A Practical Guide: Expanding Disk Space on a RedHat Linux Server with SAN Storage

Expanding SAN Storage on Red Hat: The Definitive Guide to LVM & Multipath

Adding disk space to a production Red Hat server is a routine but high-stakes task. When your storage is delivered over a SAN (Storage Area Network) with multiple paths, you must coordinate between the Linux SCSI layer, the Multipath daemon, and the Logical Volume Manager (LVM).

⚠️ Production Safety: Read Before Proceeding

In a production environment, one wrong device name can lead to catastrophic data loss.

• Identify the Correct Path: Never run pvcreate on a raw SCSI path (like /dev/sdb) if the disk is part of a multipath setup. Always use the multipath map (e.g., /dev/mapper/mpathX or /dev/dm-X).

• Backup LVM Metadata: Before modifying your Volume Group, run vgcfgbackup <VG_NAME>. This stores a recovery layout in /etc/lvm/backup/.

• Verify with blkid: Ensure the new disk/LUN is truly empty before initializing it. If blkid returns a UUID, that disk already contains data.

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Step 1: Storage Discovery (Hardware Layer)

To see the new LUN provided by the SAN team without rebooting, we use the sg3_utils package.

1. Install the Utilities

Bash

yum install sg3_utils

2. Rescan the SCSI Bus

Monitor the kernel logs in one terminal while running the scan in another to confirm the new paths are detected.

Bash

# Terminal 1: Watch the logs
tail -f /var/log/messages

# Terminal 2: Trigger the scan
rescan-scsi-bus.sh -r -i -s

Note: The -s flag is essential for detecting size changes on existing LUNs.

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Step 2: Initialize and Extend LVM

Once the OS detects the device (we will use /dev/dm-28 as per your notes), we move into LVM management.

1. Create the Physical Volume (PV)

Caution: Ensure you are targeting the correct DM (Device Mapper) node.

Bash

pvcreate /dev/dm-28

Verify with pvscan or pvs.

2. Extend the Volume Group (VG)

We add the new Physical Volume to the existing group (e.g., VG-SG-PI-DPI01).

Bash

vgextend VG-SG-PI-DPI01 /dev/dm-28

3. Expand the Logical Volume (LV)

You can use the exact number of Free Physical Extents (PE) shown in vgdisplay. In your example, this was 12799.

Bash

lvextend -l +12799 /dev/VG-SG-PI-DPI01/lv-sg-pi-dpi01

Pro-Tip: In modern RHEL versions, you can use lvextend -r -l +100%FREE <path>. The -r flag automatically resizes the underlying filesystem for you safely.

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Step 3: Filesystem Expansion

The final step is to grow the filesystem to fill the newly enlarged Logical Volume.

For ext3/ext4 Filesystems:

Use resize2fs. This is an online operation and does not require unmounting.

Bash

# Syntax: resize2fs [Device_Path]
resize2fs /dev/VG-SG-PI-DPI01/lv-sg-pi-dpi01

For XFS Filesystems (RHEL 7/8/9):

If your server uses XFS, resize2fs will not work. Use xfs_growfs.

Bash

# Syntax: xfs_growfs [Mount_Point]
xfs_growfs /sg-pi-dpi01

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Step 4: Final Verification

Always confirm the expansion was successful at the OS level:

Bash

df -h

Command	Purpose	Production Risk
rescan-scsi-bus.sh	Detects new hardware	Low (unless -i causes a reset)
pvcreate	Formats disk for LVM	High (can overwrite data)
lvextend	Increases LV size	Low (but hard to shrink later)
resize2fs	Expands filesystem	Medium (always have a backup/snapshot)