Storage

Targetcli

Scripts for creating loopback disks

targetcli /loopback create wwn=naa.5000000000000000 \
for i in {00..16}; \
    do lvcreate -y -n disk$i -L100G export; \
    targetcli /backstores/block create dev=/dev/export/disk$i name=disk$i; \
    targetcli /backstores/block/disk$i set attribute optimal_sectors=4096; \
    targetcli /loopback/naa.5000000000000000/luns create \
    storage_object=/backstores/block/disk$i; \
done

Input Oputput I/

Breef overview of current (2023) I/O mechanism supported Linux kernel.

Synchronous

Represented by system calls read()/write(), pread()/pwrite() and it’s vectored version of readv()/writev() and preadv()/pwritev().

Asynchronous

Represented either by POSIX aio (man aio), libaio and latest io_uring.

Buffered

All IO which ends up in page cache and are not directly written to but are written by the page background process.

With direct I/O data is read from or written to the storage device (e.g., HDD, SSD, NVMe) without being cached in the operating system’s buffer cache. This means that data is transferred directly between the application’s memory and the storage device, bypassing any intermediate caching layers in the operating system.

Benefits of Direct I/O:

Direct I/O is often used in scenarios where data consistency and control over I/O operations are critical, such as in databases, file systems, and some scientific computing applications.
It can help ensure that data is written to and read from the storage device without interference from the operating system’s cache, which can be especially important for applications that require strict data durability or real-time performance. By bypassing the cache, direct I/O can reduce the variability in I/O response times that can occur with cached I/O.