.print("<");
    myFile.print(var1);
    myFile.print(",");
    myFile.print(var2);
    myFile.print(",");
    myFile.print(var3);
    myFile.print(","); 
    myFile.print(var4);
    myFile.print(",");
    myFile.print(var5);       
    myFile.println(">");
    // close the file:
    myFile.close();
    Serial.println("done.");
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }


}

void loop() {
  // nothing happens after setup
}
2

The prints and writes are collected into a 512 byte buffer that is written when needed : when it’s full or when you do a flush or close so what matters to some extent is how often you dump that buffer on the card.

➜ some reading SD Card Write Cycles Overview - Knowledge Base EN - MOBOTIX Community

Also SD cards do flash wear leveling so that helps mitigate issues.

3

Thank you for the info on the 512 byte buffer. Based on your answer and the one I've linked, it seems like the only thing I need to do is limit the usage of file.close(). or file.flush(); .