sync is a standardsystem call in theUnix operating system, which commits all data from thekernelfilesystembuffers tonon-volatile storage, i.e., data which has been scheduled for writing via low-levelI/O system calls. Higher-level I/O layers such asstdio may maintain separate buffers of their own.
As a function inC, thesync() call is typically declared asvoid sync(void) in<unistd.h>. The system call is also available via acommand line utility also calledsync, and similarly named functions in other languages such asPerl andNode.js (in the fs module).
The related system callfsync() commits just the buffered data relating to a specifiedfile descriptor.[1]fdatasync() is also available to write out just the changes made to the data in the file, and not necessarily the file's related metadata.[2]
Some Unix systems run a kind offlush orupdatedaemon, which calls thesync function on a regular basis. On some systems, thecron daemon does this, and onLinux it was handled by thepdflush daemon which was replaced by a new implementation and finally removed from the Linux kernel in 2012.[3] Buffers are also flushed when filesystems areunmounted or remountedread-only,[4] for example prior to system shutdown.
Some applications, such asLibreOffice, also call thesync function to save recovery information in an interval.
In order to provide properdurability, databases need to use some form of sync in order to make sure the information written has made it tonon-volatile storage rather than just being stored in a memory-based write cache that would be lost if power failed.PostgreSQL for example may use a variety of different sync calls, includingfsync() andfdatasync(),[5] in order for commits to be durable.[6] Unfortunately, for any single client writing a series of records, a rotating hard drive can only commit once per rotation, which makes for at best a few hundred such commits per second.[7] Turning off the fsync requirement can therefore greatly improve commit performance, but at the expense of potentially introducing database corruption after a crash.
Databases also employtransaction log files (typically much smaller than the main data files) that have information about recent changes, such that changes can be reliably redone in case of crash; then the main data files can be synced less often.
To avoid any data loss return values offsync() should be checked because when performing I/O operations that are buffered by the library or the kernel, errors may not be reported at the time of using thewrite() system call or thefflush() call, since the data may not be written to non-volatile storage but only be written to the memorypage cache. Errors from writes are instead often reported during system calls tofsync(),msync() orclose().[8] Prior to 2018, Linux'sfsync() behavior under certain circumstances failed to report error status,[9][10] change behavior was proposed on 23 April 2018.[11]
Hard disks may default to using their own volatile write cache to buffer writes, which greatly improves performance while introducing a potential for lost writes.[12] Tools such ashdparm -F will instruct the HDD controller to flush the on-drive write cache buffer. The performance impact of turning caching off is so large that even the normally conservativeFreeBSD community rejected disabling write caching by default in FreeBSD 4.3.[13]
InSCSI and inSATA withNative Command Queuing (but not in plain ATA, even with TCQ) the host can specify whether it wants to be notified of completion when the data hits the disk's platters or when it hits the disk's buffer (on-board cache). Assuming a correct hardware implementation, this feature allows the disk's on-board cache to be used while guaranteeing correct semantics for system calls likefsync.[14] This hardware feature is calledForce Unit Access (FUA) and it allows consistency with less overhead than flushing the entire cache as done for ATA (or SATA non-NCQ) disks.[15] Although Linux enabled NCQ around 2007, it did not enable SATA/NCQ FUA until 2012, citing lack of support in the early drives.[16][17]
Firefox 3.0, released in 2008, introducedfsync system calls that were found to degrade its performance; the call was introduced in order to guarantee the integrity of the embeddedSQLite database.[18]Linux Foundationchief technical officerTheodore Ts'o claims there is no need to "fear fsync", and that the real cause of Firefox 3 slowdown is the excessive use offsync.[19] He also concedes however (quotingMike Shaver) that
On some rather common Linux configurations, especially using theext3 filesystem in the "data=ordered" mode, calling fsync doesn't just flush out the data for the file it's called on, but rather on all the buffered data for that filesystem.[20]