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Defrag on Large HW RAID-5 Volumes

By tony ·
What benefit would defragging a 100+GB volume that is stored on an external hardware managed RAID-5 disk system that is online 24/7/365?

I understand how defrag would help single physical disks and RAID-1 configurations, but how could it possiblyhelp multiple drive/head configuraitons, since this is invisible to the O/S.

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advantage to defrag -- reboot time

by dbesenho In reply to Defrag on Large HW RAID-5 ...

we had a scenario on a raid-5 where it went down, and because the drive was so fragmented, it took several HOURS to come back up. we scheduled some down-time over the weekend, ran defrag (which ran for 9 hours!), and reboot took 15 minutes. that'sthe only reason i can think of.

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Regardless of the underlying disk technology being used (IDE, SCSI, RAIDx), the file system sees things in terms of LOGICAL clusters. Each and every partition starts at LOGICAL cluster 0 and goes for nnn clusters. Defragmenters work at the file system level - at the logical cluster level - and ensure that a file is LOGICALLY contiguous. If a file is LOGICALLY contiguous, then only 1 LOGICAL request has to be made to retrieve the file - that is where the performance benefit is achieved. Where that file actually ends up on the hard drive (which platter of which hard drive) is unknown. Even if a file is LOGICALLY contiguous, it may or may not be PHYSICALLY contiguous on the hard drive.

- Greg/Raxco Software

Disclaimer: I work for Raxco Software, the maker of PerfectDisk - a commercial defrag utility, as a systems engineer in the support department.

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