I am having this kind of issue as the OP.
Jul 24 21:13:38 Tower kernel: Pid: 1444, comm: smbd Not tainted 3.0.35-unRAID #2 (Errors)
Jul 24 21:13:38 Tower kernel: Call Trace: (Errors)
Jul 24 21:13:38 Tower kernel: [<c105fee8>] warn_alloc_failed+0xb2/0xc4 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1060653>] __alloc_pages_nodemask+0x456/0x47f (Errors)
Jul 24 21:13:38 Tower kernel: [<c10606d0>] __get_free_pages+0xf/0x21 (Errors)
Jul 24 21:13:38 Tower kernel: [<c107dfa0>] __kmalloc+0x28/0xff (Errors)
Jul 24 21:13:38 Tower kernel: [<c103efff>] ? hrtimer_try_to_cancel+0x6e/0x77 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12910a9>] pskb_expand_head+0xca/0x1eb (Errors)
Jul 24 21:13:38 Tower kernel: [<c1291549>] __pskb_pull_tail+0x41/0x21f (Errors)
Jul 24 21:13:38 Tower kernel: [<c1298321>] dev_hard_start_xmit+0x20a/0x322 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12a6d6a>] sch_direct_xmit+0x50/0x137 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1298537>] dev_queue_xmit+0xfe/0x274 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12b3d4f>] ip_finish_output+0x237/0x272 (Errors)
Jul 24 21:13:38 Tower kernel: [<c108ccf1>] ? do_sys_poll+0x129/0x188 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12b3e2a>] ip_output+0xa0/0xa8 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12b326e>] ip_local_out+0x1b/0x1e (Errors)
Jul 24 21:13:38 Tower kernel: [<c12b3772>] ip_queue_xmit+0x2a5/0x2f2 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12c2ce8>] tcp_transmit_skb+0x4d7/0x50d (Errors)
Jul 24 21:13:38 Tower kernel: [<c12c4ff2>] tcp_write_xmit+0x2f9/0x3d7 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12c50f8>] tcp_push_one+0x28/0x2c (Errors)
Jul 24 21:13:38 Tower kernel: [<c12bad70>] do_tcp_sendpages+0x352/0x493 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12baf02>] tcp_sendpage+0x51/0x66 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12baeb1>] ? do_tcp_sendpages+0x493/0x493 (Errors)
Jul 24 21:13:38 Tower kernel: [<c12d2725>] inet_sendpage+0x82/0x9c (Errors)
Jul 24 21:13:38 Tower kernel: [<c12d26a3>] ? inet_dgram_connect+0x5e/0x5e (Errors)
Jul 24 21:13:38 Tower kernel: [<c128a95e>] kernel_sendpage+0x1a/0x2d (Errors)
Jul 24 21:13:38 Tower kernel: [<c128a998>] sock_sendpage+0x27/0x2c (Errors)
Jul 24 21:13:38 Tower kernel: [<c10995ce>] pipe_to_sendpage+0x5a/0x6c (Errors)
Jul 24 21:13:38 Tower kernel: [<c128a971>] ? kernel_sendpage+0x2d/0x2d (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099634>] splice_from_pipe_feed+0x54/0xc4 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099574>] ? splice_from_pipe_begin+0x10/0x10 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099b8a>] __splice_from_pipe+0x36/0x55 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099574>] ? splice_from_pipe_begin+0x10/0x10 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099cf1>] splice_from_pipe+0x51/0x64 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099d30>] ? default_file_splice_write+0x2c/0x2c (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099d43>] generic_splice_sendpage+0x13/0x15 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099574>] ? splice_from_pipe_begin+0x10/0x10 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099e85>] do_splice_from+0x57/0x61 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099ea6>] direct_splice_actor+0x17/0x1c (Errors)
Jul 24 21:13:38 Tower kernel: [<c109a471>] splice_direct_to_actor+0xbe/0x16b (Errors)
Jul 24 21:13:38 Tower kernel: [<c1099e8f>] ? do_splice_from+0x61/0x61 (Errors)
Jul 24 21:13:38 Tower kernel: [<c109a569>] do_splice_direct+0x4b/0x62 (Errors)
Jul 24 21:13:38 Tower kernel: [<c1080516>] do_sendfile+0x157/0x19f (Errors)
Jul 24 21:13:38 Tower kernel: [<c108059a>] sys_sendfile64+0x3c/0x7c (Errors)
Jul 24 21:13:38 Tower kernel: [<c130f525>] syscall_call+0x7/0xb (Errors)
My rig is quite similar to the OP.
UNRAID 5.0rc5 running under ESXi 5, assinged 4 GB out of 8 GB RAM to UNRAID, X8SIL, 128G SSD for ESXi, 1x M1015 (flashed in LSI IT mode) + HP SAS Expander passthrough to UNRAID. 1x3T Seagate VX as parity and 5x3T Seagate as data disks. All the 3T disks have the latest firmware CC4H.
The errors occurred between 19:31 and 21:13, when I was running the preclear_disk.sh script to clear two 2T Seagate disks, in the post_read phase. I am sure the post read phase started earlier than 17:00 and progressed 81% when it was 1:45 early in the next day morning.
I remember I was probably streaming a movie from the UNRAID server during the above time window (i.e. 19:31 - 21:13), and checked the progress of preclear a couple of times from the vSphere client.
Also I turned the unmenu and Plex media server on, though I did not visit the Plex server during the time window.
It is off the topic, but another strange thing I noticed was that I could not transfer big files to disk4. I tried to transfer a 7.7 GB video file to the disk4 from my computer over cable, the computer complained could not write to the disk. I checked the disk4 and found a file of exact 4 GB in size was written. I also remember that I had this problem on disk4 before, while the problem is gone right after I restart the UNRAID server yesterday. I haven't restarted the server since then. And I have the same problem again later on. Meanwhile I tried to write to the disk4 with 2 small files (of 244 MB and 311 MB) with success. Then I tried to transfer a folder containing one 12 GB file and several small files in KBs, the computer returned error as "Can not write to the disk", and nothing was written to the disk4. So it appears to me that right after the restart, I could write big files (probably >4 GB) to the disk4, but after a certain time point, I could not.
syslog-2012-07-25.zip