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Storage and Filesystems

Updated Mar 21, 2021 ·

Disk Layout​

Disk layout involves organizing a disk into partitions, which can then be assigned to different directories to segregate data and manage storage efficiently. The disk is identified by a device name, typically formatted as:

/dev/device-name

When setting up disk partitions, it's important to consider the system type, which can be either BIOS or UEFI.

BIOS​

BIOS (Basic Input/Output System) is an older firmware interface that initializes and tests hardware during the booting process. It uses the MBR (Master Boot Record) partitioning scheme, which has several limitations.

  • Limited to 4 primary partitions.
  • Only has 64 bytes to store partition information.
  • Because there's only 4 partitions, logical partitions are invented.
  • Logical partitions are created in the extended partition.
  • Cannot handle disks larger than 2TB.

Diagram:

UEFI​

UEFI (Unified Extensible Firmware Interface) is a modern firmware interface designed to replace BIOS. It uses the GPT (GUID Partition Table) partitioning scheme, which offers several advantages over MBR.

  • Supports up to 128 partitions.
  • Provides better support for large drives and modern hardware.
  • Includes a secure boot feature to ensure that only trusted software is loaded at boot time.
  • Faster boot times and more advanced graphical interfaces during startup.

Diagram:

Listing Block Devices​

The lsblk command lists block devices, displaying their names, sizes, and mount points. This helps in understanding the current disk layout and identifying available devices.

$ lsblk
NAME    MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda    202:0    0  50G  0 disk
├─xvda1 202:1    0   1M  0 part
└─xvda2 202:2    0  50G  0 part /
xvdb    202:16   0   5G  0 disk
xvdc    202:32   0  25G  0 disk
└─xvdc1 202:33   0  25G  0 part

Accessing Devices​

Devices are accessible in the /dev/ directory. This is where all the device files are located, allowing interaction with hardware components.

$ ll /dev/xvd*
brw-rw----. 1 root disk 202,  0 Dec 23 22:27 /dev/xvda
brw-rw----. 1 root disk 202,  1 Dec 23 22:27 /dev/xvda1
brw-rw----. 1 root disk 202,  2 Dec 23 22:27 /dev/xvda2
brw-rw----. 1 root disk 202, 16 Dec 23 22:27 /dev/xvdb
brw-rw----. 1 root disk 202, 32 Dec 23 22:27 /dev/xvdc
brw-rw----. 1 root disk 202, 33 Dec 23 22:27 /dev/xvdc1

From the kernel's perspective, block devices can also be viewed in /proc/partitions. This file shows a list of partitions recognized by the kernel.

$ ll /proc/partitions
-r--r--r--. 1 root root 0 Dec 23 22:27 /proc/partitions

$ cat /proc/partitions
major minor  #blocks  name

 202        0   52428800 xvda
 202        1       1024 xvda1
 202        2   52426735 xvda2
 202       16    5242880 xvdb
 202       32   26214400 xvdc
 202       33   26213376 xvdc1

Storage Options​

There are several storage options available, including partitions, LVM (Logical Volume Manager), Stratis, and VDO (Virtual Data Optimizer). Each option offers different features and benefits, depending on the use case.

  • Partitions

    • Classical solution which can be used in all cases.
    • It can be utilized to allocate dedicated storage to specific types of data.

  • LVM (Logical Volume Manager)

    • Used at default installation of RHEL.
    • Supports resizing volumes and adding new disks without downtime.
    • Adds flexibility to storage (resize, snapshots, and more).

  • Stratis

    • Offers features like snapshots and data integrity checking.
    • Next generation volume management filesystem that uses thin provisioning by default.
    • Creates a unified storage pool, and is implemented in user space, making API access possible.

  • VDO (Virtual Data Optimizer)

    • Focused on storing files in the most efficient way.
    • Provides block-level deduplication, compression, and thin provisioning.

GPT vs. MBR​

Comparing GPT and MBR partitioning schemes, GPT is the modern choice offering better features and flexibility, while MBR is the older, more limited option.

  • MBR

    • Limited to disks up to 2TB.
    • Supports only 4 primary partitions (extendable with logical partitions).
    • 512 bytes to store boot information, 64 bytes to store partitions.
    • Does not include redundancy.

  • GPT

    • Supports larger disks (up to 9.4 ZB).
    • Allows for more partitions (up to 128 max).
    • Includes redundancy with a backup partition table.

File Systems​

Different file systems are optimized for various use cases, such as general-purpose storage or high-performance applications. Creating a file system is similar to formatting partitions in other operating systems. The process prepares a partition to store data.

To create an XFS Filesystem:

mkfs.xfs

To create an EXT4 Filesystem:

mkfs.ext4

If no filesystem is specified, it will create an Ext2 file system.

mkfs

Use mount to mount it in runtime and umount before disconnecting device.

EXT4 Filesystem​

EXT4 is widely used for its reliability and performance. It's suitable for most general-purpose storage needs and is known for its journaling feature, which helps in preventing data corruption.

When to Use EXT4:

  • Default in RHEL 6, and is still used.
  • Backward compatible with EXT2 and EXT3.
  • Uses Journal to guarantee data integrity.
  • Size can be increased and decreased.
  • Prevents data corruption by tracking uncommitted changes.
  • Supported by most Linux distributions, ensuring broad compatibility.

Some limitations:

  • Single-threaded I/O
  • Limited resources:
    • Less than 1000 IOPS
    • Less than 200MB/s bandwidth
    • Limited CPU availability

XFS Filesystem​

XFS is designed for high-performance and large-scale data management. It's particularly effective for large storage servers due to its scalability and efficiency in handling large files.

  • Default filesystem, uses CoW to guarantee data integrity.
  • Optimized for environments with heavy I/O operations.
  • Supports extremely large files and file systems.
  • Size can be increased, but not decreased.
  • Uses extent-based allocation to reduce fragmentation.
  • Allocates inodes dynamically for better disk space utilization.

The xfsdump utility can be used for creating backups of XFS formatted devices and considers specific XFS attributes.

  • xfsdump only works on a complete XFS device.
  • xfsdump can make full backups or different levels of incremental backups.

Commands:

  • To create a full backup of the contents of the /data directory:

    xfsdump -1 0 -f /backupfiles/data.xfsdump /data

  • To restore a backup that was made with xfsdump:

    xfsrestore -f /backupfiles/data.xfsdump /data

  • To manually repair broken XFS file systems:

    xfsrepair