union mounts, mfs, and newsyslog

[matthew sporleder <msporleder%gmail.com@localhost>]

When I run newsyslog I get the following message on my union-mounted
mfs-over-ffs /var:
newsyslog: /var/log/wtmpx.1.gz: Cross-device link

From here, I think:
                PRINFO(("mv %s %s\n", file1, file2));
                if (!noaction)
                        if (rename(file1, file2))
                                err(EXIT_FAILURE, "%s", file1);


Caused by not having whiteout support on my /var filesystem.

file system: /dev/rwd0a
endian  little-endian
magic   11954 (UFS1)    time    Sun Jul  5 21:34:03 2009
superblock location     8192    id      [ 49fbaeb3 7b729202 ]
cylgrp  dynamic inodes  4.4BSD  sblock  FFSv2   fslevel 4
nbfree  8044    ndir    2021    nifree  79762   nffree  10176
ncg     10      size    466560  blocks  453503


But I'm using an MFS:
/dev/wd0b       /var/log_mfs mfs rw,hidden,-s10M 0 0
/var/log_mfs    /var/log union rw - -

So is it a limitation of mfs, union mounting, or my ffs?

Thanks,
Matt