将服务器从 centos 4 升级到 centos 6 后，系统只能使用 12 Gb 中的 8 Gb；我以为有些是有故障的，但事实并非如此（我尝试与另一台服务器交换内存）我有 6 x 2GB DDR DIMM；
free
total used free shared buffers cached
Mem: 8195024 560508 7634516 0 37388 169116
-/+ buffers/cache: 354004 7841020
Swap: 4200956 0 4200956
和/proc/meminfo
cat /proc/meminfo
MemTotal: 8195024 kB
MemFree: 7702732 kB
Buffers: 29232 kB
Cached: 106368 kB
SwapCached: 0 kB
Active: 93684 kB
Inactive: 107164 kB
Active(anon): 65476 kB
Inactive(anon): 1032 kB
Active(file): 28208 kB
Inactive(file): 106132 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 4200956 kB
SwapFree: 4200956 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 65244 kB
Mapped: 33532 kB
Shmem: 1264 kB
Slab: 211304 kB
SReclaimable: 24852 kB
SUnreclaim: 186452 kB
KernelStack: 1960 kB
PageTables: 11336 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 8298468 kB
Committed_AS: 346860 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 321300 kB
VmallocChunk: 34359408376 kB
HardwareCorrupted: 0 kB
AnonHugePages: 8192 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 10176 kB
DirectMap2M: 8378368 kB
以下是一些输出：
dmidecode -t 17
#dmidecode 2.10
SMBIOS 2.3 present.
Handle 0x003A, DMI type 17, 27 bytes
Memory Device
Array Handle: 0x0039
Error Information Handle: Not Provided
Total Width: 144 bits
Data Width: 128 bits
Size: 2048 MB
Form Factor: DIMM
Set: None
Locator: A0
Bank Locator: Bank0/1
Type: DDR
Type Detail: Synchronous
Speed: 400 MHz
Manufacturer: None
Serial Number: None
Asset Tag: None
Part Number: None
Handle 0x003B, DMI type 17, 27 bytes
Memory Device
Array Handle: 0x0039
Error Information Handle: Not Provided
Total Width: 144 bits
Data Width: 128 bits
Size: 2048 MB
Form Factor: DIMM
Set: None
Locator: A1
Bank Locator: Bank2/3
Type: DDR
Type Detail: Synchronous
Speed: 400 MHz
Manufacturer: None
Serial Number: None
Asset Tag: None
Part Number: None
Handle 0x003C, DMI type 17, 27 bytes
Memory Device
Array Handle: 0x0039
Error Information Handle: Not Provided
Total Width: 144 bits
Data Width: 128 bits
Size: 2048 MB
Form Factor: DIMM
Set: None
Locator: A2
Bank Locator: Bank4/5
Type: DDR
Type Detail: Synchronous
Speed: 400 MHz
Manufacturer: None
Serial Number: None
Asset Tag: None
Part Number: None
Handle 0x003D, DMI type 17, 27 bytes
Memory Device
Array Handle: 0x0039
Error Information Handle: Not Provided
Total Width: 144 bits
Data Width: 128 bits
Size: 2048 MB
Form Factor: DIMM
Set: None
Locator: A3
Bank Locator: Bank6/7
Type: DDR
Type Detail: Synchronous
Speed: 400 MHz
Manufacturer: None
Serial Number: None
Asset Tag: None
Part Number: None
Handle 0x003E, DMI type 17, 27 bytes
Memory Device
Array Handle: 0x0039
Error Information Handle: Not Provided
Total Width: 144 bits
Data Width: 128 bits
Size: 16384 MB
Form Factor: DIMM
Set: None
Locator: A4
Bank Locator: Bank8/9
Type: DDR
Type Detail: Synchronous
Speed: 400 MHz
Manufacturer: None
Serial Number: None
Asset Tag: None
Part Number: None
Handle 0x003F, DMI type 17, 27 bytes
Memory Device
Array Handle: 0x0039
Error Information Handle: Not Provided
Total Width: 144 bits
Data Width: 128 bits
Size: 16384 MB
Form Factor: DIMM
Set: None
Locator: A5
Bank Locator: Bank10/11
Type: DDR
Type Detail: Synchronous
