Accusys ExaRAID GUI - Manual (Page 216)

Advanced Functions
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• File system defragmentation might be needed before resizing
• Comparisons with thin provisioning
Dynamic LUN resizing is better than thin provisioning in terms of reliability
and performance because it retains the linear and contiguous data
layout:
• Thin provisioning needs to maintain address translation tables, which
could cause disastrous data loss when corrupted.
• The over-advertised capacity of thin provisioning misleads the space
allocation algorithm of a file system.
• The scrambled data layout of thin provisioning cause bad
performance, especially when sharing storage of different types of
workloads or host computers.
• Thin provisioning is very likely to be misled to allocate unnecessary
space by data movement or data scrubbing applications, like file
system defragmentation.
5.5.1 Free chunk defragmentation
The free space on a disk group is managed as free chunks. When there is
no logical disk on a disk group, all the available space forms a free chunk.
Later, free chunks are created when you delete logical disks or shrink the
capacity of logical disks, but adjacent free chunks will be merged
automatically. You can use free chunks for creating new logical disks or
expanding a logical disk.
A logical disk has to use contiguous space on a disk group, so you need
to merge all free chunks into one by the disk group defragmentation
utility, which starts a background task to move the data of all logical disks
on the disk group to the beginning space of the hard disks, such that all
free space is consolidated to form single free chunk located at the
ending space of the hard disks. Two common scenarios are illustrated as
below:
• Disk group expansion to expand the last existing free chunk
All existing free chunks except the one at the end of the disk group are
deleted, and the last free chunk is expanded.
Note
The software listed above is only for your information; no warranty
should be assumed. Please contact the software vendors to learn
how to use the software to support LUN resizing.