Changes between Version 12 and Version 13 of MMC

Timestamp:

05/22/2020 07:14:26 PM (4 years ago)

Author:

Tim Harvey

Comment:

added section on reliability and longevity

MMC

@@ -241 +241 @@
  * http://en.wikipedia.org/wiki/Secure_Digital#Ultra_High_Speed_bus
 
+[=#reliability]
+[=#longevity]
+== Reliability / Longevity and Program / Erase cycles
+MMC devices are managed NAND FLASH devices meaning there is a controller inside the part that deals with wear-leveling to move blocks around to avoid over-use of blocks. Parts contain extra blocks such that over time blocks that have to be excluded either because their program/erase cycle max has been reached or blocks fail the devices still retain their original size.
+
+The reliability or longevity of a manged device depends on your use case (rate of program/erase cycles as well as how much of the device is available for use) and the number of extra blocks and therefore varies greatly per device and use case.
+
+Therefore, endurance is expressed in TBW (Terabytes written), which represents the total amount of data that can be written to a storage device during its lifetime of use. A bigger TBW number means longer life.
+
+The value of TBW is inversely proportional to another variable WAF (Write Amplification Factor).
+WAF is an undesirable phenomenon associated with flash memory, where the actual amount of information physically written to the storage media is a multiple of the logical amount intended to be written.
+
+The WAF of flash memory can be affected by many factors like Garbage collection, Erase, TRIM, wear leveling, sequential writes, random writes.... Therefore, WAF is a variable base on system workload.
+
+For example, workloads of Random Writes will generate a bigger WAF number than workloads of Sequential Writes.
+
+As a simple example here is a calculation for a specific workload on a specific eMMC device:
+ * Workload description
+  1. 84% Sequential write, 16%Random write
+  2. Chunk Size IOs Distribution: 30%: 4KB, 27%: 16KB, 42%: Mix of 8KB, 32KB-256KB, 1%: 512KB
+  3. eMMC Cache on
+ * specific eMMC device specs (from datasheet):
+  - MLC device
+  - physical capacity = 0.0037(TB) for 4GB device
+  - endurance cycle = 3000 for MLC
+ * Write Amplication Factor (WAF):
+  - WAF = 4.5 (estimated from the workload description above with simulation)
+ * TBW = physical capacity * endurance cycle / WAF
+  - TBW =  0.0037(TB) * 3000(cycles) / 4.5(WAF) ~= 2.5 TBW
+  - 2.5 Terabytes is the total amount of data written to the device during its lifetime of use, depending on the workload.
+
+For additional detail including Write Amplification Factor for your specific workload you would need to consult the particular storage device manufacturer.
+
+As far as partition block pool isolation goes the boot0/boot1 eMMC partitions share the same block pool for wear-leveling. Therefore if Boot0 is no longer writable because the PE cycles of all boot blocks are over-spec, Boot1 will no longer be writable as well. The User area has its own block pool for wear leveling therefore it would be unaffected by overuse of Boot0/Boot1. While the the User area can be partitioned into an additional 4 Generic Purpose Partitions (GPPs) they still all would use the same block pool assigned to User.
+
+Reference:
+ * https://www.slideshare.net/VijayGESYS/emmc-embedded-multimedia-card-overview
+
 
 [=#newport]