how to configure persistent memory as local disks - Elastic Compute Service

Persistent memory on Elastic Compute Service (ECS) instances can be used as memory or local disks based on instance types. This topic describes how to configure persistent memory as local disks. This topic also describes how to resolve the following issue: An ecs.re7p or ecs.i4p persistent memory-optimized instance on which persistent memory can be used as local disks failed to allocate a memory pool by using Low-Level Persistence Library (LLPL).

Prerequisites

Persistent memory is suitable for the following instance types and image versions:

Instance types
- Instance family on which persistent memory can be used as memory: ecs.re6p-redis
  Important
  If you use persistent memory as memory on an instance, the following situations occur:
  - You can use persistent memory immediately after you purchase the instance, without the need to initialize persistent memory.
  - The persistent memory used as memory does not provide data persistence, and the data stored in the persistent memory is lost when the instance is stopped or restarted.
- Instance families on which persistent memory can be used as local disks: ecs.re6p and ecs.i4p
  Important
  If you use persistent memory as local disks on an instance, the following situations occur:
  - You can initialize persistent memory after you purchase the instance. For more information, see the Configure persistent memory as a local disk section in this topic.
  - The persistent memory used as local disks provides data persistence but may cause data loss. We recommend that you back up data in advance. For more information about local disks, see Local disks.
Image versions
- Alibaba Cloud Linux 2
- CentOS 7.6 and later
- Ubuntu 18.04 and 20.04

Background information

Persistent memory has higher access latency compared to regular memory, but it is cost-effective and suitable for use as local storage. The data stored in persistent memory is retained when you stop or restart instances. Persistent memory can be used as memory or local disks.

When persistent memory is used as memory, you can move some data from regular memory to persistent memory, such as non-hot data that does not require high-speed storage access. Persistent memory offers large capacity at a lower price per GiB and helps reduce the total cost of ownership (TCO) per GiB of memory.
When persistent memory is used as local disks, it provides ultra-high I/O performance and a read/write latency as low as 170 nanoseconds. You can use persistent memory for core application databases that require consistent response time. You can replace Non-Volatile Memory Express (NVMe) SSDs with persistent memory-based local disks to provide higher IOPS, higher bandwidth, and lower latency and resolve performance bottlenecks.

Important

The reliability of data stored in persistent memory depends on the reliability of persistent memory devices and the physical servers to which these devices are attached. This increases the risks of single points of failure (SPOFs). To ensure the reliability of application data, we recommend that you implement data redundancy at the application layer and use cloud disks for long-term data storage.

Configure persistent memory as a local disk

In this example, an instance that has the following configurations is used:

Instance type: ecs.re6p.2xlarge
Image: Alibaba Cloud Linux 2.1903 LTS 64-bit

Log on to a created instance.
For more information, see Connect to a Linux instance by using a password or key.

Install the utilities that are used to manage persistent memory, and delete all namespaces and labels.

sudo yum install -y ndctl daxctl
sudo ndctl disable-namespace all && sudo ndctl destroy-namespace all #Delete all namespaces.
sudo ndctl disable-region all && sudo ndctl zero-labels all && sudo ndctl enable-region all #Delete all labels.

Check the size of persistent memory.
```
ndctl list -R
```
The following figure shows a sample command output, in which the size parameter indicates the size of persistent memory.
Set the usage mode to fsdax.
```
sudo ndctl create-namespace --reconfig=namespace0.0 -m fsdax --size={region-size} --force
```
Note
Replace the {region-size} variable with the size value obtained in the previous step.

Format and mount the persistent memory (/dev/pmem) device.

sudo mkfs -t ext4 /dev/pmem0 && \
sudo mkdir /mnt/sdb && \
sudo mount -o dax,noatime /dev/pmem0 /mnt/sdb

View the mounted /dev/pmem device.

df -h

pmem-as-ssd

After the /dev/pmem device is mounted, you can use disk performance test tools to test the performance of the device.

For information about how to test the performance of local disks on an i4p or re6p instance, see Test the performance of local disks on an i4p instance .

The following table describes the performance comparison between local NVMe SSDs, enhanced SSDs (ESSDs), and persistent memory-based local disks.

Note

The performance data in the following table is only for reference. Data in the results of your tests prevails.

Metric	128-GiB persistent memory	1,788-GiB NVMe SSD	800-GiB ESSD at performance level 1 (PL1)
Read bandwidth	8 GB/s to 10 GB/s	2 GB/s to 3 GB/s	0.2 GB/s to 0.3 GB/s
Read/write bandwidth	8 GB/s to 10 GB/s	1 GB/s to 2 GB/s	0.2 GB/s to 0.3 GB/s
Write bandwidth	2 GB/s to 3 GB/s	1 GB/s to 2 GB/s	0.2 GB/s to 0.3 GB/s
Read IOPS	1,000,000	500,000	20,000 to 30,000
Read/write IOPS	1,000,000	300,000	20,000 to 30,000
Write IOPS	1,000,000	300,000	20,000 to 30,000
Read latency	300 nanoseconds to 400 nanoseconds	100,000 nanoseconds	250,000 nanoseconds
Write latency	300 nanoseconds to 400 nanoseconds	20,000 nanoseconds	150,000 nanoseconds

Resolve the issue that an instance failed to allocate a memory pool by using LLPL

Problem description

An ecs.re7p or ecs.i4p instance on which persistent memory can be used as local disks failed to allocate a memory pool by using LLPL. The Failed to create heap. Cannot read unsafe shutdown count** error message is returned, as shown in the following figure. 问题现象

Cause

By default, unsafe shutdown detection is enabled in the LLPL source code. However, virtualized non-volatile memory (NVM) does not support unsafe shutdown detection. For more information, see llpl.

Solution

Perform the following steps to disable unsafe shutdown detection in the LLPL source code.

Add the following code to the src/main/cpp/com_intel_pmem_llpl_AnyHeap.cpp file of the LLPL source code:
```
intsds_write_value=0;
pmemobj_ctl_set(NULL,"sds.at_create",&sds_write_value)
```
The following figure shows that the preceding code is added to the file.
Log on to the instance.
For more information, see Connect to a Linux instance by using a password or key.
Run the following command to run a test case by using LLPL:
```
mvn clean && mvn test -Dtest.heap.path=/mnt/sdb
```
If the "Failed to create heap. Cannot read unsafe shutdown count**" error message is not returned, you can proceed to allocate a memory pool.