A storage device is a term used loosely to mean anything that’s used for storing data. Storage devices comes in various forms, but they most commonly in the form of a partition (e.g. /dev/sdb2), or an unpartitioned block device (e.g. /dev/sdc), or a logical volume (e.g. /dev/mapper/centos-home). Before you can start storing data in storage device, you first need to:
- Format the storage device – Aka install a filesystem.
- Mount the filesystem – Attach the filesystem to a folder on your system
File system types
There are several main filesystem types that are available:
- XFS – This is now the default filesystem in RHEL 7.
- Ext4 – This used to be the default file system up to and including RHEL 6
- vfat – This is typically used for removable media, e.g. USB pens
To view a complete list of supported filesystems, check out:
$ man fs
Each file system type has their own man page as well, e.g. to learn more about xfs, you do:
$ man xfs
This is a bit like a library that doesn’t have any bookshelves, and hence unable to store any books. Installing a filesystem is a bit like filling the library up with book shelves.
Install a file system
To install a filesystem on a partition (or an unpartitioned hdd) you need to use one of the mkfs.* commands:
$ mkfs # I pressed the tab key twice, write after typing "mkfs" mkfs mkfs.cramfs mkfs.ext3 mkfs.fat mkfs.msdos mkfs.xfs mkfs.btrfs mkfs.ext2 mkfs.ext4 mkfs.minix mkfs.vfat
Let’s now install the xfs filesystem on a partition:
$ mkfs.xfs -L "DummyStorage" /dev/sdb7 meta-data=/dev/sdb7 isize=256 agcount=4, agsize=64064 blks = sectsz=512 attr=2, projid32bit=1 = crc=0 finobt=0 data = bsize=4096 blocks=256256, imaxpct=25 = sunit=0 swidth=0 blks naming =version 2 bsize=4096 ascii-ci=0 ftype=0 log =internal log bsize=4096 blocks=853, version=2 = sectsz=512 sunit=0 blks, lazy-count=1 realtime =none extsz=4096 blocks=0, rtextents=0
Notice that I used the “L” switch. This is optional, but is recommended because it lets you assign a human friendly name to your partition/hdd. This is important because the current name, (which in this case is /dev/sdb7) was a name that was auto-assigned to do the device, and if you then plug in a usb-pen, then reboot your machine, then there is a chance that the usb-pen will change which devices are auto-assigned which name. Hence labels solve this problem.
Having a label or knowing the UUID (which we will cover next) is important, since we need this info as part of the 3rd and final step, which is mounting the partition.
As soon as a hdd/partition is formatted, a unique ID (UUID) is assigned to that block device. You can use the blkid command to view a list of all formatted block devices along with their UUID value:
$ blkid /dev/sda1: UUID="547c78ce-34b5-4e31-84d2-bf72b196fa57" TYPE="xfs" /dev/sda2: UUID="JnfbeO-XPAG-AS0O-Mrtw-hnwn-7hqf-Mhk46t" TYPE="LVM2_member" /dev/sdb1: LABEL="DummyExt4Storage" UUID="eeb45452-3e56-4455-a7c0-0e2e41d8a3cf" TYPE="ext4" /dev/sdb7: LABEL="DummyStorage" UUID="f252bb11-abaa-4744-8001-e82b0e066b17" TYPE="xfs" /dev/mapper/centos-swap: UUID="94263d84-5357-4566-9987-fc74ca946a96" TYPE="swap" /dev/mapper/centos-root: UUID="17f09dae-88d1-4813-9738-8996819cf6ba" TYPE="xfs"
Any unformatted block devices (e.g. /dev/sdb2) won’t appear in the above list. The blkid command only displays formatted block devices, along with UUID, Label (if any), and the type of filesystem installed. All of this info is useful when adding entries to the
Using the “file” command to analyse block devices
If you have a hdd/partition and want to find out which file system type is installed on it, then the best way to do this is by using the “file” command with the (s)pecial file option enabled so that it does additional analysis of it’s target. I also enabled the symbolic (L)ink option, so to auto-resolve any sybmolic links. Here are a few examples:
$ file -sL /dev/sda1 /dev/sda1: SGI XFS filesystem data (blksz 4096, inosz 256, v2 dirs)
Next we have:
$ file -sL /dev/sdb2 /dev/sdb2: data
Here it means that sdb2 is not formatted. Here’s an example of a partition with an ext4 filesystem installed on it:
$ file -sL /dev/sdb1 /dev/sdb1: Linux rev 1.0 ext4 filesystem data, UUID=eeb45452-3e56-4455-a7c0-0e2e41d8a3cf, volume name "DummyExt4Storage" (extents) (64bit) (huge files)
Next we have an example of an LVM’s physical volume:
$ file -sL /dev/sda2 /dev/sda2: LVM2 PV (Linux Logical Volume Manager), UUID: JnfbeO-XPAG-AS0O-Mrtw-hnwn-7hqf-Mhk46t, size: 20949499904
Delete a file system
You might think that deleting a file system could be done by deleting entire partition (and recreate it again) using fdisk. However that doesn’t work since deleting a partition doesn’t delete any data on the disk, it just deletes the artificial partition boundaries. In fact deleting a file system is best done with the help of the /dev/zero file:
$ cat /dev/zero > /dev/sdb1
Here’s the approach in action:
$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 20G 0 disk ├─sda1 8:1 0 500M 0 part /boot └─sda2 8:2 0 19.5G 0 part ├─centos-swap 253:0 0 2G 0 lvm [SWAP] └─centos-root 253:1 0 17.5G 0 lvm / sdb 8:16 0 2.1G 0 disk └─sdb1 8:17 0 200M 0 part sr0 11:0 1 55.4M 0 rom $ blkid | grep sdb1 /dev/sdb1: UUID="9fa9e491-8e21-4322-a1c2-dc269825e824" TYPE="xfs" $ cat /dev/zero > /dev/sdb1 cat: write error: No space left on device $ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 20G 0 disk ├─sda1 8:1 0 500M 0 part /boot └─sda2 8:2 0 19.5G 0 part ├─centos-swap 253:0 0 2G 0 lvm [SWAP] └─centos-root 253:1 0 17.5G 0 lvm / sdb 8:16 0 2.1G 0 disk └─sdb1 8:17 0 200M 0 part sr0 11:0 1 55.4M 0 rom $ blkid | grep sdb1
As you can see the filesystem has been deleted but the partition itself has remained intact.
Take the RHCSA Quiz
This article is part of our RHCSA Study guide (click on the yellow tab on the far left). By the end of this article you should be able to answer the following questions:
$ man mount
$ man fs
$ man xfs
$ mkfs # then type tab+tab
$ mkfs.xfs -L “DummyStorage” /dev/sdb7
It displays filesystems rather than devices or partitions. If a device/partition does not have filesystem on it, then it doesn’t appear in the list.
$ xfs_admin -L “storage” /dev/sdb7
$ e2label /dev/sdb1 webdata
$ cat /dev/zero > /dev/sdb1