My work

NetBSD has UFS implementation that allows to mount UFS file systems created on architecture with different endianess. In other words, one can do newfs on sparc64 and mount it on i386 or vice versa. Very cool. It works by detecting what endianess file system uses and byteswapping fields in UFS structures at run-time as needed. I wanted to see how hard will it be to implement something simlar on FreeBSD. After one-night hacking mounting file systems read-only seems to works fine. I decided to work a bit lower than NetBSD and I replace bread()s with special functions that byteswap fields when needed. This saves quite a lot of code, but not everything can be implemented that way. I can byteswap the superblock, cylinder groups and inodes, but I can’t do the same for dirents, because ufs_readdir() use plain VOP_READ() to read directory entires, so I need to do the same NetBSD does for dirents. My method is most likely slower than NetBSD’s, because when file system reads one block of inodes, I byteswap them all and only one inode may be used later. We will see if performance impact is too high at some point. On the other hand you probably don’t want to use this functionality very often.

This entry was posted on Sunday, January 28th, 2007 at 1:39 am and is filed under FreeBSD. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.