FFS

UFS的目录项中的文件名字段是定长的(参见sys/ufs/dir.h)，这样也许我们可以把symlink的内容(如果够短的话)保存到目录项，而不是inode所指定的fragment上，从而访问symlink可以节省最多两次I/O操作。

And finally, it happend.

On Monday, one of our server, running FreeBSD, panic’ed with “ffs_clusteralloc: map mismatch”. This message is quite uncommon and should only happen when there is some “event” with the memory or hard disk.

I have suggested our administrator to reboot and do “fsck” on the volume, since it has very small block size and can not be checked background (I have made a change to warn about this to FreeBSD, and was MFC’ed to 5-STABLE so you will get the feature in 5.4-RELEASE). Unfortunatelly, the check is so slow and we start to suspect whether there is some hardware problem.

While reading the snapshot code in FFS Soft Updates’s FreeBSD implementation, I got a question about the following code:

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		/*
		 * If there is a fragment, clear it here.
		 */
		blkno = 0;
		loc = howmany(xp->i_size, fs->fs_bsize) - 1;
		if (loc < NDADDR) {
			__len = fragroundup(fs, blkoff(fs, xp->i\_size));__
			if (len < fs->fs_bsize) {
				ffs_blkfree(copy_fs, vp, DIP(xp, i_db[loc]),
				    len, xp->i_number);
				blkno = DIP(xp, i_db[loc]);
				DIP_SET(xp, i_db[loc], 0);
			}
		}