Speed: 400 MHz
Manufacturer: None
Serial Number: None
Asset Tag: None
Part Number: None
最后两个显示为 16384MB，应该是 2048MB
我还设置了 lm_sensors + i2c-tools：
decode-dimms
# decode-dimms version 5929 (2011-02-16 14:58:38 +0100)
Memory Serial Presence Detect Decoder
By Philip Edelbrock, Christian Zuckschwerdt, Burkart Lingner,
Jean Delvare, Trent Piepho and others
Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/2-0050
Guessing DIMM is in bank 1
---=== SPD EEPROM Information ===---
EEPROM Checksum of bytes 0-62 OK (0xE2)
# of bytes written to SDRAM EEPROM 128
Total number of bytes in EEPROM 256
Fundamental Memory type DDR SDRAM
SPD Revision 1.0
---=== Memory Characteristics ===---
Maximum module speed 400MHz (PC3200)
Size 2048 MB
Voltage Interface Level SSTL 2.5V
Module Configuration Type ECC
Refresh Rate Reduced (7.8 us) - Self Refresh
tCL-tRCD-tRP-tRAS 3-3-3-8
Supported CAS Latencies 3T, 2.5T, 2T
Supported CS Latencies 0
Supported WE Latencies 1
Minimum Cycle Time 5 ns at CAS 3
6 ns at CAS 2.5
7.5 ns at CAS 2
Maximum Access Time 0.5 ns at CAS 3
0.5 ns at CAS 2.5
0.5 ns at CAS 2
Module Height 1.125" to 1.25"
---=== Manufacturing Information ===---
Manufacturer Infineon (former Siemens)
Manufacturing Location Code V
Part Number 72D256220GBR5B
Revision Code 0x030C
Manufacturing Date 2006-W01
Assembly Serial Number 0x0600C212
Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/2-0051
Guessing DIMM is in bank 2
---=== SPD EEPROM Information ===---
EEPROM Checksum of bytes 0-62 OK (0xB5)
# of bytes written to SDRAM EEPROM 128
Total number of bytes in EEPROM 256
Fundamental Memory type DDR SDRAM
SPD Revision 1.0
---=== Memory Characteristics ===---
Maximum module speed 400MHz (PC3200)
Size 2048 MB
Voltage Interface Level SSTL 2.5V
Module Configuration Type ECC
Refresh Rate Reduced (7.8 us) - Self Refresh
tCL-tRCD-tRP-tRAS 3-3-3-8
Supported CAS Latencies 3T, 2.5T
Supported CS Latencies 0
Supported WE Latencies 1
Minimum Cycle Time 5 ns at CAS 3
6 ns at CAS 2.5
Maximum Access Time 0.65 ns at CAS 3
0.7 ns at CAS 2.5
---=== Manufacturing Information ===---
Manufacturer Kingston
Manufacturing Location Code 0x04
Part Number Undefined
Manufacturing Date 2006-W14
Assembly Serial Number 0x08108AB0
Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/2-0052
Guessing DIMM is in bank 3
---=== SPD EEPROM Information ===---
EEPROM Checksum of bytes 0-62 OK (0xE2)
# of bytes written to SDRAM EEPROM 128
Total number of bytes in EEPROM 256
Fundamental Memory type DDR SDRAM
SPD Revision 1.0
---=== Memory Characteristics ===---
Maximum module speed 400MHz (PC3200)
Size 2048 MB
Voltage Interface Level SSTL 2.5V
Module Configuration Type ECC
Refresh Rate Reduced (7.8 us) - Self Refresh
tCL-tRCD-tRP-tRAS 3-3-3-8
Supported CAS Latencies 3T, 2.5T, 2T
Supported CS Latencies 0
Supported WE Latencies 1
Minimum Cycle Time 5 ns at CAS 3
6 ns at CAS 2.5
7.5 ns at CAS 2
Maximum Access Time 0.5 ns at CAS 3
0.5 ns at CAS 2.5
0.5 ns at CAS 2
Module Height 1.125" to 1.25"
---=== Manufacturing Information ===---
Manufacturer Infineon (former Siemens)
Manufacturing Location Code V
Part Number 72D256220GBR5B
Revision Code 0x030C
Manufacturing Date 2006-W01
