Q: How can I erase all quota project informations on my XFS file system?
Q: Should I use xfs_fsr to defragment my filesystem?
xfs_fsr(1) says "xfs_fsr impoves the organization of mounted filesystems." It does this by defragmenting individual files, one a time. However this has the side effect of fragmenting the free space. This makes the filesystem 'age' faster, which may adversely affect performance. So running xfs_fsr on a regular basis is generally not a useful thing to do.
On older kernels (2.6.34 and earlier) xfs_fsr can corrupt filesystems if they encounter a file with a specific layout. It can also produce bad results if you are using subsystems that store information in attributes (like selinux). Kernels 2.6.38 and later should be much more resistent to fragmentation due to the dynamic speculative allocation changes that went into 2.6.38.
The best way to avoid fragmentation on XFS is to not run your filesystems close to full for extended periods.