Howto obtain memory usage of persistence table

one simple persistence record is about 112 bytes. anyway, you may grep "persist" from "b memory", "tmsh show sys memory" or "tmctl memory_usage_stat" command output.

sol5779: The amount of memory the BIG-IP LTM allocates for a simple persistence record

https://support.f5.com/kb/en-us/solutions/public/5000/700/sol5779.html

by the way, you are not running 10.x, are you?

sol10094: Running the 'bigpipe persist show' command on a system with a large number of persistent connections can cause system instability

http://support.f5.com/kb/en-us/solutions/public/10000/000/sol10094.html

[root@ve10:Active] config  tmsh list ltm virtual bar
ltm virtual bar {
    destination 172.28.19.252:http
    ip-protocol tcp
    mask 255.255.255.255
    persist {
        source_addr {
            default yes
        }
    }
    pool foo
    profiles {
        http { }
        tcp { }
    }
    snat automap
}

 no persistence record

[root@ve10:Active] config  b persist
No Persistence Table Entries were found.

[root@ve10:Active] config  tmsh show sys memory|grep -i "subsystem\|persist"
Sys::SubSystem Memory
SubSystem                     Alloc (bytes)  Max Alloc (bytes)  Object Size
http_persist                              0                  0           72
persist                                   0                  0          112
persistence (variable)                    0                  0            1

 1 persistence record

[root@ve10:Active] config  b persist
PERSISTENT CONNECTIONS
    Type           Virtual                Node
    source addr    172.28.19.252:80   200.200.200.101:80

[root@ve10:Active] config  tmsh show sys memory|grep -i "subsystem\|persist"
Sys::SubSystem Memory
SubSystem                     Alloc (bytes)  Max Alloc (bytes)  Object Size
http_persist                              0                  0           72
persist                                 112                112          112
persistence (variable)                    0                  0            1