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I am looking for technical details of how software RAID (specifically RAID 1 and RAID 5) is implemented in Windows operating systems, particularly Server 2008 R2 and Windows 7. I'm sure this information is out there, but I haven't been able to find it on Google. I am looking for:

  • At what level does RAID 1/mirroring operate? File, cluster, sector, or something else? That is, how identical are two disks in a mirrored pair, at the sector level?
  • What triggers a resynchronization of a mirror?
  • Once Windows determines a resynchronization is needed, how does it decide what order to resynchronize volumes in?
  • What checks are done on read data to ensure the data is correct, and should not be reconstructed from the redundancy information?
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up vote 2 down vote accepted
  • RAID operates at the block level, typically a 64k block.
  • A resync is triggered when a disk is replaced.
  • It probably does them in disk ID order. (I'm just guessing.)
  • No checks are done on the data to ensure that the data is correct. You are protecting against a disk failure not against corrupt data.
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"RAID operates at the block level, typically a 64k block." Source for this, as applied to Windows software RAID? I have weak evidence to the contrary, and I'd like to sort out exactly what's going on. – kquinn Feb 3 '11 at 2:37
kquinn, I can't find anything useful online. Let me contact some people I know at Microsoft and get a firm answer. – mrdenny Feb 3 '11 at 20:05
@kquinn, I heard back from my contacts at Microsoft. They passed around the question until it got to someone who used to work on the NTFS team. The response was "The mirroring is done at the block level. NTFS knows the LBNs associated with each file. When the data is modified NTFS modifies the master file table and the relevant 512 byte block. When the block updates are made NTFS updates both physical disks at the same time. The disk blocks correspond to the sectors that windows sees from the disks which are comprised of the allocation units (ie. disk clusters)." – mrdenny Feb 5 '11 at 22:04

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