Defrag on RAID 1 and RAID 5 arrays
This is really two questions: 1. Does Defrag actually do anything useful on a RAID 5 array? 2. What about a RAID 1 array? What got me thinking about this was thinking about what Defrag actually does, and why it does it: It combines fragments of files that may be located on different parts of the disk platter, thus reducing disk thrashing and improving disk performance. Now what about a RAID 5 array? When a file is written to it, it could be split between physical drives, transparently, by the RAID controller. How does Defrag improve this? In a RAID 1 array, one could imagine that Defrag does the same thing as it would on a simple single drive, just mirrored. But will the second disk actually physically mirror the first? Surely the RAID controller is in command, and would put the files just where it saw fit? Any ideas? Grant Ward, a.k.a. Bigteddy
June 30th, 2012 5:11am

Except for SSD, defrag would help in native server environment. In my understanding defrag is doing its job vertically while RAID take care on vertical transfer. Though Windows 2008 R2 is doing maintenance itself, I have used Diskeeper*) which may optimize process (... and you see the results) Regards Milos *) www.Condusiv.com
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June 30th, 2012 6:48am

For any OS, connected disks looks like a plain disks and any defrag application treats disk as a plain disk irrespective of disk RAID levels. Fragmentation would be the same across disks irrespective of underlying storage technology. A RAID 5 fragments exactly the same as a RAID 1 volume or a RAID 10 volume or a plain drive. In my perception, its safe to run defrag on both RAID 5 and RAID 1 volumes. However during defragmentation process, servers would likely to respond slowly. If you wish to perform disk defragmentation, schedule the defrag task to run during Off business hours. here are couple of similar discussions, which might give you some more insight on this aspect. Defragging RAID5 server partitions - yes or no? http://community.spiceworks.com/topic/96775-defragging-raid5-server-partitions-yes-or-no Can a Raid5 array be optimized using defragmenting or other maintenance tools? http://social.technet.microsoft.com/Forums/en-US/winservergen/thread/c26aa69e-d1f0-4606-b888-a294e288b192 Press any key... What the ... Where's any key ? This posting is provided "AS IS" with no warranties or guarantees and confers no rights. About Me ?
June 30th, 2012 7:07am

I'm not convinced, because no-one has touched on my argument that the RAID controller abstracts the physical disk to a logical disk, and when the computer writes to a position pointed to by the FAT, the operating system has no way of translating this into where the bits will acutally be written on the physical disk. In other words, the RAID controller presents a map of a logical drive, which Windows sees as a single disk. But this map is translated by the RAID controller according to its own algorithm. This is why I don't think defrag actually defrags the physical disk, but rather the logical disk. Because the logical disk is not what is actually being read and written to by the the disk heads, this need not necessarily result in real defragmentation.Grant Ward, a.k.a. Bigteddy
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July 1st, 2012 4:17am

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