¿Las bases de datos utilizan archivos mapeados en memoria?

Do databases use memory mapped files?

Memory mapping is the use of virtual memory to map files stored on secondary storage into primary storage so that the data is directly accessible by the processor’s instructions. When the working set of the data structure can be kept in memory, performance begins to approach that of memory-resident databases.

No, Sqlite will read data for hard disk by paging. The Sqlite document said: “In order to return data from the database to the user, for example as the results of a SELECT query, SQLite must at some point read data from the database file.

Memory-mapped files are accessed through the operating system’s memory manager, so the file is automatically partitioned into a number of pages and accessed as needed. You do not have to handle the memory management yourself.

The benefit of memory mapping a file is increasing I/O performance, especially when used on large files. For small files, memory-mapped files can result in a waste of slack space as memory maps are always aligned to the page size, which is mostly 4 KiB.

But there are also disadvantages: An I/O error on a memory-mapped file cannot be caught and dealt with by SQLite. Instead, the I/O error causes a signal which, if not caught by the application, results in a program crash.

I/O operations in memory-mapped computers only use part of the full memory address, to make their location more distinctive. Isolated-memory systems are more complex because, unlike memory mapped computers, they can’t use the same decoding and control systems for memory and I/O devices.

They report that as overall the in-memory database Redis provides the best performance. Also, they report that column family databases Cassandra and HBase showed good update performance since they are optimized for update operations.

Memory-mapped I/O uses the same address space to address both memory and I/O devices. The memory and registers of the I/O devices are mapped to (associated with) address values. Thus, the CPU instructions used to access the memory can also be used for accessing devices.

There are advantages and disadvantages to using memory-mapped I/O. Advantages include: Many operations, especially I/O intensive operations, can be faster since content does need to be copied between kernel space and user space.

As a CPU needs to communicate with the various memory and input-output devices (I/O) as we know data between the processor and these devices flow with the help of the system bus. Have common bus (data and address) for I/O and memory but separate control lines. …

Second, SQLite uses a read-only memory map to prevent stray pointers in the application from overwriting and corrupting the database file. After all needed changes are completed, xWrite () is used to move the content back into the database file.

The default mechanism by which SQLite accesses and updates database disk files is the xRead () and xWrite () methods of the sqlite3_io_methods VFS object. These methods are typically implemented as “read ()” and “write ()” system calls which cause the operating system to copy disk content between the kernel buffer cache and user space.

When updating the database file, SQLite always makes a copy of the page content into heap memory before modifying the page. This is necessary for two reasons. First, changes to the database are not supposed to be visible to other processes until after the transaction commits and so the changes must occur in private memory.

The Compiling is executed by the add of command -l sqlite3 . std::cout << “Opened Database Successfully!” << std::endl;