如何在同步/ fsync / syncfs到便携式设备后确保数据完整性 [英] How to make sure data integrity after sync/fsync/syncfs to portable device

问题描述

基于同步手册页，无法保证在调用同步后光盘将刷新其缓存：
根据标准规范（例如，POSIX.1-2001），sync（）会调度写入，但是在实际写入完成之前可能会返回。但是，由于版本1.3.20 Linux确实在等待。（这仍然不能保证数据完整性：现代磁盘有大量缓存。）

并且，在fsync手册中，没有提到这一点。

有没有办法确保所有写入光盘特别是便携式设备（USB）的写入调用同步后完成？我遇到过调用sync / fsync后数据和元数据信息没有完全写入光盘的情况。
我很好奇Windows / linux中的安全删除设备如何知道所有数据都已由设备本身完全写入。

解决方案

我很好奇Windows / linux中的安全删除设备是如何知道所有数据都已由设备本身完全写入。

对于IXish系统：

使用 umount 命令或 umount（）系统调用。

---

做

  blockdev --flushbufs 
  

可能会刷新设备的缓冲区，但不会让任何人再次访问设备和补充缓冲区。

---

此外， / proc 文件系统：

  / proc / sys / vm / drop_caches 
  

可用于冲洗不同的缓冲区：

从 https://www.kernel.org/doc/Documentation/sysctl/vm.txt

[...]

要释放dentries和inode：

  echo 2> / proc / sys / vm / drop_caches 
  

[...]

Based on sync manual page, there is no guarantee the disc will flush its cache after calling sync: "According to the standard specification (e.g., POSIX.1-2001), sync() schedules the writes, but may return before the actual writing is done. However, since version 1.3.20 Linux does actually wait. (This still does not guarantee data integrity: modern disks have large caches.) "

And, in fsync manual, there is no mention about this.

Is there ways to makes sure all writes to disc especially portable device (USB) has been finished after calling sync? I have encountered cases that data and metadata information has not fully written to disc after calling sync/fsync. I am curious how "Safely remove device" in windows/linux knows that all data has been fully written by the device itself.

解决方案

I am curious how "Safely remove device" in windows/linux knows that all data has been fully written by the device itself.

For IXish systems:

Unmount the USB-device's partitions using the umount command or the umount() system call.

---

Doing

blockdev --flushbufs

might flush the device's buffer, but does not keep anybody from accessing the devices again and refill buffers.

---

Also there is this kernel interface in the /proc file system:

/proc/sys/vm/drop_caches

which can be used to flush different buffers:

Verbatim from https://www.kernel.org/doc/Documentation/sysctl/vm.txt

[...]

To free dentries and inodes:

  echo 2 > /proc/sys/vm/drop_caches

[...]

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