Adding a Drive to a RAID 5 array

[Osarhan]

Hi,

Sorry if this is a bit of a newbie question to ask, but to be honest i'm learning so don't kill me.
At the moment I have a linux server setup (using clarkconnect) for home use, movies games and videos etc. and its got 3 SATA drives setup in RAID 5 on an adaptec 2610sa card (dell branded CERC card) and its running along happily, however I am thinking of adding another 2 drives to the system in order to add capacity.
Is it possible to add the drives to the array in such a way that the drive mounted in linux will appear bigger? or set it up as a second array and mount it seperately in linux?
If they can be added to the array will I have to reinitialize ALL the drives ? OR will I have to format the whole thing in linux again?
II heard about stretching a parition but was a bit unsure if it applies in this instance.
Basically I want to make my RAID setup mounted in linux bigger without impacting on the data already there.

Thanks

 

[ddrueding]

Provided your RAID card supports "On-line RAID-5 expansion" or something similar, it should be fine. I'm in the middle of doing the same (going from 5 to 6 drives). It will have to rebuild the entire array, mine has taken 4 days so far (out of an estimated 6).

 

[Osarhan]

Provided your RAID card supports "On-line RAID-5 expansion" or something similar, it should be fine. I'm in the middle of doing the same (going from 5 to 6 drives). It will have to rebuild the entire array, mine has taken 4 days so far (out of an estimated 6).

thanks for the pointer, I'll have to check On-line RAID-5 expansion and my adaptec card.
I guess i could add another 3 drives and have 2 RAID 5 arrays?
is it possible to mount 2 arrays to the same directory in linux?

 

[Osarhan]

turns out card does support online capacity expansion

 

[P5-133XL]

thanks for the pointer, I'll have to check On-line RAID-5 expansion and my adaptec card.
I guess i could add another 3 drives and have 2 RAID 5 arrays?
is it possible to mount 2 arrays to the same directory in linux?

One mount point per directory. You can, however, use a sub-directory as a mount-point.

 

[MaxBurn]

I would recommend a backup before doing this too. Might even be faster to shove the data somewhere else, nuke the partition & RAID set and create fresh.

 

[ddrueding]

Also, doing RAID-5 arrays of 3 drives is simply not efficient. Use larger numbers of drives or RAID-10.

 

[Osarhan]

Thanks for all the replies, al i need now is to find someone with a drive to lend me so i can back up the data!

Also, doing RAID-5 arrays of 3 drives is simply not efficient. Use larger numbers of drives or RAID-10.

How inefficient are we talking, like as long a I can drag a movie cross the network in under 5 min its grand!

 

[ddrueding]

RAID-5 arrays have many problems; slow write speed, difficulty of data recovery if more than one drive fails, slow rebuilds/verifys. These are only "worth it" IMO if you can get some form of redundancy with very little drive overhead. For my array, only 17% of my drive capacity is taken up with redundancy, for yours it's 33%. In order to get even more redundancy and a much faster array in RAID-10; I would need to add 4 more drives, but you would only need to add 1 more drive.

 

[Osarhan]

hmm , good point about the redunancy percentage, would the writes be much slower or are we talking about up to 20% slower? is rebuild/very used at all times or is it only at the array creation stage? on a completely seperate topic you know if there is a way to use apt-get to install something in a custom location?

sorry for the newbie questions, just gotta learn somewhere , you'd think being in my 4th year of a computers degree i'd have some exposure to all this. anyway better get back and study my decision trees!

 

[ddrueding]

Writes on RAID-5 depend on quite a lot. Because of the checksum calculation, you will never get write speeds that are faster than one drive; and if the processor on your RAID card isn't fast enough, it may be much slower than that. My Bufallo Terastation is CPU limited to 6.5MB/sec for writes. Both writes and reads in RAID-10 are as fast as half the number of drives (a 4 drive array would read and write twice as fast as a single drive). Rebuild/verify is used any time the array is changed or becomes degraded; this may not happen at all, but if it does it will take a long time. If one of your drives fails and you replace it, any additional drive failure before the rebuild completes means your data is toast.

You'll have to ask someone else about the Linux stuff, I have no idea.

CS degrees are, well, none of my friends in the business have one.

 

[Osarhan]

thanks man!