25 Persistent Memory #

It is important to note that this form of memory access is not transactional. In the event of a power outage or other system failure, data may not be completely written into storage. PMEM storage is only suitable if the application can handle the situation of partially-written data.

This is useful when you want to use the persistent memory on a set of NVDIMMs as a disk-like pool of very fast storage.

To applications, such devices just appear as very fast SSDs and can be used like any other storage device. For example, LVM can be layered on top of the non-volatile storage and will work as normal.

The advantage of BTT is that sector write atomicity is guaranteed, so even sophisticated applications that depend on data integrity will keep working. Media error reporting works through standard error-reporting channels.

Although it is more robust against single-device failure, this requires additional management, as each NVDIMM appears as a separate device. Thus, PMEM with BTT is generally preferred.

Note

BLK storage is deprecated and is not supported in later versions of SUSE Linux Enterprise Server.

The ndctl list command can be used to list all available NVDIMMs in a system.

In the following example, the system has three NVDIMMs which are in a single, triple-channel interleaved set.

root # ndctl list --dimms

[
 {
  "dev":"nmem2",
  "id":"8089-00-0000-12325476"
 },
 {
  "dev":"nmem1",
  "id":"8089-00-0000-11325476"
 },
 {
  "dev":"nmem0",
  "id":"8089-00-0000-10325476"
 }
]

With a different parameter, ndctl list will also list the available regions.

Note

Regions may not appear in numerical order.

Note that although there are only three NVDIMMs, they appear as four regions.

root # ndctl list --regions

[
 {
  "dev":"region1",
  "size":68182605824,
  "available_size":68182605824,
  "type":"blk"
 },
 {
  "dev":"region3",
  "size":202937204736,
  "available_size":202937204736,
  "type":"pmem",
  "iset_id":5903239628671731251
  },
  {
   "dev":"region0",
   "size":68182605824,
   "available_size":68182605824,
   "type":"blk"
  },
  {
   "dev":"region2",
   "size":68182605824,
   "available_size":68182605824,
   "type":"blk"
  }
]

The space is available in two different forms: either as three separate 64 GB regions of type BLK, or as one combined 189 GB region of type PMEM which presents all the space on the three interleaved NVDIMMs as a single volume.

Note that the displayed value for available_size is the same as that for size. This means that none of the space has been allocated yet.

For the first example, we will configure our three NVDIMMs into a single PMEM namespace with Direct Access (DAX).

The first step is to create a new namespace.

root # ndctl create-namespace --type=pmem --mode=fsdax --map=memory
{
 "dev":"namespace3.0",
 "mode":"memory",
 "size":199764213760,
 "uuid":"dc8ebb84-c564-4248-9e8d-e18543c39b69",
 "blockdev":"pmem3"
}

This creates a block device /dev/pmem3, which supports DAX. The 3 in the device name is inherited from the parent region number, in this case region3.

The --map=memory option sets aside part of the PMEM storage space on the NVDIMMs so that it can be used to allocate internal kernel data structures called struct pages. This allows the new PMEM namespace to be used with features such as O_DIRECT I/O and RDMA.

The reservation of some persistent memory for kernel data structures is why the resulting PMEM namespace has a smaller capacity than the parent PMEM region.

Next, we verify that the new block device is available to the operating system:

root # fdisk -l /dev/pmem3
Disk /dev/pmem3: 186 GiB, 199764213760 bytes, 390164480 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Before it can be used, like any other drive, it must be formatted. In this example, we format it with XFS:

root # mkfs.xfs /dev/pmem3
meta-data=/dev/pmem3      isize=256    agcount=4, agsize=12192640 blks
         =                sectsz=4096  attr=2, projid32bit=1
         =                crc=0        finobt=0, sparse=0
data     =                bsize=4096   blocks=48770560, imaxpct=25
         =                sunit=0      swidth=0 blks
naming   =version 2       bsize=4096   ascii-ci=0 ftype=1
log      =internal log    bsize=4096   blocks=23813, version=2
         =                sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none            extsz=4096   blocks=0, rtextents=0

Next, we can mount the new drive onto a directory:

root # mount -o dax /dev/pmem3 /mnt/pmem3

Then we can verify that we now have a DAX-capable device:

root # mount | grep dax
/dev/pmem3 on /mnt/pmem3 type xfs (rw,relatime,attr2,dax,inode64,noquota)

The result is that we now have a PMEM namespace formatted with the XFS file system and mounted with DAX.

Any mmap() calls to files in that file system will return virtual addresses that directly map to the persistent memory on our NVDIMMs, completely bypassing the page cache.

