- explain what buckets are - rework text about storage backends - explain redundancy concept Signed-off-by: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> --- Documentation/user/state.rst | 33 ++++++++++++++++++++++++++------- 1 file changed, 26 insertions(+), 7 deletions(-) diff --git a/Documentation/user/state.rst b/Documentation/user/state.rst index 5dd5c486e2..73c4be8159 100644 --- a/Documentation/user/state.rst +++ b/Documentation/user/state.rst @@ -23,16 +23,35 @@ available, ``raw`` and ``dtb``. Both format the state data differently. Basically these are serializers. The raw serializer additionally supports a HMAC algorithm to detect manipulations. +The data is always stored in a logical unit called ``bucket``. A ``bucket`` has +its own size depending on some external contraints. These contraints are listed +in more detail below depending on the used memory type and storage backend. A +``bucket`` stores exactly one state. A default number of three buckets is used +to store data redundantely. + +Redundancy +---------- + +The state framework is safe against powerfailures during write operations. To +archieve that multiple buckets are stored to disk. When writing all buckets are +written in order. When reading, the buckets are read in order and the first +one found that passes CRC tests is used. When all data is read the buckets +containing invalid or outdated data are written with the data just read. Also +NAND blocks need cleanup due to excessive bitflips are rewritten in this step. +With this it is made sure that after successful initialization of a state the +data on the storage device is consistent and redundant. + Storage Backends ---------------- -The serialized data can be stored to different backends which are automatically -selected depending on the defined backend in the devicetree. Currently two -implementations exist, ``circular`` and ``direct``. ``circular`` writes the -data sequentially on the backend storage device. Each save is appended until -the storage area is full. It then erases the block and starts from offset 0. -``circular`` is used for MTD devices with erase functionality. ``direct`` -writes the data directly to the file without erasing. +The serialized data can be stored to different backends. Currently two +implementations exist, ``circular`` and ``direct``. The state framework automatically +selects the correct backend depending on the storage medium. Media requiring +erase operations (NAND, NOR flash) use the ``circular`` backend, others use the ``direct`` +backend. The purpose of the ``circular`` backend is to save erase cycles which may +wear out the flash blocks. It continuously fills eraseblocks with updated data +and only when an eraseblock if fully written erases it and starts over writing +new data to the same eraseblock again. For all backends multiple copies are written to handle read errors. -- 2.11.0 _______________________________________________ barebox mailing list barebox@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/barebox
