blocks|key|2839405|text|rsync有几种比较文件的方法--权威的来源是rsync算法描述：https://www.andrew.cmu.edu/course/15-749/READINGS/required/cas/tridgell96.pdf。rsync上的wikipedia+article也非常好。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2839406|对于本地文件，rsync比较元数据，如果它看起来不需要复制文件，因为源和目标之间的大小和时间戳匹配，则不会进一步查看。如果它们不匹配，cp就是文件。但是，如果元数据确实匹配，但文件实际上并不相同，该怎么办？那么rsync可能没有做您想要的事情。|2839407|大小相同的文件可能仍已更改。一个简单的示例是一个文本文件，您可以在其中更正一个拼写错误--比如将"teh“更改为"the”。文件大小相同，但更正后的文件将具有较新的时间戳。--size-only说“不要看时间；如果大小匹配，就假定文件匹配”，这在这种情况下是错误的选择。|style|CODE|2839408|另一方面，假设你昨天不小心做了一个大的cp+-r+A+B，但是你忘了保存时间戳，现在你想用反向rsync+B+A来做这个操作。您cp‘s的所有文件都有昨天的时间戳，即使它们在昨天并没有真正被修改，rsync在默认情况下会复制所有这些文件，并将时间戳更新到昨天。在这种情况下，--size-only可能是你的朋友(在上面的例子中取模)。|2839409|--ignore-times说，无论文件的修改时间是否相同，都要比较这些文件。考虑一下上面的拼写错误示例，但是您不仅更正了拼写错误，而且还使用了touch来使更正后的文件具有与原始文件相同的修改时间--可以这么说，您在这方面是偷偷摸摸的。即使文件的大小和时间匹配，--ignore-times也会对文件进行比较。|2839410|entityMap|0|LINK|mutability|MUTABLE|url|https://www.andrew.cmu.edu/course/15-749/READINGS/required/cas/tridgell96.pdf|1|http://en.wikipedia.org/wiki/Rsync^0|X|25|0|3A|H|1|0|0|2E|B|0|J|9|1B|9|3T|B|0|0|E|20|5|3O|E|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@$A|Y|B|Z|1|10]|$A|11|B|12|1|13]]|C|$]]|$1|D|3|E|5|6|7|14|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|15|8|@$A|16|B|17|H|I]]|9|@]|C|$]]|$1|J|3|K|5|6|7|18|8|@$A|19|B|1A|H|I]|$A|1B|B|1C|H|I]|$A|1D|B|1E|H|I]]|9|@]|C|$]]|$1|L|3|M|5|6|7|1F|8|@$A|1G|B|1H|H|I]|$A|1I|B|1J|H|I]|$A|1K|B|1L|H|I]]|9|@]|C|$]]|$1|N|3|-4|5|6|7|1M|8|@]|9|@]|C|$]]]|O|$P|$5|Q|R|S|C|$T|U]]|V|$5|Q|R|S|C|$T|W]]]]

There are several ways rsync compares files -- the authoritative source is the rsync algorithm description: <a href="https://www.andrew.cmu.edu/course/15-749/READINGS/required/cas/tridgell96.pdf" rel="noreferrer">https://www.andrew.cmu.edu/course/15-749/READINGS/required/cas/tridgell96.pdf</a>. The <a href="http://en.wikipedia.org/wiki/Rsync" rel="noreferrer">wikipedia article</a> on rsync is also very good.

For local files, rsync compares metadata and if it looks like it doesn't need to copy the file because size and timestamp match between source and destination it doesn't look further. If they don't match, it cp's the file. However, what if the metadata do match but files aren't actually the same? Then rsync probably didn't do what you intended.

Files that are the same size may still have changed. One simple example is a text file where you correct a typo -- like changing "teh" to "the". The file size is the same, but the corrected file will have a newer timestamp. <code>--size-only</code> says "don't look at the time; if size matches assume files match", which would be the wrong choice in this case.

On the other hand, suppose you accidentally did a big <code>cp -r A B</code> yesterday, but you forgot to preserve the time stamps, and now you want to do the operation in reverse <code>rsync B A</code>. All those files you cp'ed have yesterday's time stamp, even though they weren't really modified yesterday, and rsync will by default end up copying all those files, and updating the timestamp to yesterday too. <code>--size-only</code> may be your friend in this case (modulo the example above).

