Echelon SmartServer 2.2 - Manual (Page 107)

108 Configuring and Managing the SmartServer
FPM Licensed Indicates whether custom app programmability is licensed on your
SmartServer. If “Activated” appears in this field, custom app
programmability is licensed on your SmartServer. If “Not Licensed”
appears in this field, a programming license is not installed on your
SmartServer. A programming license must be installed on your
SmartServer in order to deploy and use custom apps on it.
You can click the Not Licensed link to activate custom app
programmability on your SmartServer. Alternatively, you can order an
programming license (Echelon model number 72161) from the Echelon
Web site at
www.echelon.com/products/cis/activate .
Using the SmartServer Flash Memory
The SmartServer uses flash memory to store data associated with its applications. This non-volatile
memory appears to the SmartServer applications as a logical disk drive (also called a flash disk). The
flash disk is extremely reliable and will last a long period of time, but it has some limitations that may
affect the lifetime and runtime configuration of the SmartServer.
The SmartServer flash memory is physically composed of 4,096 blocks of 16 KB each (these are not
the same as disk blocks/sectors, which are much smaller). Of the 4,096 blocks, 4,000 are mapped to
the logical disk driver, 2 are required for overhead, and the remaining ones (up to 94) are spares.
Once a flash block has been written to, it must be completely erased before it can be written to again.
As the SmartServer applications write data to the logical flash disk, physical flash blocks are erased as
needed to allow data to be stored. Each flash block can be erased a limited number of times before it
fails, and flash manufacturers specify a minimum expected value for this limit. When a flash block
fails, it is permanently marked as failed and no longer used by the SmartServer. The spare flash blocks
allow for a limited number of block failures to occur without affecting the reliability of the flash disk.
The amount of time it takes for the flash blocks on the SmartServer to reach their erase limit depends
on how much and how often data is written to the flash disk. Writes to the logical flash disk cause
erases of physical flash blocks, but there is no simple formula to precisely describe this relationship.
The flash erase rates reported by the SmartServer apply to the entire flash memory, not to individual
flash blocks. The SmartServer contains a sophisticated flash file system that distributes flash erases
across all available blocks, in order to optimize the flash life. Over time, the actual number of erases
of each block will remain approximately equal, thus maximizing the life span of all flash blocks.
According to the flash manufacturers’ rated limits, all flash blocks should last at least 10 years if the
total block erase rate averages no more than 39 erases per minute for that entire time. Exceeding this
limit may cause the flash disk to wear out sooner.
The block erase limit specified by the flash manufacturers is very conservative. Empirical testing of at
least some flash parts indicates that a typical limit under normal conditions may be at least 10 times
longer, which translates to a much longer life for the SmartServer flash disk, and/or erase rates much
greater than 39 erases per minute. Informally, flash manufacturers will confirm these findings, but
they cannot be guaranteed. You can trade off expected life span for increased erase rates.
The configuration of your SmartServer may have a significant impact on the number of flash erases per
minute. You can monitor both the rate at which you are erasing the flash blocks on your SmartServer
and how many spare disk blocks are available. You can do so using the information on this Web page.
If the number of spare block falls below 8 blocks, you should replace the SmartServer.
Short-term increases or decreases in the rate of flash block erases value are not meaningful to the life
span of the flash memory. Only long-term, sustained rates are truly significant. However, if you are
trying to adjust your data logging to determine the affect this has on the erase rate, a long-term number
is not very helpful. To accommodate these different needs, this Web page reports four different values
for the flash block erase rate.
The most important one is computed using a two-day moving average, which will reflect only
sustained erase rates. However, this average is slow to respond to changes in the rate, and it may take