Very slow writing to file

[czarnikjak]

I am not sure what i am doing wrong, but I am experiencing very slow writing to a file located on built-in flash of Pico.
The file is 250KB in size, and I am writing only 128 bytes to it (after seeking to a specific offset).

I measured the time it takes to write with the ticks_us diff, and it takes over 3 seconds! Please tell me I am doing something wrong and this should be much faster...

     with open('myfile', mode='r+b') as file:
          file.seek(position)
          t = time.ticks_us()
          file.write(myByteArray[128:256])
      
      print(time.ticks_diff(time.ticks_us(), t))

[jimmo]

@czarnikjak Is this easily reproducible? Does the slow write happen every time. Are you able to provide a simple script I can run here to test the same thing?

For some background... 3 seconds seems too long. Sometimes writes will be slower as the LittleFS filesystem has to rebalance the flash and do housekeeping, but this should not occur on every write.

[czarnikjak]

Hi @jimmo ,
Repeatable every time on my Pico W,
Below is a test script. File is 250KB in size.

import time

myByteArray = bytearray(512)
position = 0
myFile = '1.file'

for i in range(20): 
  print('Position ',position)
  file = open(myFile, mode='r+b')
  file.seek(position)  
  file.write(myByteArray[128:256])
  t = time.ticks_us()
  file.flush()
  print(time.ticks_diff(time.ticks_us(), t))
  file.close()
  position = position + 10000

And here is the output, you can see writing 128bytes near the beginning of the file takes 3.5s, as you go further down the file its faster, about 1second if you writing 128bytes near the 200KB offset.

Position 0
3501963
Position 10000
3431295
Position 20000
3302977
Position 30000
3148699
Position 40000
3003530
Position 50000
2876461
Position 60000
2720220
Position 70000
2557618
Position 80000
2484944
Position 90000
2334595
Position 100000
2145765
Position 110000
2074833
Position 120000
1866241
Position 130000
1752181
Position 140000
1596832
Position 150000
1490915
Position 160000
1326186
Position 170000
1180414
Position 180000
1032127
Position 190000
926753

[robert-hh]

It seems to be a matter of the file system. With LFS I get the similar numbers on RP2 or e.g. MIMXRT. With a FAT file system the write times are consistent at about 140ms. It seems that with LFS the whole tail of the file is rewritten.

[czarnikjak]

Hmm...with LFS i get file system with size of about 860KB or so, but when trying to create FAT fs, my size seems to be only 89KB with the following parameters...

os.umount('/')
os.VfsFat.mkfs(flash)
os.mount(flash, '/')
print(os.statvfs('/'))
(512, 512, 178, 178, 178, 0, 0, 0, 0, 255)

There seems to be no options to customize anything, like a block size. Any ideas how to create larger FAT fs on Pico?

[robert-hh]

The standard block size for FAT is 4096. To get a FAT file system, try:

import os, rp2
bdev = rp2.Flash()
os.VfsFat.mkfs(bdev)
vfs = os.VfsFat(bdev)
os.mount(vfs, "/")

For frequent writes, FAT is bad, since it does not support wear leveling. The flash vendor usually guarantee ~100000 block erase cycles. In a test I tried to verify that. The flash chip broke after ~5 Million at best conditions.
If you need to write frequently to that file, better use another option, like either the one Jimmo suggested, of an external Micro-SD card. These support wear-leveling even with FAT and are generally way faster.

[czarnikjak]

Thanks, it defo creates 512 block size, with an overall FS size of only 89KB

os.umount('/')
bdev = rp2.Flash()
os.VfsFat.mkfs(bdev)
vfs = os.VfsFat(bdev)
os.mount(vfs, "/")
print(os.statvfs('/'))
(512, 512, 178, 178, 178, 0, 0, 0, 0, 255)

I will try splitting the file into little chunks like jimmo suggested.

[robert-hh]

Somehow the wring block size is reported. At a ARDUINO_RP2040_NANO_CONNECT the block size is reported as 4096, and bdev.ioctl() also reports 4096 as block size. maybe that's a bug to be fixed.
Anyhow, LFS is the better choice for flash based small file systems. When splitting files, you have to find the balance between the number of files and the file size. With many files, LFS slows down as well. But I just ran a quick test on a PICO W, creating, reading & deleting a series of files. With 100 files it was not a problem. Creating a short file filled with random numbers took ~74ms in average. With a FAT file system, it took ~260ms.

[jimmo]

FYI @czarnikjak @robert-hh this behaviour in LittleFS is described here littlefs-project/littlefs#244

Suggested workaround is to break your data up into smaller files.

[jimmo]

Also thank you @czarnikjak for the very detailed report and repro!