XFS: no space left on device, (but I have 850GB available!)

I'm using a combination of mdadm, lvm2, and XFS on Amazon EC2.

So far, I've had success running a RAID 5 volume built from a number of EBS volumes. The EBS volumes are attached and used with mdadm to create the RAID 5. Then, I use LVM to present the resulting RAID as a single physical volume and single logical volume.

In the past, I've been able to grow the file system by adding a new EBS volume, attaching it, and then running the following process.

mdadm --add /dev/md0 /dev/xvdi

# grow the raid... (could take a while for a large disk!)
mdadm --grow /dev/md0 --raid-devices=4

# grow the LVM physical volume
pvresize /dev/md0

# grow the LVM logical volume ... fairly certain
# -l100%PVS will make the extents use as much space
# as possible on physical disks (and hopefully won't overwrite anything)
lvresize -l100%PVS /dev/data_vg/data_lv

# resize the file system, too!
xfs_growfs /dev/data_vg/data_lv

# yay!
df -h

My most recent attempt at doing this has worked just fine, ostensibly. Running df -ih/fh shows that I have a mounted filesystem with an additional terabyte available, as expected. Also, the total number of inodes used is ONLY 1%. Also, pvdisplay and lvdisplay also show the proper volume sizes.

I've even been able to add some data (about 150GB) to the volume since growing it. However, today I attempt to create a directory and get

mkdir: no space left on device

Why would I encounter this problem if I allegedly have plenty of inodes availble?

I've unmounted the disk and run xfs_check, but that does not report any issues.

Thanks!