Topic: Four 300GB disks in RAID 5 only gives me 550GB disk space

[weblance]

I just made a fresh install of SME in RAID 5 mode. I have four 300GB disks installed but the RAID array only gives med about 550GB of space - only half of what is installed. Did I do something wrong in the installation? Shouldn't it give me about 830GB usable disk space?

/Carl

[Stefano]

Carl, please take some time to read the documentation

without the "spares=0" flag at boot time, with 4 hd you have a raid5 array with 1 spare.. so you are using 3 hds, then 300x(3-1) Gb.. et voilà, yours 550Gb available

mdadm that comes with SME7 doesn't support the (AFAIK) --grow parameter

with SME8b6 a simple

Code: [Select]

mdadm --grow /dev/md2 --raid-devices=4

would do the trick, promoting the spare disk to active

at boot time, press F2 to check the available flags

[weblance]

so you are using 3 hds, then 300x(3-1) Gb.. et voilà, yours 550Gb available

I don't understand the math. 4x300GB = 1.2TB . . . 3x300GB = 900GB ?? What am I missing?

[weblance]

When does the spare come to use?

[Stefano]

I don't understand the math. 4x300GB = 1.2TB . . . 3x300GB = 900GB ?? What am I missing?

you are missing some theory

[Stefano]

When does the spare come to use?

as the name says, the spare disck become active when one of the active disks fails

[idp_qbn]

Hi weblance
Do a re-install, using the "spares=0" option at boot time (the install boot time, that is) and all 4 disks will be in raid 5.
That will give you (4-1) x 300Gb ie approx 900GB. In actuality, depending on the disk manufacturer and other factors, count on getting more like 800Gb.

Why (4-1) disks? Read about RAID - http://en.wikipedia.org/wiki/Standard_RAID_levels
With a 5 disk RAID5, you get 4 data disks ie (5-1) x 300 Gb = 1200GB.
I had 4 x 500G disks and I got about 1400Gb or so.

Cheers
Ian