Embedded Sharing #26: Why Use Sync?

Cover ImageThe Southern Right Whale Dolphin (Lissodelphis peronii) is a unique species of dolphin that only lives in the Southern Hemisphere, characterized by the absence of a dorsal fin and its black and white coloration.Main ContentIn Linux systems, we often need to execute the sync command after modifying files to save the changes. Why is this necessary? What is the underlying logic?Let’s first look at the literal meaning,sync is short for “Synchronize”, which means “to synchronize”. In other words, executing the sync command in a Linux system is for synchronization. Synchronization? Why is synchronization needed? With whom do we synchronize?

This is because the Linux system uses a page cache mechanism to improve performance and efficiency: when writing files, data is first temporarily stored in memory and is only written to disk in batches when certain conditions are met (such as high cache usage or system scheduled tasks).The core function of sync is to force the immediate synchronization of modified data in memory to the disk, preventing data loss due to the caching mechanism.

In simpler terms, when we modify file data in a Linux system, it is first temporarily stored in memory and not immediately written to disk. This is done to enhance system performance and efficiency (as we know, the read and write speeds of memory and disk are not on the same level). However, this mechanism carries certain risks: for example, if the system suddenly loses power, a program crashes, or a device is hot-swapped, the data that has not been written to disk in time will be permanently lost.

Therefore, after modifying files, we can use the sync command to immediately synchronize the data in memory to the disk, preventing the occurrence of the aforementioned risks.

In practice, as long as the system is normally shut down or rebooted, these commands already include the sync operation, so it is not necessary to manually execute sync. However, for safety’s sake, we are accustomed to executing sync again.

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