I got this error when trying to update a debian's kernel by aptitude, this error seems to be critical, luckily you can ignore it simply.

The error message is:
Code:
[8945066.158195] INFO: task taper:8468 blocked for more than 120 seconds.
[8945066.158223] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[8945066.158261] taper         D 0000000000000002     0  8468   8422
[8945066.158267]  ffff81042c845cb8 0000000000000082 ffff81042a5f37d8 ffff81042a5f3a20
[8945066.158273]  ffff81042a4f1920 ffff810426188ab0 ffff81029eaaa480 ffff810426188d38
[8945066.158278]  ffff81042a4f1a10 ffff810423da89e8 ffff81042a4f1800 0000000000000000
[8945066.158283] Call Trace:
[8945066.158426]  [<ffffffff804649db>] schedule_timeout+0x1e/0xad
[8945066.158437]  [<ffffffff8031ecdf>] blk_execute_rq_nowait+0x6b/0x82
[8945066.158488]  [<ffffffffa00280b4>] :scsi_mod:scsi_execute_async+0x322/0x36f
[8945066.158533]  [<ffffffff80463ff3>] wait_for_common+0xcf/0x13a
[8945066.158550]  [<ffffffff8022c1d8>] default_wake_function+0x0/0xe
[8945066.158614]  [<ffffffffa015be5c>] :st:st_do_scsi+0x235/0x265
[8945066.158653]  [<ffffffffa015b65b>] :st:st_sleep_done+0x0/0x60
[8945066.158714]  [<ffffffffa015ee8d>] :st:st_write+0x5cf/0xa71
[8945066.158906]  [<ffffffff802a6a6d>] vfs_write+0xad/0x156
[8945066.158939]  [<ffffffff802a700f>] sys_write+0x45/0x6e
[8945066.158943]  [<ffffffff8020bf89>] sysret_signal+0x2b/0x45
[8945066.158981]  [<ffffffff8020beda>] system_call_after_swapgs+0x8a/0x8f
[8945066.159063] 
[8945151.785610] st0: Sense Key : Medium Error [current] 
[8945151.785610] Info fld=0x8000
[8945151.785610] st0: Add. Sense: Write error
[8945151.800703] st0: Sense Key : Medium Error [current] 
[8945151.800709] Info fld=0x1
[8945151.800711] st0: Add. Sense: Write error
[8945151.800716] st0: Error on write filemark.
[8957775.692972] Buffer I/O error on device dm-9, logical block 0
[8957775.693006] Buffer I/O error on device dm-9, logical block 1
[8957775.693034] Buffer I/O error on device dm-9, logical block 2
[8957775.693060] Buffer I/O error on device dm-9, logical block 3
[8957775.699464] Buffer I/O error on device dm-9, logical block 0
[8957775.699510] Buffer I/O error on device dm-9, logical block 786431
[8957775.699549] Buffer I/O error on device dm-9, logical block 786431
[8957775.699646] Buffer I/O error on device dm-9, logical block 0
[8957775.699675] Buffer I/O error on device dm-9, logical block 1
[8957775.699702] Buffer I/O error on device dm-9, logical block 2
Although the error messages were not fatal, since no actual hardware was damaged, and no data loss was likely, it was still very annoying to see these Buffer I/O error messages every time I did anything related to LVM. Rebooting would have fixed the problem of course, but I'm deeply averse to utilising Windows-style solutions on Linux systems that should be repairable without rebooting. Also, this is a server, and I hated the thought of losing uptime, and having to restart everything and check all the services were working properly, just to solve some stupid "non-existent logical volumes" problem.

A more detailed explanation is available here: http://slated.org/device_mapper_weirdness