Any fsync or msync calls on files in that file system will still ensure that modified data has been fully written to the NVDIMMs. These calls flush the processor cache lines associated with any pages that have been modified in userspace via mmap mappings.

root # umount /mnt/pmem3

root # ndctl disable-namespace namespace3.0
disabled 1 namespace

root # ndctl destroy-namespace namespace3.0
destroyed 1 namespace

In the next example, we create a PMEM namespace that uses BTT.

root # ndctl create-namespace --type=pmem --mode=sector
{
 "dev":"namespace3.0",
 "mode":"sector",
 "uuid":"51ab652d-7f20-44ea-b51d-5670454f8b9b",
 "sector_size":4096,
 "blockdev":"pmem3s"
}

Next, verify that the new device is present:

root # fdisk -l /dev/pmem3s
Disk /dev/pmem3s: 188.8 GiB, 202738135040 bytes, 49496615 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Like the DAX-capable PMEM namespace we previously configured, this BTT-capable PMEM namespace consumes all the available storage on the NVDIMMs.

Note

The trailing s in the device name (/dev/pmem3s) stands for sector and can be used to easily distinguish PMEM and BLK namespaces that are configured to use the BTT.

The volume can be formatted and mounted as in the previous example.

The PMEM namespace shown here cannot use DAX. Instead it uses the BTT to provide sector write atomicity. On each sector write through the PMEM block driver, the BTT will allocate a new sector to receive the new data. The BTT atomically updates its internal mapping structures after the new data is fully written so the newly written data will be available to applications. If the power fails at any point during this process, the write will be completely lost and the application will have access to its old data, still intact. This prevents the condition known as "torn sectors".

This BTT-enabled PMEM namespace can be formatted and used with a file system just like any other standard block device. It cannot be used with DAX. However, mmap mappings for files on this block device will use the page cache.

Note

In both these examples, space from all the NVDIMMs is combined into a single volume. Just as with a non-redundant disk array, this means that if any individual NVDIMM suffers an error, the contents of the entire volume could be lost. The more NVDIMMs are included in the volume, the higher the chance of such an error.

root # ndctl disable-namespace namespace3.0
disabled 1 namespace

root # ndctl destroy-namespace namespace3.0
destroyed 1 namespace

In this example, we will create three separate BLK devices: one per NVDIMM.

One advantage of this approach is that if any individual NVDIMM fails, the other volumes will be unaffected.

Note

The commands must be repeated for each namespace.

root # ndctl create-namespace --type=blk --mode=sector
{
 "dev":"namespace1.0",
 "mode":"sector",
 "uuid":"fed466bd-90f6-460b-ac81-ad1f08716602",
 "sector_size":4096,
 "blockdev":"ndblk1.0s"
}
   
root # ndctl create-namespace --type=blk --mode=sector
{
 "dev":"namespace0.0",
 "mode":"sector",
 "uuid":"12a29b6f-b951-4d08-8dbc-8dea1a2bb32d",
 "sector_size":4096,
 "blockdev":"ndblk0.0s"
}
    
root # ndctl create-namespace --type=blk --mode=sector
{
 "dev":"namespace2.0",
 "mode":"sector",
 "uuid":"7c84dab5-cc08-452a-b18d-53e430bf8833",
 "sector_size":4096,
 "blockdev":"ndblk2.0s"
}

Next, we can verify that the new devices exist:

root # fdisk -l /dev/ndblk*
Disk /dev/ndblk0.0s: 63.4 GiB, 68115001344 bytes, 16629639 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Disk /dev/ndblk1.0s: 63.4 GiB, 68115001344 bytes, 16629639 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Disk /dev/ndblk2.0s: 63.4 GiB, 68115001344 bytes, 16629639 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

The block devices generated for BLK namespaces are named /dev/ndblkX.Y where X is the parent region number and Y is a unique namespace number within that region. So, /dev/ndblk2.0s is child namespace number 0 of region 2.

As in the previous example, the trailing s means that this namespace is configured to use the BTT—in other words, for sector-based access. Because they are accessed via a block window, programs cannot use DAX, but accesses will be cached.

As ever, these devices must all be formatted and mounted before they can be used.

A set of NVDIMMs are located in DIMM slots on the motherboard of the server.

The platform (the combination of server hardware and firmware) allocates the memory on these NVDIMMs to one or more regions, such as region0.

Then within those regions, the operating system defines namespaces, for example /dev/pmem1 or /dev/dax0.

In the diagram below, there are three regions. One is a PMEM region, composed of part of the space from three NVDIMMs, interleaved. The remaining space on two of the NVDIMMs has been configured as two additional BLK regions. In each of these, a namespace has been created.

Figure 25.1: NVDIMM Region Layout #

 

In our example, the part of region0 labeled as [X] has been damaged or become defective.

You must:

Note: Prerequisites for Fault-Finding

For testing, the nfit_test kernel module is required.

The testing procedure is described in detail on the GitHub page for the ndctl command, in steps 1-4 of the section Unit test. See Section 25.7, “For More Information” at the end of this chapter.