Syncing to SD Card is too slow

Question

I'm writing some bytes to the filesystem. Afterward I'm calling sync to ensure the buffered data is flushed to the SD card. This command takes several seconds!

I'm working with raspbian. My SD Card is a 16GB Kingston Class 10 (industrial). Logging the disk io with sudo iotop -o -d 1 -a say, that only some 100 KBytes are written to disk.

To find out the source of the problem I'm doing following:

• Watching the output of sudo iotop -o -d 1 -a in one Terminal
• Run some application that writes about every minute some 100Kbytes
• Calling time sync in an other terminal after the write is done. This one takes sometimes several seconds (up to 5sec). (if nothing is written to disk it takes only some milliseconds)

Whats wrong that sync takes as long?

Answer 1

First, sync synchronizes all mounted file systems, not just the one you are writing to. Using Linux-specific syncfs call instead of sync could help here: if syncfs is always fast, the problem is likely with a different filesystem not the SD card.

The second possible offender could be the journal. Disabling the journal on your SD card will likely make sync much more deterministic.

In any case, check for background tasks which could be modifying files, like unattended upgrades. You cannot synchronize just the 100KB your program has written, you always synchronize the entire filesystem. You should have seen this in iotop though.

Answer 2

Until sync or flush is called, the writes to the filesystem might be cached in RAM. Or perhaps metadata to the filesystem isn't written yet.

Once sync is called, all this data must be really written to the filesystem.

This is why it's a bad idea to turn any linux system off until you've shutdown, which does the equivalent of a sync. Some cached data might not have been written yet, or you could be turning it off in the middle of a write.