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Online spare memory configuration

Online spare memory provides protection against degrading FBDIMMs by reducing the likelihood of uncorrectable memory

errors.

An understanding of single-rank and dual-rank FBDIMMs is required to understand memory usage in online spare mode.

FBDIMMs can either be single-rank or dual-rank. Certain FBDIMM configuration requirements are based on these

classifications. A dual-rank FBDIMM is similar to having two separate FBDIMMs, or ranks, on the same module. Although

only a single FBDIMM module, a dual-rank FBDIMM acts as two separate FBDIMMs. The purpose of dual-rank FBDIMMs is

to provide the largest capacity FBDIMM for the current DRAM technology. If the current DRAM technology allows for 2-GB

single-rank FBDIMMs, a dual-rank FBDIMM using the same technology would be 4-GB.

In online spare mode, a single rank of memory acts as the spare memory. For single-rank FBDIMMs, the entire FBDIMM

acts as the spare memory. For a dual-rank FBDIMM, only half of the FBDIMM acts as the spare memory while the other half

is available for operating system and application usage.

If one of the non-spare FBDIMMs receives correctable memory errors at a higher rate than a specific threshold, the server

automatically copies the memory contents of the degraded rank to the online spare rank. The server then deactivates the

failing rank and automatically switches over to the online spare. Because FBDIMMs that experience a high rate of

correctable memory errors have a higher probability of receiving an uncorrectable memory error, this effectively reduces the

likelihood of uncorrectable memory errors that would result in server downtime.

Online spare is performed per branch of the memory controller. For a server populating both branches, two ranks are utilized

for online spare memory. One branch can have a fail over to the associated online spare while the other branch is still

protected.

Each branch is made up of two banks:

Branch 0 contains banks A and C

Branch 1 contains banks B and D

Online spare FBDIMM configuration requirements (in addition to general configuration requirements):

When only bank A is being used, it must be fully populated with dual-rank FBDIMMs.

If banks A and C are being used, they must be fully populated.

If installed, bank A and bank C must contain FBDIMMs with identical part numbers.

If installed, bank B and bank D must also contain FBDIMMs with identical part numbers.

In online spare mode, FBDIMMs must be populated as specified in the following table:

Configuration

Branch 0

Bank A

1A and 3A

Branch 0

Bank C

2C and 4C

Branch 1

Bank B

5B and 7B

Branch 1

Bank D

6D and 8D

- - -

X X

- -

X X X X

NOTE:

Configuration 1 is only supported if using bank A, populated with dual-rank FBDIMMs.

After installing FBDIMMs, use RBSU to configure the system for online spare memory support ("Configuring online spare

memory" on page 57).

Mirrored memory configuration

Mirroring provides protection against uncorrectable memory errors that would otherwise result in server downtime.

Mirroring is performed on the branch level. Branch 0 and branch 1 mirror each other.

Each branch maintains a copy of all memory contents. Memory writes go to both branches. Memory reads come from only

one of the two branches (unless an uncorrectable error occurs). If a memory read on one branch returns incorrect data due

to an uncorrectable memory error, the system automatically retrieves the proper data from the other branch. A branch is not

necessarily disabled (thus losing mirroring protection) because of a single uncorrectable error. Mirroring protection is not lost

due to transient and soft uncorrectable errors, resulting in systems that maintain mirroring protection (and thus improved

uptime) unless there is a true FBDIMM failure.

Because both branches are active, concurrent reads from both branches at the same time are possible. Therefore, mirroring

provides superior read performance compared to all other Advanced Memory Protection modes.

Mirrored memory FBDIMM configuration requirements (in addition to general configuration requirements):

Banks A and B must be fully populated.

Bank A and bank B must contain FBDIMMs with identical part numbers. If installed, bank C and bank D must also

contain FBDIMMs with identical part numbers.

When using mirrored memory mode, FBDIMMs must be populated as specified in the following table: