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How much space does one lose when formatting a HDD in windows

gigeorge asked
Medium Priority
Last Modified: 2010-04-03
Hi there,

I am planning on making a a raid 5 setup in win XP using 4 x 400 gb drives, i understand that for the raid 5 config i will lose 1/3 of the overall space available to me because of the redundancy. I know that in win xp when one formats a 200 or 400 gb drive they dont get the full space labelled on the disk due to sector size etc.

Is there an easy way to work out the loss of a drives space after formatting in win on the assumption that one uses the NTFS fs?


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raid 5 uses 1 drive for redundancy, so with 4 drives that would total 1/4th (except if one of the 4 drives will be used as an online spare, then you loose the spare and another drive). Remember that you will need to use a raid controller, as XP doesn't support software raid5. What you loose with the format I'm not sure, it depends on the cluster sizes you use etc. Normally windows will select the optimal size so you don't have to change any settings, but the overall loss isn't really that big an issue. If you create more than one partition the clusters will be smaller and use up less space, if your clusters are too smal on a large partition it'll decrease your access speed, so I'd leave those sizes at their optimum level which you get with an automatic format.

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You are slightly incorrect in your calculations.

For ease of maths, you can remove 1 drive out of your RAID-5 set for the parity bits. RAID-5e makes use of the parity drive, and spreads the parity bits across all drives for more resilience and faster recovery but enough of that for now.

Therefore, 4x400GB drives would give you 1200GB (ie 3 x 400GB) of raw volume with a single parity drive. This assumes you are not allocating a hot spare.

MS use 512B blocks and so you can calculate what would be presented in 1/2 MB sizes. Also, drive manufacturers present capacity in different ways to make their drive look the biggest and best in class. This is done using a mix between US and everyone else measurements.

For example, in the US (generalisation here) an advertised 400GB drive would actually be a 400,000,000,000 byte drive. (ie 1 byte x 400 x 1000 x 1000 x 1000). The rest of the planet uses 1024 Byte chunks so in the above 400,000,000,000 byte drive, it would display in Microsoft as 372.5GB useable. Cheeky, but true. You need to be sure you look at the actual capacity and how it is advertised !!! This is the biggest 'loss' of space you are likely to see.

Back to your question. In Windows XP, I would expect see 99+% of a volume available after NTFS formatting due to the small cluster sizes vs the large capacity.

Hope this helps.
IPKON Networks Ltd

You are not losing much by formatting HDD in Windows.  Windows only took very small portion, just don't include system files and other when you chose it.
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