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<p><?php<br>
/*<br>
What causes performance degradation when using the fsync function in PHP? How to optimize it?<br>
*/</p>
<p>/*<br>
In PHP, the fsync function is used to force the file buffer’s data to be written to disk, ensuring data durability and integrity. While fsync guarantees data safety, it often causes performance degradation in practice, especially in scenarios with frequent writes. This article explores the reasons for this performance drop and suggests optimization methods.<br>
*/</p>
<p>/*</p>
<ol>
<li>
<p>Reasons for performance degradation</p>
</li>
<li>
<p>Overhead of synchronous disk writes<br>
fsync directly invokes the system’s low-level synchronous operation, forcing the kernel buffer data to be physically written to disk and waiting for the disk to complete the write. Disk I/O physical operations are much slower than memory speed, so each fsync call can introduce significant delay.</p>
</li>
<li>
<p>Blocking nature<br>
fsync is a blocking function; it blocks the current process or thread until the write completes. Frequent calls to fsync cause long pauses in the program, impacting overall performance.</p>
</li>
<li>
<p>Differences in disk devices<br>
Different types of disk devices (HDD vs SSD) have significant differences in fsync response times. Mechanical HDDs have longer physical seek times, making fsync delays more noticeable.</p>
</li>
<li>
<p>Impact of file system and OS level<br>
Different file systems implement and optimize fsync differently. Some file systems reduce fsync costs via journaling or other mechanisms, while others have higher overhead.<br>
*/</p>
</li>
</ol>
<p>/*<br>
2. How to optimize fsync performance</p>
<ol>
<li>
<p>Reduce the number of fsync calls<br>
Batch multiple write operations before calling fsync, avoiding calling fsync after every small write.</p>
</li>
<li>
<p>Asynchronous and batch writes<br>
Use asynchronous queues or caching mechanisms to write data first to cache, then call fsync periodically or in batches to reduce blocking frequency.</p>
</li>
<li>
<p>Use faster storage devices<br>
Replacing mechanical HDDs with SSDs can significantly reduce fsync write latency.</p>
</li>
<li>
<p>Choose appropriate file systems<br>
Select file systems optimized for synchronous writes, such as ext4 with journaling mode, or others that support more efficient writes.</p>
</li>
<li>
<p>Evaluate if fsync is necessary<br>
If data durability requirements are not strict, consider using fflush or delaying fsync calls, balancing performance and data safety.<br>
*/</p>
</li>
</ol>
<p>/*<br>
3. Sample code optimization<br>
*/</p>
<p>// Non-optimized example: Frequent calls to fsync<br>
$file = fopen('data.log', 'a');<br>
for ($i = 0; $i < 1000; $i++) {<br>
fwrite($file, "Line <span class="hljs-subst">$i\n");<br>
fflush($file);<br>
// Forces synchronization after every write, very poor performance<br>
fsync(fileno($file));<br>
}<br>
fclose($file);</p>
<p>// Optimized example: Batch writes and call fsync once at the end<br>
$file = fopen('data.log', 'a');<br>
for ($i = 0; $i < 1000; $i++) {<br>
fwrite($file, "Line <span class="hljs-subst">$i\n");<br>
}<br>
fflush($file);<br>
fsync(fileno($file)); // Calls fsync only once, reducing blocking<br>
fclose($file);</p>
<p data-is-last-node="" data-is-only-node="">/*<br>
Summary:<br>
fsync ensures data safety, but its synchronous disk write nature causes performance bottlenecks. By designing writing strategies reasonably, reducing the frequency of calls, and choosing appropriate storage devices, it’s possible to optimize performance while ensuring data safety.<br>
*/<br>
