关于jvm:Try-to-Avoid-XX-UseGCLogFileRotation

Try to Avoid -XX:+UseGCLogFileRotation

Source：http://link.zhihu.com/?target=https%3A//dzone.com/articles/try-to-avoid-xxusegclogfilerotation

Developers take advantage of the JVM argument -XX:+UseGCLogFileRotation to rotate GC log files.

开发人员利用JVM参数-XX:+UseGCLogFileRotation来旋转GC日志文件。

-XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:/home/GCEASY/gc.log -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=5 -XX:GCLogFileSize=20M"

As shown above, the JVM will rotate the GC log file whenever its size reaches 20MB. It will generate up to five files, with extensions gc.log.0, gc.log.1, gc.log.2, gc.log.3, and gc.log.4.

如上所示的配置会产生5个日志文件，并且每个日志文件有20M。

Losing Old GC Logs

Suppose you configured -XX:NumberOfGCLogFiles=5, then over a period of time, five GC log files will be created:

假如你配置了-XX:NumberOfGCLogFiles=5，那么在一段期间内，将创立五个GC日志文件。

gc.log.0 ← _oldest GC Log content_
gc.log.1
gc.log.2
gc.log.3
gc.log.4 ← _latest GC Log content_

The most recent GC log contents will be written to gc.log.4 and old GC log contents will be present in gc.log.0.

最新的GC日志内容将被写入gc.log.4，旧的GC日志内容将呈现在gc.log.0。

When the application starts to generate more GC logs than the configured -XX:NumberOfGCLogFiles, in this case, five, then old GC log contents in gc.log.0 will be deleted. New GC events will be written to gc.log.0. It means that you will end up not having all the generated GC logs. You will lose the visibility of all events.

当应用程序配置的-XX:NumberOfGCLogFiles产生更多的GC日志时（在本例中是5个），gc.log.0中的旧GC日志内容将被删除。新的GC事件将被写入gc.log.0。这意味着你最终将无奈取得所有生成的GC日志。你将失去所有事件的可见性。

Mixed-Up GC Logs

Suppose an application has created five GC log files, including:

假如一个应用程序创立了五个GC日志文件，包含：

gc.log.0
gc.log.1
gc.log.2
gc.log.3
gc.log.4

Then, let’s say you are restarting the application. Now, new GC logs will be written to gc.log.0 file and old GC log content will be present in gc.log.1, gc.log.2, gc.log.3, gc.log.4, etc.

而后，假如你正在重启应用程序。当初，新的GC日志将被写入gc.log.0文件，而旧的GC日志内容将呈现在gc.log.1、gc.log.2、gc.log.3、gc.log.4等等。

gc.log.0 ← GC log file content after restart
gc.log.1 ← GC log file content before restart
gc.log.2 ← GC log file content before restart
gc.log.3 ← GC log file content before restart
gc.log.4 ← GC log file content before restart

So, your new GC log contents get mixed up with old GC logs. Thus, to mitigate this problem, you might have to move all the old GC logs to a different folder before you restart the application.

所以你的新GC日志和旧的GC日志会混合在一起，为了缓解这个问题，你可能要在重启应用程序之前把所有旧的GC日志移到一个不同的文件夹里。

Forwarding GC Logs to a Central Location（将GC日志转发到一个核心地位）

In this approach, the current active file to which GC logs are written is marked with the extension .current. For example, if GC events are currently written to the file gc.log.3, it would be named as: gc.log.3.current.

在这种办法中，以后写入GC日志的流动文件被标记为扩展名.current。例如，如果GC事件以后被写入文件gc.log.3，它将被命名为。 gc.log.3.current。

If you want to forward GC logs from each server to a central location, then most DevOps engineers use rsyslog. However, this file naming convention poses a significant challenge to use rsyslog, as described in this blog.

如果你想把每台服务器的GC日志转发到一个核心地位，那么大多数DevOps工程师会应用rsyslog。然而，这种文件命名常规给应用rsyslog带来了微小的挑战，正如[本博客所述]（http://www.planetcobalt.net/s...）。

Tooling

Now, to analyze the GC log file using the GC tools such as (GCeasy, GCViewer, etc.), you will have to upload multiple GC log files instead of just one single GC Log file.

当初，为了应用GC工具剖析GC日志文件，如（GCeasy, GCViewer等），你将不得不上传多个GC日志文件，而不是只有一个GC日志文件。

Try to Avoid -XX:+UseGCLogFileRotation

Losing Old GC Logs

Mixed-Up GC Logs

Forwarding GC Logs to a Central Location（将GC日志转发到一个核心地位）

Tooling

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