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kikinovak 10-12-2013 06:32 AM

How to calculate effective disk capacity with RAID 5?
 
Hi,

I'm planning to install a storage server for a client, using an HP Proliant Microserver with 4 x 2 TB harddisks and RAID 5.

I have a similar configuration in my office, only with much smaller disks. Each disk is 250 GB. Here's what this looks like:

Code:

[root@nestor:~] # cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath]
md3 : active raid5 sda3[0] sdd3[3] sdc3[2] sdb3[1]
      729317376 blocks level 5, 512k chunk, algorithm 2 [4/4] [UUUU]
     
md2 : active raid1 sda2[0] sdd2[3] sdc2[2] sdb2[1]
      995904 blocks [4/4] [UUUU]
     
md1 : active raid1 sda1[0] sdd1[3] sdc1[2] sdb1[1]
      96256 blocks [4/4] [UUUU]
     
unused devices: <none>

[root@nestor:~] # df -h
Sys. fich.    Taille Util. Dispo Uti% Monté sur
/dev/md3        685G  517G  133G  80% /
/dev/md1          92M  38M  49M  44% /boot
tmpfs            1,5G    0  1,5G  0% /dev/shm

[root@nestor:~] # fdisk -l | grep sd | grep Disque
Disque /dev/sda*: 250.1*Go, 250059350016*octets
Disque /dev/sdb*: 250.1*Go, 250059350016*octets
Disque /dev/sdc*: 250.1*Go, 250059350016*octets
Disque /dev/sdd*: 250.1*Go, 250059350016*octets

In the example above, combining four 250 GB disks with RAID 5 results in 685 GB effective storage space.

Q: is there any formula or rule of thumb to calculate this storage space for disks with a different size, in my case 2 TB each?

Cheers,

Niki

chemfire 10-12-2013 06:57 AM

Raid 5 should give you the capacity of N_disks - 1. You need to keep in mind though formatted capacity is not the same as partition size. The filesystem itself uses some space.

Also df, ls etc report values in powers of 2, it looks like fdisk in your screen shot is using powers of 10. So what is sold to you as 250GB disk is really more like 238.

kikinovak 10-12-2013 01:26 PM

OK thanks. It's just to have a rough idea. So basically my 4 x 2 TB array with RAID 5 will offer 4 x 2 - 2 = 6 TB minus some weird math powers stuff, so let's give it something between 5.0 and 5.5 TB. ;)

Cheers,

Niki

jtsn 10-12-2013 07:39 PM

Just use df -H (capital H) to get correct numbers with SI prefixes.


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