Shared memory is a method of inter-process communication (IPC), where two processes communicate with each other through shared blocks of RAM. Because communication is resident in RAM, shared memory allows for very fast communication between processes. There are significant drawbacks to shared memory; one obvious limitation is that all communicating processes must exist on the same box. Additional complexities with the implementation of shared memory means that it is typically relegated to lower-level, performance oriented systems, such as databases or backup systems.
In OS X, these settings MUST be tweaked if you are expecting to backup significant amounts of data with any semblance of speed or stability. I can confirm that both TiNa and NetVault use shared memory for IPC. Other products such as Retrospect or PresStore utilize other IPC methods, such as named pipes.
kern.sysv.shmall
shmall represents the maximum number of pages able to be provisioned for shared memory. It determines the total amount of shared memory that the system can allocate. To determine total system shared memory, multiply this value by the size of the page file. The page file size can be determined via `vm_stat` or `getconf PAGE_SIZE`. A typical page size is 4KB, 4096 bytes.
In OS X, Apple uses extremely conservative settings for shmall. At 1024, OS X defaults to only 4MB of shared memory.
kern.sysv.shmseg
shmseg represents the maximum number of shared memory segments each process can attach. Default in OS X is 8.
kern.sysv.shmmni
shmmni limits the number of shared memory segments across the system, representing the total number of shared memory segments. Default in OS X is 32.
kern.sysv.shmmin
shmmin is the minimum size of a shared memory segment, this should pretty much never need modification. Default is 1.
kern.sysv.shmmax
shmmax is the maximum size of a segment. Default in OS X is 4 MB, 4194304.
Suggested Settings:
512MB of shared memory
kern.sysv.shmall: 131072
kern.sysv.shmseg: 32
kern.sysv.shmmni: 128
kern.sysv.shmmin: 1
kern.sysv.shmmax: 536870912
1GB Shared memory
kern.sysv.shmall: 262144
kern.sysv.shmseg: 32
kern.sysv.shmmni: 128
kern.sysv.shmmin: 1
kern.sysv.shmmax: 1073741824
