ext4如何使用fallocate

Question

最近，我正在测试ext4文件系统的正确用法。我的专家是：

当系统崩溃时，数据已经被写回ok不能丢失，但是元数据可以。

下面是我的用法：

   1. call fallocate to alloc centain space          fallocate(fd, 0, 0, 4\*1024\*1024);    //4MB
   2. call fsync(fd)  let data and metadata write to disks
   3. then i call function to randomly write the file with 4k   size(random data but not 0).  with O\_DRICT flag，but not call fsync.  I log the offset with return write ok.
   4. check the offset that logged. but i find in some offset, read 4k data, is 0. It seems mean that offset isn't used like hole files.  

我的问题是：

<1. why after calling fallocate and fsync the metadata of the file still seems indicate some blocks is not used, so when read it return null. It is my understand .

<2. have other api to call, can make sure that in allocate space with file is not holes ,after that  when write data return ok with O\_DIRECT  can make sure the data will not be loss even the system crashed. 

Thanks.

Answer 1

只有写入文件空间才能消除漏洞。如果不写，就没有脏页，fsync什么也不做。

我想知道你是如何执行你的第四步的。看起来你是手动崩溃的，是吗？如果你在没有崩溃的情况下读完它，它不应该是零，只要你写了非零。如果您在崩溃后读取它，如果存在磁盘缓存，则可能发生零。然而，这种类型的零不像孔，它们是从磁盘读取的零(很可能磁盘包含零)。