English 中文(简体)
How to check if the block is present in a sparse file (for simple copy-on-write)?
原标题:

How to get sparse block size and check if data is present at the given offset in sparse file in reiserfs/ext3 in Linux?

I want to use it to implement simple copy-on-write block device using FUSE.

Or I should better keep a bitmap in a separate file?

最佳回答

/usr/src/linux/Documentation/filesystems/fiemap.txt

The fiemap ioctl is an efficient method for userspace to get file extent mappings. Instead of block-by-block mapping (such as bmap), fiemap returns a list of extents.

There s a quick example of usage in git://kernel.ubuntu.com/cking/debug-code/fiemap/. A sparse file will lack extents for the "missing" portions.

问题回答

Since Linux 3.1, lseek provides flags SEEK_HOLE and SEEK_DATA to navigate to the beginning or end of a hole, so this might be an alternative to the ioctl based solution. Haven t tried either in practice, so I don t have any real experience to compare the two.

Well, http://lxr.linux.no/#linux+v2.6.33/arch/um/drivers/cow_user.c indicates that User Mode Linux uses an explicit bitmap for this, FWIW.





相关问题
Signed executables under Linux

For security reasons, it is desirable to check the integrity of code before execution, avoiding tampered software by an attacker. So, my question is How to sign executable code and run only trusted ...

encoding of file shell script

How can I check the file encoding in a shell script? I need to know if a file is encoded in utf-8 or iso-8859-1. Thanks

How to write a Remote DataModule to run on a linux server?

i would like to know if there are any solution to do this. Does anyone? The big picture: I want to access data over the web, using my delphi thin clients. But i´would like to keep my server/service ...

How can I use exit codes to run shell scripts sequentially?

Since cruise control is full of bugs that have wasted my entire week, I have decided the existing shell scripts I have are simpler and thus better. Here is what I have so far svn update /var/www/...

Good, free, easy-to-use C graphics libraries? [closed]

I was wondering if there were any good free graphics libraries for C that are easy to use? It s for plotting 2d and 3d graphs and then saving to a file. It s on a Linux system and there s no gnuplot ...

热门标签