HP 3PAR RedHat and Oracle Linux Implementation Guide

WARNING! While removing the device, make sure the correct underlying device is used. Use
the multipath command to identify the underlying device.
CAUTION: For iSCSI devices, do not remove the last iSCSI device in / proc/scsi/scsi
without first stopping multipathing, and then stopping the iSCSI daemon ( /etc/init.d/iscsi
stop). Otherwise, data corruption can occur and the host will hang.
Any change to the /etc/multipath configuration file requires running the multipathd
command to be effective. If the change is not reflected, try stopping and starting the multipathd
script.
# /etc/init.d/multipathd stop
# /etc/init.d/multipathd start
NOTE: The removed SCSI device is updated in /proc/scsi/scsi, /proc/partitions,
and /sys/device path.
UNMAP Storage Hardware Primitive Support for RHEL 6.x
HP 3PAR OS 3.1.1 or later supports the UNMAP storage primitive (operation code 42h) which is
supported by RHEL 6.x OS with the ext4 file system. UNMAP causes to free up space on a
thinly-provisioned virtual volume (TPVV) storage volume when data or files are deleted on the ext4
file system, and requires that the file system be mounted with the -o discard option. This feature
is useful in maintaining the volume as a thin volume with no storage disk space allocated for files
that are deleted. Space is released on the TPVV storage volume when minimum deletions of 16
kilobytes occur in the file system.
Example:
# mount -t ext4 -o discard /dev/mapper/350002ac000020121p1 /mnt
This will cause the RHEL 6.x OS to issue the UNMAP command, which in turn causes space to be
released back to the array from the TPVV volumes for any deletions in that ext4 file system. This
is not applicable for fully-provisioned virtual volumes.
In RHEL 6.x, the default option for creating the ext2/ext3/ext4 file system has the -E discard
option enabled for thinly-provisioned virtual volumes (TPVV). This discard option basically causes
the host to issue the UNMAP command to unmap all the blocks on the storage volume before the
file system is created.
Because the UNMAP commands are issued sequentially, and because there is no need to release
blocks on a newly created TPVV (since the storage will not have allocated any space on a TPVV),
these UNMAP commands do not serve any purpose for initial file system creation on a new TPVV.
Because of the sequential nature of the UNMAP commands issued from the host, file system creation
takes a long time on a TPVV by comparison to a fully-provisioned volume.
Therefore, to create the ext2/ext3/ext4 file system quickly on a newly created TPVV, use the
nodiscard option. Testing has shown that on a 100 G TPVV, it takes around 3 minutes 30
seconds with a default discard option, and only about 10-12 seconds with nodiscard option
for the ext4 default file system.
For example, on a newly created TPVV, use the -E nodiscard option:
# mkfs.ext4 -E nodiscard /dev/mapper/350002ac000020121p1
124 Modifying HP 3PAR Devices on the Host