6
With the persistent memory key-value store, values are accessed by the application
directly, without the need to first allocate buffers in memory. The kvprint() routine in
Listing 1-1 will be called with references to the actual keys and values, directly where
they live in persistence – something that is not possible with traditional storage. In
fact, even the data structures used by the key-value store library to organize its data are
accessed directly. When a storage-based key-value store library needs to make a small
update, for example, 64 bytes, it must read the block of storage containing those 64 bytes
into a memory buffer, update the 64 bytes, and then write out the entire block to make it
persistent. That is because storage accesses can only happen using block I/O, typically
4K bytes at a time, so the task to update 64 bytes requires reading 4K and then writing
4K. But with persistent memory, the same example of changing 64 bytes would only
write the 64 bytes directly to persistence.
The Performance Difference
Moving a data structure from storage to persistent memory does not just mean smaller
I/O sizes are supported; there is a fundamental performance difference. To illustrate this,
Figure 1-3 shows a hierarchy of latency among the different types of media where data
can reside at any given time in a program.
Figure 1-2. A key-value store in persistent memory
CHApter 1 IntroduCtIon to persIstent MeMory proGrAMMInG
With the persistent memory key-value store, values are accessed by the application
directly, without the need to first allocate buffers in memory. The kvprint() routine in
Listing 1-1 will be called with references to the actual keys and values, directly where
they live in persistence – something that is not possible with traditional storage. In
fact, even the data structures used by the key-value store library to organize its data are
accessed directly. When a storage-based key-value store library needs to make a small
update, for example, 64 bytes, it must read the block of storage containing those 64 bytes
into a memory buffer, update the 64 bytes, and then write out the entire block to make it
persistent. That is because storage accesses can only happen using block I/O, typically
4K bytes at a time, so the task to update 64 bytes requires reading 4K and then writing
4K. But with persistent memory, the same example of changing 64 bytes would only
write the 64 bytes directly to persistence.
The Performance Difference
Moving a data structure from storage to persistent memory does not just mean smaller
I/O sizes are supported; there is a fundamental performance difference. To illustrate this,
Figure 1-3 shows a hierarchy of latency among the different types of media where data
can reside at any given time in a program.
Figure 1-2. A key-value store in persistent memory
CHApter 1 IntroduCtIon to persIstent MeMory proGrAMMInG