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Reported disk size reduced after create RAID1+0

 
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Michael BUTOW
Frequent Advisor

Reported disk size reduced after create RAID1+0

Hello all,

we have DL380 G6 with P410 RAID controller and 146GB haard disks (Seagate ST9146803SS SAS 10 kTpm).

We configure 2 disks each in RAID1+0 configuration, and have noticed that before setting up the logical disk, the siyes of the physical disks are reported as 146GB in the ROM-based RAID setup utility.
Afterwards, the size indicated for the logical disks is somewhere around 137GB (I do not remember the exact number).


However, it is a little less than before, even thought also reported in GB.

Does anyone know for sure what causes this reduction in disk space?

Thanks in advance,
Michael
6 REPLIES
Suman_1978
HPE Pro

Re: Reported disk size reduced after create RAID1+0

Hi,

I hope this link helps you find difference between gigabyte and gibibyte value.
http://en.wikipedia.org/wiki/Gigabyte

http://www.diskview.com/disk-size-discrepancy.htm

Thanks
David Claypool
Honored Contributor

Re: Reported disk size reduced after create RAID1+0

The number on the hard disks refers to the RAW capacity of the drive. Once an operating system has formatted it the efficiency of the formatting comes into play (256/512/1024 byte sectors) and how much space the operating system reserves for its own use is also a factor.

There is additionally some potential for confusion as some references are to a binary number and some references can be to a decimal number, as in

1K = 2^10 = 1024
1K = 1000
Michael BUTOW
Frequent Advisor

Re: Reported disk size reduced after create RAID1+0

Hello,

Thanks for responding, but this size difference is reported by RAID setup *before* an OS is installed, and in both cases the reported unit is GB.

Also, the exact figure left is reported as 136.7 GB - this does not fit a GB / GiB mismatch.

I am hoping for further clarification.

Regards,
Michael
kris rombauts
Honored Contributor
Solution

Re: Reported disk size reduced after create RAID1+0

Michael,

looking at ACU with older 73 GB disks, they are reported as 72,8 GB before they are configured in an array.
When i.e. two such disk are added into an array , it reports 138928 MB available space (this is already less than 2 * 72,8 GB = 145,6 GB). If i then create a logical drive RAID0/1 within that array, i get a end result of 69463 MB.

There is always some overhead when creating an array/logical drive and sometimes there is a small amount of space left unused for optimizing performance of the array.

This has always been the case but i don't have a % of how much this takes to calculate what to expect as available space upfront.

Doing the same with two 300 GB disks, gives me 286101 MB when selecting a logical drive in raid 0/1 which is approx 4,6% less then raw capacity.
This is all without converting the GB into MB.

HTH

Kris
Michael BUTOW
Frequent Advisor

Re: Reported disk size reduced after create RAID1+0

Thanks Kris.

I thought there was some space used by the RAID itself and I had also noticed this before on disks of other sizes.

Regards,
Michael
Michael BUTOW
Frequent Advisor

Re: Reported disk size reduced after create RAID1+0

Closing this thread as I have received enough information.