SLES 11 SP1 x864

My Customer is using a NAS device for backup.

192.168.0.y:/mnt/HD_a2 on /external_disk type nfs (rw,addr=192.168.0.y)

also the the directory they backed up is on a local file system "/backup"
/dev/cciss/c0d0p8 on /backup type ext3 (rw,acl,user_xattr)
this /backup file system contains some very large file like 129 GB.

Problem is that when we try to copy a very large file "/backup/19sep/large-file" which is about 129 GB in size to the NAS we found that
1 - blocks in(bi) and blocks out(bo) remains very low
HTML Code:
procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  1      0 11230940  91200 19138368    0    0   102     7   18  156  0  0 98  2  0
 0  1      0 11231312  91200 19139952    0    0     0     0 1116 1321  0  0 96  4  0
 0  1      0 11231296  91200 19140744    0    0     0     0  350  475  0  0 96  4  0
 2  1      0 11231296  91200 19141592    0    0     0     0  767  876  0  0 95  5  0
 0  1      0 11231544  91208 19143008    0    0     0    12  801  961  0  0 97  3  0
 0  1      0 11231544  91208 19144184    0    0     0   268  727  990  0  0 95  5  0
 1  1      0 11232108  91208 19145680    0    0     0    16 1020 1498  0  0 96  3  0
 0  1      0 11231744  91208 19148480    0    0     0     0 1125 1311  1  0 93  5  0
 0  1      0 11231496  91208 19150044    0    0     0     0  863 1034  0  0 96  4  0
 0  1      0 11231744  91216 19152192    0    0    32     0 1318 1965  0  0 94  5  0
 1  1      0 11231480  91216 19154592    0    0     0   944 1385 1563  0  0 96  4  0
 0  1      0 11231736  91216 19155932    0    0     0     0 1050 1091  0  0 96  4  0
 0  1      0 11231116  91308 19157936    0    0   516   136 1176 2495  0  0 93  6  0
 2  1      0 11231116  91308 19158368    0    0     0     0  217  343  0  0 95  5  0
 0  1      0 11227396  91308 19164352    0    0     0     0 1504 1717  1  0 95  4  0
 0  2      0 11227396  91324 19166280    0    0    80   372 1373 1706  0  0 95  5  0
 0  1      0 11227520  91324 19167488    0    0     8     0  885 1121  0  0 96  4  0
 1  1      0 11227528  91324 19169596    0    0     0     0 1254 1473  0  0 95  4  0
 0  1      0 11226784  91324 19173440    0    0     0     0 1479 2022  1  0 96  4  0
 0  1      0 11225372  91324 19176268    0    0     0     0 1265 1545  0  0 94  6  0
 1  1      0 11225372  91336 19178540    0    0    12   468 1067 1220  0  0 96  4  0
2 - then after few hours like 2 hours we found that
HTML Code:
procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  4      0 11230940  91200 19138368    0    0   108     12   40  131  0  0 95  5  0
 1  3      0 11231312  91200 19139952    0    0     576     288 1571 2068  0  0 85  14  0
 1  3      0 11231296  91200 19140744    0    0     216     344  1585  3430  1  0 86  13  0
i.e block jobs(b), in, cs, and wa are all high, while bi and bo remains low. Also the copy operation becomes uninterruptable (D+)
"ps aux" shows
root 8866 1.0 0.0 12576 804 pts/0 D+ 11:13 0:08 cp largefile /external_disk/BMP/19sep12/
and we have to reboot the server to recover.

My questions
1 - why it start copying very slow('bi' and 'bo' values of vmstat, remains too low when we copy the 129 Gig file from local disk to the NAS)
2 - then why the copy operation becomes hang/free or D+

and where is the problem.. is there something wrong with the NAS device or with our local disk/file system(/backup)

please help