Linux Memory

Linux Memory Analysis

Specifics - Linux Memory Analysis

This is Linux memory/storage from the z/VM perspective:

Linux needs to use swapping or all of its initialization pages will remain in memory, which is wasteful.
Swap should be done to virtual disk (not real DASD).
Reducing the size of the virtual machine on z/VM reduces caching of old data.
Start with 1 small VDISK (100M) and 1 larger VDISK (2G). Set the priority to favor the smaller VDISK. Set an alert if the larger VDISK is used.
Note: In Linux the lower number has the higher prioritization and will be used first.
Use DIAG driver instead of FBA. FBA is 512 bytes. DIAG will group those into 4K pages which reduces I/O by a factor of 8.
Virtual disk can get paged out when not in use (goodness). It has no overhead until 'touched'.
Rule of thumb - if a server is not using swap, it is oversized. If a server is using some swap, it is sized correctly. If a server is using a lot of swap, it is undersized.
See Linux Storage for more details.

If Linux memory is an issue, consider using zVRM

ESAUCD2 - Linux UCD memory analysis. Both screen and report samples:

Real Storage Avail - This shows available RAM not currently in use. If a server shows high availability of memory not in use, they may be over allocated (over 5%). The memory size can be dropped by 1-2% over ~5 minutes until some swapping occurs. Or, zVRM can be used to do this automatically, without changing the allocated memory. It can also return the memory automatically if needed. However, be very careful if running MongoDB, which has an underlying cache system built in that needs to be understood before removing memory.

SWAP Storage Used - This shows SWAP space in use. Watch for high real memory availability with no swap or swapping over 50%. Both could mean the a virtual machine is sized incorrectly.

Storage in Use Buffer - This shows memory in use for data buffers that really should be written to disk. Watch for greater than 20mb. If this is the case, may need to use CMM (see above) to change memory size on Linux.

Storage in Use Cache - This shows memory in use for page cache.

Error Message - This shows an error message if the threshold has been exceeded or some other error has occurred. (The mail server is a proprietary component and doesn't use swap space.)