Linux 7 Storage Concepts: Difference between revisions
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Revision as of 20:13, 3 April 2018
Internal
Block
A block is a fixed-size chunk of data. The size is determined by the kernel, and depends on the system's architecture and the filesystem being used. The block size the kernel uses to access a specific device can be obtained with:
blockdev --getbsz /dev/sda2
Sector
A sector is a small block whose size is usually determined by the underlying hardware.
Block Devices
Unlike a character device, a block device provides random access to fixed-size blocks of data. On Linux, the block devices can be either "mapped", offering access to a logical volume in a volume group (/dev/mapper/VolGroup00-LogVol01), or "static", which is a traditional storage volume (/dev/sdba).
Block Driver
A block driver provides access to block devices - devices that transfer randomly accessible data in fixed-size blocks. These are primarily hard drives. Aside providing user space processes read and writing access to block storage, the block drivers act as conduit between the core memory of the system and secondary storage, therefore they can be seen as making part of the virtual memory subsystem.
Much of the design of the block layer is centered on performance, as the entire system cannot run well if the block I/O subsystem is not well tuned.
Alos see:
File System
File System Concepts
inode
An inode is a file system data structure that describes a filesystem object such as a file or directory. It has an unique identifier. Each inode stores file attributes (creation, last modification and last access time, permissions, etc.) and disk location of the fileystem object data. The inode does not store the name if the file, nor the file data.
The name of the file
Hard Link
Symbolic Link
Redirected at open() time.
File System Types
ext4
An ext4 filesystem is created with mkfs.ext4.
Journal Recovery is done with e2fsck in userspace at boot time.
Metadata Error Behavior. When metadata errors are encountered, the behavior is configurable. The default is to continue.
Resize
XFS
Device Mapper
Device Mapper is a kernel-based framework for volume management. It has thin provisioning and snapshotting capabilities. Used as one of Docker storage backend options.
Loopback Device
A loopback device is an O/S mechanism that allows exposing a file as a block device. The loopback devices are usually named /dev/loop0, /dev/loop1, etc.
Loopback devices are managed with losetup, which can associate a loopback device with a file:
losetup /dev/loop0 ./lvm0.img
Once setup, the loopback devices should be reported as block devices by lsblk.
Loopback devices are used, among other things, to set up storage with Docker devicemapper driver.
External resources: