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libnvdimm: documentation clarifications

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A bunch of changes that I hope will help in understanding it
better for first-time readers.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
This commit is contained in:
Konrad Rzeszutek Wilk 2015-11-10 16:10:45 -08:00 committed by Dan Williams
parent 589e75d157
commit 8de5dff8ba
1 changed files with 28 additions and 21 deletions
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49
Documentation/nvdimm/nvdimm.txt
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@ -62,6 +62,12 @@ DAX: File system extensions to bypass the page cache and block layer to
mmap persistent memory, from a PMEM block device, directly into a
process address space.
DSM: Device Specific Method: ACPI method to to control specific
device - in this case the firmware.
DCR: NVDIMM Control Region Structure defined in ACPI 6 Section 5.2.25.5.
It defines a vendor-id, device-id, and interface format for a given DIMM.
BTT: Block Translation Table: Persistent memory is byte addressable.
Existing software may have an expectation that the power-fail-atomicity
of writes is at least one sector, 512 bytes. The BTT is an indirection
@ -133,16 +139,16 @@ device driver:
registered, can be immediately attached to nd_pmem.
2. BLK (nd_blk.ko): This driver performs I/O using a set of platform
defined apertures. A set of apertures will all access just one DIMM.
Multiple windows allow multiple concurrent accesses, much like
defined apertures. A set of apertures will access just one DIMM.
Multiple windows (apertures) allow multiple concurrent accesses, much like
tagged-command-queuing, and would likely be used by different threads or
different CPUs.
The NFIT specification defines a standard format for a BLK-aperture, but
the spec also allows for vendor specific layouts, and non-NFIT BLK
implementations may other designs for BLK I/O. For this reason "nd_blk"
calls back into platform-specific code to perform the I/O. One such
implementation is defined in the "Driver Writer's Guide" and "DSM
implementations may have other designs for BLK I/O. For this reason
"nd_blk" calls back into platform-specific code to perform the I/O.
One such implementation is defined in the "Driver Writer's Guide" and "DSM
Interface Example".
@ -152,7 +158,7 @@ Why BLK?
While PMEM provides direct byte-addressable CPU-load/store access to
NVDIMM storage, it does not provide the best system RAS (recovery,
availability, and serviceability) model. An access to a corrupted
system-physical-address address causes a cpu exception while an access
system-physical-address address causes a CPU exception while an access
to a corrupted address through an BLK-aperture causes that block window
to raise an error status in a register. The latter is more aligned with
the standard error model that host-bus-adapter attached disks present.
@ -162,7 +168,7 @@ data could be interleaved in an opaque hardware specific manner across
several DIMMs.
PMEM vs BLK
BLK-apertures solve this RAS problem, but their presence is also the
BLK-apertures solve these RAS problems, but their presence is also the
major contributing factor to the complexity of the ND subsystem. They
complicate the implementation because PMEM and BLK alias in DPA space.
Any given DIMM's DPA-range may contribute to one or more
@ -220,8 +226,8 @@ socket. Each unique interface (BLK or PMEM) to DPA space is identified
by a region device with a dynamically assigned id (REGION0 - REGION5).
1. The first portion of DIMM0 and DIMM1 are interleaved as REGION0. A
single PMEM namespace is created in the REGION0-SPA-range that spans
DIMM0 and DIMM1 with a user-specified name of "pm0.0". Some of that
single PMEM namespace is created in the REGION0-SPA-range that spans most
of DIMM0 and DIMM1 with a user-specified name of "pm0.0". Some of that
interleaved system-physical-address range is reclaimed as BLK-aperture
accessed space starting at DPA-offset (a) into each DIMM. In that
reclaimed space we create two BLK-aperture "namespaces" from REGION2 and
@ -230,13 +236,13 @@ by a region device with a dynamically assigned id (REGION0 - REGION5).
