Kernel: Ata1.00

recently,my dedicated server down frequently,

i can not find any important info from /var/log/messages

but i find some records many time on it,those like
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Jun 15 05:30:40 server kernel: ata1.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x0
Jun 15 05:30:40 server kernel: ata1.00: (irq_stat 0x40000001)
Jun 15 05:30:40 server kernel: ata1.00: cmd 25/00:08:42:23:d2/00:00:2c:00:00/e0 tag 0 cdb 0x0 data 4096 in
Jun 15 05:30:40 server kernel: res 51/40:00:42:23:d2/00:00:2c:00:00/e0 Emask 0x9 (media error)
Jun 15 05:30:40 server kernel: ata1.00: configured for UDMA/133
Jun 15 05:30:40 server kernel: ata1: EH complete
Jun 15 05:30:42 server kernel: ata1.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x0
Jun 15 05:30:42 server kernel: ata1.00: (irq_stat 0x40000001)
Jun 15 05:30:42 server kernel: ata1.00: cmd 25/00:08:42:23:d2/00:00:2c:00:00/e0 tag 0 cdb 0x0 data 4096 in
Jun 15 05:30:42 server kernel: res 51/40:00:42:23:d2/00:00:2c:00:00/e0 Emask 0x9 (media error)
Jun 15 05:30:42 server kernel: ata1.00: configured for UDMA/133
Jun 15 05:30:42 server kernel: ata1: EH complete
Jun 15 05:30:44 server kernel: ata1.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x0
Jun 15 05:30:51 server kernel: ata1.00: (irq_stat 0x40000001)
Jun 15 05:30:51 server kernel: ata1.00: cmd 25/00:08:42:23:d2/00:00:2c:00:00/e0 tag 0 cdb 0x0 data 4096 in
Jun 15 05:30:51 server kernel: res 51/40:00:42:23:d2/00:00:2c:00:00/e0 Emask 0x9 (media error)
Jun 15 05:30:51 server kernel: ata1.00: configured for UDMA/133
Jun 15 05:30:51 server kernel: ata1: EH complete
Jun 15 05:30:51 server kernel: ata1.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x0
Jun 15 05:30:51 server kernel: ata1.00: (irq_stat 0x40000001)
Jun 15 05:30:51 server kernel: ata1.00: cmd 25/00:08:42:23:d2/00:00:2c:00:00/e0 tag 0 cdb 0x0 data 4096 in
Jun 15 05:30:51 server kernel: res 51/40:00:42:23:d2/00:00:2c:00:00/e0 Emask 0x9 (media error)
Jun 15 05:30:51 server kernel: ata1.00: configured for UDMA/133
Jun 15 05:30:51 server kernel: ata1: EH complete
Jun 15 05:30:51 server kernel: ata1.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x0
Jun 15 05:30:51 server kernel: ata1.00: (irq_stat 0x40000001)
Jun 15 05:30:51 server kernel: ata1.00: cmd 25/00:08:42:23:d2/00:00:2c:00:00/e0 tag 0 cdb 0x0 data 4096 in
Jun 15 05:30:51 server kernel: res 51/40:00:42:23:d2/00:00:2c:00:00/e0 Emask 0x9 (media error)
Jun 15 05:30:51 server kernel: ata1.00: configured for UDMA/133
Jun 15 05:30:52 server kernel: ata1: EH complete

Jun 15 05:31:26 server kernel: ata1.00: configured for UDMA/133
Jun 15 05:31:30 server kernel: sd 0:0:0:0: SCSI error: return code = 0x08000002
Jun 15 05:31:33 server kernel: sda: Current [descriptor]: sense key: Medium Error
Jun 15 05:31:36 server kernel: Add. Sense: Unrecovered read error - auto reallocate failed
Jun 15 05:31:36 server kernel:
Jun 15 05:31:39 server kernel: Descriptor sense data with sense descriptors (in hex):
Jun 15 05:31:46 server kernel: 72 03 11 04 00 00 00 0c 00 0a 80 00 00 00 00 00
Jun 15 05:31:51 server kernel: 2c d2 23 42
Jun 15 05:31:56 server kernel: end_request: I/O error, dev sda, sector 751969090
Jun 15 05:31:57 server kernel: ata1: EH complete
Jun 15 05:31:57 server kernel: SCSI device sda: 976773168 512-byte hdwr sectors (500108 MB)
Jun 15 05:31:58 server kernel: sda: Write Protect is off
Jun 15 05:31:58 server kernel: SCSI device sda: drive cache: write back
Jun 15 05:31:59 server kernel: SCSI device sda: 976773168 512-byte hdwr sectors (500108 MB)
Jun 15 05:32:03 server kernel: sda: Write Protect is off
Jun 15 05:32:04 server kernel: SCSI device sda: drive cache: write back
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is it safe ? or any hardware error?