<code>--ignore-times</code> says to compare the files regardless of whether the files have the same modify time. Consider the typo example above, but then not only did you correct the typo but you used <code>touch</code> to make the corrected file have the same modify time as the original file -- let's just say you're sneaky that way. Well <code>--ignore-times</code> will do a diff of the files even though the size and time match.

blocks|key|205300|text|您错过了rsync还可以通过校验和比较文件。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|205301|--size-only意味着rsync将跳过大小匹配的文件，即使时间戳不同。这意味着它将同步比默认行为更少的文件。它将丢失任何更改不会影响整个文件大小的文件。如果您需要在不更改文件的情况下更改文件的日期，并且不希望rsync花费大量时间对这些文件进行校验和以发现它们没有更改，则可以使用此选项。|offset|length|style|CODE|205302|--ignore-times意味着rsync将校验每个文件，即使时间戳和文件大小匹配也是如此。这意味着它将同步比默认行为更多的文件。它将包括对文件的更改，即使文件大小相同，并且修改日期/时间已重置为原始值。校验和每个文件意味着它必须完全从磁盘读取，这可能会很慢。一些构建管道会将时间戳重置为特定的日期(如1970-01-01)，以确保最终的构建文件是逐位可复制的，例如，当打包成保存时间戳的tar文件时。|205303|entityMap^0|0|0|B|0|0|E|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@$D|Q|E|R|F|G]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|K|$]]

You are missing that rsync can also compare files by checksum. 

<code>--size-only</code> means that rsync will skip files that match in size, even if the timestamps differ. This means it will synchronise fewer files than the default behaviour. It will miss any file with changes that don't affect the overall file size. If you have something that changes the dates on files without changing the files, and you don't want rsync to spend lots of time checksumming those files to discover they haven't changed, this is the option to use.

<code>--ignore-times</code> means that rsync will checksum every file, even if the timestamps and file sizes match. This means it will synchronise more files than the default behaviour. It will include changes to files even where the file size is the same and the modification date/time has been reset to the original value. Checksumming every file means it has to be entirely read from disk, which may be slow. Some build pipelines will reset timestamps to a specific date (like 1970-01-01) to ensure that the final build file is reproducible bit for bit, e.g. when packed into a tar file that saves the timestamps.

blocks|key|206813|text|在科学Linux+6.7系统上，rsync上的手册页上写道：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|206814|--ignore-times++++++++++don't+skip+files+that+match+size+and+time|code-block|syntax|javascript|206815|我有两个文件，内容相同，但创建日期不同：|206816|[root@windstorm+~]#+ls+-ls+/tmp/master/usercron+/tmp/new/usercron
4+-rwxrwx---+1+root+root+1595+Feb+15+03:45+/tmp/master/usercron
4+-rwxrwx---+1+root+root+1595+Feb+16+04:52+/tmp/new/usercron

[root@windstorm+~]#+diff+/tmp/master/usercron+/tmp/new/usercron
[root@windstorm+~]#+md5sum+/tmp/master/usercron+/tmp/new/usercron
368165347b09204ce25e2fa0f61f3bbd++/tmp/master/usercron
368165347b09204ce25e2fa0f61f3bbd++/tmp/new/usercron|206817|对于--size-only，这两个文件被视为相同的文件：|offset|length|style|CODE|206818|[root@windstorm+~]#+rsync+-v+--size-only+-n++/tmp/new/usercron+/tmp/master/usercron

sent+29+bytes++received+12+bytes++82.00+bytes/sec
total+size+is+1595++speedup+is+38.90+(DRY+RUN)|206819|对于--ignore-times，这两个文件被视为不同的文件：|206820|[root@windstorm+~]#+rsync+-v+--ignore-times+-n++/tmp/new/usercron+/tmp/master/usercron
usercron

sent+32+bytes++received+15+bytes++94.00+bytes/sec
total+size+is+1595++speedup+is+33.94+(DRY+RUN)|206821|所以看起来--ignore-times一点效果都没有。|206822|entityMap^0|0|0|0|0|2|B|0|0|2|E|0|0|5|E|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|12|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|14|8|@$M|15|N|16|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|18|8|@$M|19|N|1A|O|P]]|9|@]|A|$]]|$1|U|3|V|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|W|3|X|5|6|7|1C|8|@$M|1D|N|1E|O|P]]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]]|Z|$]]

On a Scientific Linux 6.7 system, the man page on rsync says:

<pre><code>--ignore-times don't skip files that match size and time
</code></pre>

I have two files with identical contents, but with different creation dates:

<pre><code>[root@windstorm ~]# ls -ls /tmp/master/usercron /tmp/new/usercron
4 -rwxrwx--- 1 root root 1595 Feb 15 03:45 /tmp/master/usercron
4 -rwxrwx--- 1 root root 1595 Feb 16 04:52 /tmp/new/usercron

[root@windstorm ~]# diff /tmp/master/usercron /tmp/new/usercron
[root@windstorm ~]# md5sum /tmp/master/usercron /tmp/new/usercron
368165347b09204ce25e2fa0f61f3bbd /tmp/master/usercron
368165347b09204ce25e2fa0f61f3bbd /tmp/new/usercron
</code></pre>

With <code>--size-only</code>, the two files are regarded the same:

<pre><code>[root@windstorm ~]# rsync -v --size-only -n /tmp/new/usercron /tmp/master/usercron

sent 29 bytes received 12 bytes 82.00 bytes/sec
total size is 1595 speedup is 38.90 (DRY RUN)
</code></pre>