2. In the last portion of DIMM0 and DIMM1 we have an interleaved
system-physical-address range, REGION1, that spans those two DIMMs as
well as DIMM2 and DIMM3. Some of REGION1 allocated to a PMEM namespace
named "pm1.0" the rest is reclaimed in 4 BLK-aperture namespaces (for
well as DIMM2 and DIMM3. Some of REGION1 is allocated to a PMEM namespace
named "pm1.0", the rest is reclaimed in 4 BLK-aperture namespaces (for
each DIMM in the interleave set), "blk2.1", "blk3.1", "blk4.0", and
"blk5.0".
3. The portion of DIMM2 and DIMM3 that do not participate in the REGION1
interleaved system-physical-address range (i.e. the DPA address below
interleaved system-physical-address range (i.e. the DPA address past
offset (b) are also included in the "blk4.0" and "blk5.0" namespaces.
Note, that this example shows that BLK-aperture namespaces don't need to
be contiguous in DPA-space.
@ -252,15 +258,15 @@ LIBNVDIMM Kernel Device Model and LIBNDCTL Userspace API
What follows is a description of the LIBNVDIMM sysfs layout and a
corresponding object hierarchy diagram as viewed through the LIBNDCTL
api. The example sysfs paths and diagrams are relative to the Example
API. The example sysfs paths and diagrams are relative to the Example
NVDIMM Platform which is also the LIBNVDIMM bus used in the LIBNDCTL unit
test.
LIBNDCTL: Context
Every api call in the LIBNDCTL library requires a context that holds the
Every API call in the LIBNDCTL library requires a context that holds the
logging parameters and other library instance state. The library is
based on the libabc template:
https://git.kernel.org/cgit/linux/kernel/git/kay/libabc.git/
https://git.kernel.org/cgit/linux/kernel/git/kay/libabc.git
LIBNDCTL: instantiate a new library context example
@ -409,7 +415,7 @@ Bit 31:28 Reserved
LIBNVDIMM/LIBNDCTL: Region
----------------------
A generic REGION device is registered for each PMEM range orBLK-aperture
A generic REGION device is registered for each PMEM range or BLK-aperture
set. Per the example there are 6 regions: 2 PMEM and 4 BLK-aperture
sets on the "nfit_test.0" bus. The primary role of regions are to be a
container of "mappings". A mapping is a tuple of <DIMM,
@ -509,7 +515,7 @@ At first glance it seems since NFIT defines just PMEM and BLK interface
types that we should simply name REGION devices with something derived
from those type names. However, the ND subsystem explicitly keeps the
REGION name generic and expects userspace to always consider the
region-attributes for 4 reasons:
region-attributes for four reasons:
1. There are already more than two REGION and "namespace" types. For
PMEM there are two subtypes. As mentioned previously we have PMEM where
@ -698,8 +704,8 @@ static int configure_namespace(struct ndctl_region *region,
Why the Term "namespace"?
1. Why not "volume" for instance? "volume" ran the risk of confusing ND
as a volume manager like device-mapper.
1. Why not "volume" for instance? "volume" ran the risk of confusing
ND (libnvdimm subsystem) to a volume manager like device-mapper.
2. The term originated to describe the sub-devices that can be created
within a NVME controller (see the nvme specification:
@ -774,13 +780,14 @@ block" needs to be destroyed. Note, that to destroy a BTT the media
needs to be written in raw mode. By default, the kernel will autodetect
the presence of a BTT and disable raw mode. This autodetect behavior
can be suppressed by enabling raw mode for the namespace via the
ndctl_namespace_set_raw_mode() api.
ndctl_namespace_set_raw_mode() API.
Summary LIBNDCTL Diagram
------------------------
For the given example above, here is the view of the objects as seen by the LIBNDCTL api:
For the given example above, here is the view of the objects as seen by the
LIBNDCTL API:
+---+
|CTX| +---------+ +--------------+ +---------------+
+-+-+ +-> REGION0 +---> NAMESPACE0.0 +--> PMEM8 "pm0.0" |