Whenever I hear copy-on-write my brain immediately starts thinking RAID. How is COW any different from RAID-x version ?

Both need more than 1 HDD to be useful, both make a copy of data when data is modified and both also help in recovering data in case of something happening in primary storage i.e. /dev/sda, /dev/sdb etc. ?

  • Raid is for redundancy, while CoW is a technique to improve performance and reduce disk storage usage. Different animals here – user34720 Jan 20 '17 at 13:20
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    COW does not need more than one disk and does not help recovering data. RAID does not make a copy of data when it is modified. Where on earth did you get these? – Gilles 'SO- stop being evil' Jan 20 '17 at 22:39

How is an Apple different from an Pig?

RAID-x and COW are completely different concepts. Copy-on-write is a technique to reduce disk storage and improve the efficiency of buffers. Currently it is particularly attractive for files providing the images to virtual machines and containers. If you have say a 200MB file system image, and you start 2 different virtual machines from it the straight forward way would be to make a copy of the 200MB for each and let them alter it however they wish. This will need 600MB (200 for the original and 200 each for the copies).With a COW based system then again you make a copy but this time with the cow-copy command. This just will need 200MB plus some bookkeeping overhead, until the VMs actually start writing.

COW can run fine on a single disk, although systems running lots of VMs will benefit from additional drives to allow multiple I/O operations at the same time. Wikipedia

RAID allows you to have multiple disks to improve one or more of size, speed and redundancy compared to a single drive. Wikipedia

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