Hot Standby?

Question

I recently worked with a server that had 5 drives setup with Raid 5 using only 4 of the drives. The 5th drive was on 'hot standby'.   If at any time one of the drives failed, the 5th drive (hot standby drive) would instantly jump in and become an active part of the big 4.   The bad drive could then be pulled out 'hot' and sent back for replacement.   I just ordered a Poweredge2800 with 5 drives intending to set this same scenario up and find that all 5 drives are already apparently configured using all 5. I would like to configure the Raid like I have described above.  I'm not sure where to begin, i probably can figure this out if someone can just give me a good start.   Also, will it matter if I have already setup Active directory and such?   Thanks

ShaneRoss · Answer

I do not think this is possible with out reinstalling your OS. As the the RAID is your primary partition.   If it is hardware RAID when the computer starts up there will be a prompt asking you to press control + M or A, this is the place you would reconfigure you RAID.

galois · Answer

Okay, point taken. I don't mind going back in and reinstalling the OS.

But this brings the question: In your opinion, would it be worth it? Does the setup I described offer an advantage or is it superflous?

What happens when one drive fails in a group of 5 drives setup with Raid 5? Does the server go down until the drive is replaced? And if I were to buy a drive for emergancy, could I just swap that new drive hot?

Thanks

pcmeiners · Answer

You would need to setup the array from scratch.   How critical is the data, can you wait a min 4 hours before drive replacement. For critical uptime I always use a hotspare. Then again, for every drive in an array set over the min. of 3 , performance increases, up to the point of scsi channel saturation, 5 drives will just about be the maximum. You could purchase another drive, and add it to the present setup as a hotspare at any time, with no data loss.

galois · Answer

Adding another drive and leaving the present 5 with their present configuration would be a wonderful solution for me. The amount of time to reconfigure would cost more than just buying another drive.   Thank you so much for the extra info on performance.    I'm not entirely ignorant but I've never setup RAID after the OS is installed an configured. I've only once setup RAID and the server was brand new out of the box. Seemed easy enough.   I have to ask though, are you sure that I can add this extra drive as a standby without having to redo the current OS config? And if yes, then how do I configure this? Do I restart and enter the RAID setup? Or, do I just connect the new drive and it automatically becomes a standby?     Thanks

pcmeiners · Answer

All my client's arrays have hot spares.   You would place a new drive drive in an available slot. I have the LSI  u320-2x adapter (Perc4 DC ), which  has a different interface, same functions. Basically once the drive shows up as online, there is a hotkey or a menu choice to add the drive as a hotspare. This not a destructive process, as once added, it can also be removed as a hotspare. Small note, I would check the drive's firmware is the same as your other drives. Call Dell, they will walk you through adding the hotspare, not difficult but you could make a mistake. The Perc4E, also acts the same. Done this numerous times.   In case Dell tells you it is unnecessary to have a hotspare read this. I not advocating raid 6, but , the sooner a 'dead' disk is replace in raid 5, the less chance of failure. Lisilogic has instituted patrol read, and better raid 5 recovery ability in the newer adapters, so don't be too threatened by the Intel raid 6 article. Dell will not rebuild your AD, if it fails, you will.

galois · Answer

The client cannot afford 4 hours downtime. They would rather pay for the hot standby. So, I will follow your advice.   I do have yet another question: It seems that the 5 scci drives are separated into to I/O's. At least the Disk Managment utility displays them as disk 0 and disk 1. It appears that Disk 0 is comprised of two 36GB drives that are setup to mirror.   Disk 1 is  the remaining 3 setup Raid 5.   Does this mean that my ability to add the hot standby without fuss is thwarted? I suppose I could add two hot standbys for the two effective drives?? I prefer though to have one collective drive with a single stnadby. Ugh!

pcmeiners · Answer

Does not change the ability to add a hotspare easily....   Yes a global hotspare can be used for more than one logical array, but I would  concentrate on the raid 5. Raid 1 is an extremely safe, the odds of losing two physical drives in raid 1  before a bad drive is replaced, in a short period of time (under two days) is almost astronomical, unless excessive heat was the cause, as in fan failure; even then I have seen scsi drive run hot enough to fry eggs on, run for months before failure (not recommended), in 23 years I have never seen two raid 1 disks fail within the parameters above, even with total fan failure. Dedicate the hotspare to the raid 5; the chances of an array loss in raid 5 is so much higher, as due to physical drive failure, and  the chance of a disk being failed or offlined due to 'bad blocks' .   One thing to remember, a hotspare is not total protection..raid 5 is still  vulnerable during rebuild, and multiple drives can fail during a rebuild, ...just array failure is less likely with a hotspare. Tape backup is still critical. As a note, dust out your servers regularly , best to have  spare fans in storage.   http://lsionline2.lsil.com/esupport/esupportlsi/consumer/esupport.asp?id=18237e62-0765-4da8-98df-f7884591bb29&resource=&number=1&isExternal=0&nShowFacts=&nShowCause=&nShowChange=&nShowAddInfo=&activepage=statement.asp&bForceMatch=False&strCurrentSymptom=&searchtype=normal&searchclass=QuickSearch&bnewsession=false&selecttype=match

PowerEdge HDD/SCSI/RAID

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