Assembly Serial Number 0x0600AE27
Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/2-0053
Guessing DIMM is in bank 4
---=== SPD EEPROM Information ===---
EEPROM Checksum of bytes 0-62 OK (0xB5)
# of bytes written to SDRAM EEPROM 128
Total number of bytes in EEPROM 256
Fundamental Memory type DDR SDRAM
SPD Revision 1.0
---=== Memory Characteristics ===---
Maximum module speed 400MHz (PC3200)
Size 2048 MB
Voltage Interface Level SSTL 2.5V
Module Configuration Type ECC
Refresh Rate Reduced (7.8 us) - Self Refresh
tCL-tRCD-tRP-tRAS 3-3-3-8
Supported CAS Latencies 3T, 2.5T
Supported CS Latencies 0
Supported WE Latencies 1
Minimum Cycle Time 5 ns at CAS 3
6 ns at CAS 2.5
Maximum Access Time 0.65 ns at CAS 3
0.7 ns at CAS 2.5
---=== Manufacturing Information ===---
Manufacturer Kingston
Manufacturing Location Code 0x04
Part Number Undefined
Manufacturing Date 2006-W14
Assembly Serial Number 0x011046B4
Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/2-0054
Guessing DIMM is in bank 5
---=== SPD EEPROM Information ===---
EEPROM Checksum of bytes 0-62 OK (0xE2)
# of bytes written to SDRAM EEPROM 128
Total number of bytes in EEPROM 256
Fundamental Memory type DDR SDRAM
SPD Revision 1.0
---=== Memory Characteristics ===---
Maximum module speed 400MHz (PC3200)
Size 2048 MB
Voltage Interface Level SSTL 2.5V
Module Configuration Type ECC
Refresh Rate Reduced (7.8 us) - Self Refresh
tCL-tRCD-tRP-tRAS 3-3-3-8
Supported CAS Latencies 3T, 2.5T, 2T
Supported CS Latencies 0
Supported WE Latencies 1
Minimum Cycle Time 5 ns at CAS 3
6 ns at CAS 2.5
7.5 ns at CAS 2
Maximum Access Time 0.5 ns at CAS 3
0.5 ns at CAS 2.5
0.5 ns at CAS 2
Module Height 1.125" to 1.25"
---=== Manufacturing Information ===---
Manufacturer Infineon (former Siemens)
Manufacturing Location Code V
Part Number 72D256220GBR5B
Revision Code 0x030C
Manufacturing Date 2006-W01
Assembly Serial Number 0x04048112
Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/2-0055
Guessing DIMM is in bank 6
---=== SPD EEPROM Information ===---
EEPROM Checksum of bytes 0-62 OK (0xE2)
# of bytes written to SDRAM EEPROM 128
Total number of bytes in EEPROM 256
Fundamental Memory type DDR SDRAM
SPD Revision 1.0
---=== Memory Characteristics ===---
Maximum module speed 400MHz (PC3200)
Size 2048 MB
Voltage Interface Level SSTL 2.5V
Module Configuration Type ECC
Refresh Rate Reduced (7.8 us) - Self Refresh
tCL-tRCD-tRP-tRAS 3-3-3-8
Supported CAS Latencies 3T, 2.5T, 2T
Supported CS Latencies 0
Supported WE Latencies 1
Minimum Cycle Time 5 ns at CAS 3
6 ns at CAS 2.5
7.5 ns at CAS 2
Maximum Access Time 0.5 ns at CAS 3
0.5 ns at CAS 2.5
0.5 ns at CAS 2
Module Height 1.125" to 1.25"
---=== Manufacturing Information ===---
Manufacturer Infineon (former Siemens)
Manufacturing Location Code V
Part Number 72D256220GBR5B
Revision Code 0x030C
Manufacturing Date 2006-W01
Assembly Serial Number 0x0600BA20
Number of SDRAM DIMMs detected and decoded: 6
这里全部都有 2048MB 并且被检测到（字节 0-62 的 EEprom 校验和全部正常）
今晚我要进行一次记忆测试
感谢您的提示
编辑：当然我在 64 位 centos 上：
uname -r
2.6.32-71.el6.x86-64
答案1
如果在 POST 时仅显示 8GB，那么这不是 CentOS 的问题。很有可能是在升级过程中偶然发生了与硬件相关的问题，或者之前发生了一些事情而您没有注意到。
您应该检查以确保您运行的是硬件的最新 BIOS 版本。如果不符合要求，您应该联系他们寻求支持。