With <code>--ignore-times</code>, the two files are regarded different:

<pre><code>[root@windstorm ~]# rsync -v --ignore-times -n /tmp/new/usercron /tmp/master/usercron
usercron

sent 32 bytes received 15 bytes 94.00 bytes/sec
total size is 1595 speedup is 33.94 (DRY RUN)
</code></pre>

So it does not looks like <code>--ignore-times</code> has any effect at all.

blocks|key|205352|text|简而言之，--ignore-times所做的比它的名字所暗示的更多。它忽略了时间和大小。相比之下，--size-only完全做到了它说的话。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|205353|205354|长篇大论的答案是，rsync有三种方法来确定文件是否过时：|205355|对源的大小和源的时间戳进行比较，并对源和目标的静态校验和进行destination.|205356|Compare。|unordered-list-item|205357|205358|这些检查在传输数据之前执行。值得注意的是，这意味着静态校验和不同于流校验和-后者是在传输数据时计算的。|205359|默认情况下，rsync仅使用1和2。单个stat可以同时获取1和2，而3需要读取整个文件(这独立于读取文件进行传输)。假设只指定了一个修饰符，这意味着：|205360|205361|通过使用--size-only，仅执行1-忽略时间戳和校验和。除非文件两端的大小相同，否则文件将被复制。|205362|通过使用--ignore-times，1、2或3都不执行。始终复制文件。通过使用--checksum执行|205363|，除1之外还使用%253，但不执行%252。除非大小和校验和匹配，否则将复制文件。仅当大小匹配时才计算校验和。|205364|205365|entityMap^0|5|E|1D|B|0|0|9|5|0|0|0|0|0|6|5|K|4|0|0|4|B|0|4|E|14|A|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|17|8|@]|D|@]|E|$]]|$1|G|3|H|5|6|7|18|8|@$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|I|3|J|5|6|7|1B|8|@]|D|@]|E|$]]|$1|K|3|L|5|M|7|1C|8|@]|D|@]|E|$]]|$1|N|3|-4|5|6|7|1D|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|1E|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1F|8|@$9|1G|A|1H|B|C]|$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|S|3|-4|5|6|7|1K|8|@]|D|@]|E|$]]|$1|T|3|U|5|M|7|1L|8|@$9|1M|A|1N|B|C]]|D|@]|E|$]]|$1|V|3|W|5|M|7|1O|8|@$9|1P|A|1Q|B|C]|$9|1R|A|1S|B|C]]|D|@]|E|$]]|$1|X|3|Y|5|M|7|1T|8|@]|D|@]|E|$]]|$1|Z|3|-4|5|6|7|1U|8|@]|D|@]|E|$]]|$1|10|3|-4|5|6|7|1V|8|@]|D|@]|E|$]]]|11|$]]

The short answer is that <code>--ignore-times</code> does more than its name implies. It ignores both the time and size.
In contrast, <code>--size-only</code> does exactly what it says.

<hr>

The long answer is that <code>rsync</code> has three ways to decide if a file is outdated:

<ol>
<li>Compare the size of source and destination.</li>
<li>Compare the timestamp of source and destination.</li>
<li>Compare the static checksum of source and destination.</li>
</ol>

These checks are performed before transferring data. Notably, this means the static checksum is distinct from the stream checksum - the later is computed while transferring data.

By default, <code>rsync</code> uses only 1 and 2. Both 1 and 2 can be acquired together by a single <code>stat</code>, whereas 3 requires reading the entire file (this is independent from reading the file for transfer). Assuming only one modifier is specified, that means the following:

<ul>
<li>By using <code>--size-only</code>, only 1 is performed - timestamps and checksum are ignored. A file is copied unless its size is identical on both ends.</li>
<li>By using <code>--ignore-times</code>, neither of 1, 2 or 3 is performed. A file is always copied.</li>
<li>By using <code>--checksum</code>, 3 is used in addition to 1, but 2 is not performed. A file is copied unless size and checksum match. The checksum is only computed if size matches.</li>
</ul>

I'm trying to understand what the difference is between two options 

<pre><code>rsync --size-only
</code></pre>

and

<pre><code>rsync --ignore-times
</code></pre>

It is my understanding that by default rsync will compare both the timestamps and the file sizes in order to decide whether or not a file should be synchronized or not. The options above allow the user to influence this behavior.

Both options seem, at least verbally to result in the same thing: comparing by size only.

Am I missing something subtle here?

rsync: difference between --size-only and --ignore-times

Linux

我在试着理解两种选择之间的区别rsync --size-only和rsync --ignore-times据我所知，默认情况下，rsync会比较时间戳和文件大小，以决定是否应该同步文件。上面的选项允许用户影响此行为。这两个选项似乎至少在口头上产生了相同的结果:仅按大小比较的。我是不是漏掉了什么微妙的东西？

问rsync：--size-only和--ignore-time之间的差异
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