RAID config

[Gaz]

I've got a server with x5 600GB SAS drives, I think I have dismissed RAID6 but what are peoples thoughts on having x2 drives in a RAID1 array and then x3 drives for storage in RAID5 or would you just stick them all in a RAID5 array?

The controller is an HP P420i 1GB FBWC integrated into a HP Proliant ML350p Gen8, which I'm not sure actually supports having 2 RAID arrays or if even good to do if it does support 2 RAID arrays.

[tmcd35]

What do you want the two arrays to do? Is it your only server? How many users? etc. Gut instinct, just on info given, RAID-1 mirror would be a waste of 600Gb.

[SHimmer45]

RAID 10 with a hot spare would give you 1.2TB

RAID 5 with a hot spare 1.8TB (you might want to consider rebuild time discussed in depth in other threads)

RAID 6 without the hot spare gives you the same storage space as R5 but with extra resilience

 

what is the server doing as this can dictate what type of raid you use for it, imo RAID10 has become the go to for alot of deployments

[Gaz]

Its just a standard domain controller running the usual roles, AD, DNS, DHCP, IIS, (Possibly Hyper-V), Group Policy etc etc etc. My idea was that the RAID1 array would host the OS and roles and the RAID5 would host the storage for the user areas.

[synaesthesia]

Fairly pointless, however the OS does a lot of constant random reads/writes, especially on domain controllers, which can slow down the performance of file servers also running in the same place. However I wouldn't recommend running raid 5 with only 3 drives, you'd at least want a hot spare there. The last primary I set up was raid1 for OS and raid6 for the file storage.

[Gaz]

Unfortunately I've got 5 drives so RAID1 and RAID6 isn't possible without having 6 drives and my budget wont go that far. I'm just trying to make the best of a bad situation, the server was meant to have an SSD but it wont arrive in good time so I have changed it to, 2 more SAS drives and RAM instead.

[win]

I assume you have something like 2x 120GB as a mirror for the C drive (OS). So if we're talking about the other drive..

 

I would go with a RAID 5 + hot spare. It will be slow during the RAID rebuild but unless you're working at the stock exchange i think they can live with it for a while. HDD's don't fail too often - once every 3 years maybe?

[Gaz]

I assume you have something like 2x 120GB as a mirror for the C drive (OS). So if we're talking about the other drive..

 

I would go with a RAID 5 + hot spare. It will be slow during the RAID rebuild but unless you're working at the stock exchange i think they can live with it for a while. HDD's don't fail too often - once every 3 years maybe?

 

There are no drives in place this is a new server build that contains x5 600GB SAS drives. So If I don't have enough drives for a RAID1 and RAID6 array which seems to be the recommendation so far, would it be better to just stick them all in a RAID6 array?

[synaesthesia]

I assume you have something like 2x 120GB as a mirror for the C drive (OS). So if we're talking about the other drive..

 

I would go with a RAID 5 + hot spare. It will be slow during the RAID rebuild but unless you're working at the stock exchange i think they can live with it for a while. HDD's don't fail too often - once every 3 years maybe?

 

Unless you use Seagate drives, then you best have a lot spare...

 

RAID6 will be slower but a lot more resilient. I doubt anyone would notice. RAID10 would certainly be quicker but not quite as sturdy - you can lose up to 2 drives as long as they're not in the same stripe. Less risky than raid5 though, so maybe 10 with a hotspare for all of them will be the best compromise IMO. Just goes without saying that you should keep on top of backups, but that's a grandmother-egg-sucking-situation

Edited September 23, 2014 by synaesthesia

[tmcd35]

If there's space/support in the server, I'd buy a couple of cheap SATA drives to mirror for the OS and leave the 5 SAS drives in RAID-5 for the data.

[win]

Raid 6 means you're using the extra disk all the time i.e. wearing down all disks all the time. I prefer raid 5 + hot spare as the spare is untouched until needed. 600GB HDD's are not cheap, but if your budget allows then the raid 6 will hotswap instantly without the rebuild (athough you're using a used disk as a hotswap).

 

I would go with tmcd35 suggestion of buying a couple of cheap lower storage drives to mirror the OS. Even 2x 80 GB drives will suffice.

[Gaz]

Hehe, RAID10 was something I dismissed because its impossible with 5 drives, but you are right I could do it with 4 drives plus a hot spare! 1.2TB will be plenty of space I'm sure. That would give me good speeds in both reading and writing plus data protection.

The Drives are HP drives with no tell tale signs of manufacturer.

[DMcCoy]

Assuming you backup, and monitor disk alerts, then I'd be fine with a 5 disk raid5 array without hotspare.

 

I tend to prefer 6 for RAID5, 5+1 Hostspare and 8 for RAID6, it's a good balance for risk/cost/speed.

 

Someone will be along in a minute to tell you how evil RAID5 is, but if your SAS disks have a BER or 1x10^16 then the theoretical chances of a successful rebuild are at least 99.7% and as we all know RAID isn't backup.

[tmcd35]

Given the size, price and number of drives - I wouldn't bother with either 2 drive redundancy (RAID10 or RAID6) or Hot Spares. It's just doesn't make economic sense. If you was talking a 10-12 disk array it's a little different.

 

Personally, K.I.S.S. - 5 Drive, RAID-5. Budget to replace 1 drive each year. This will help lesson the chance of 2 drives failing at once and give you spares you can swap in if a drive does fail.

[Gaz]

We have been running a server with a RAID5 array (x4 450GB SAS drives) for ~6 years 24/7 (not including reboots) with only 1 disk being replaced in that time due to predicted failure and the rebuild didn't take too long. So I'm pretty happy with it.

 

It does seem that RAID type is almost a personal preference over there actually being a best practice going off some of these posts.

[Bobby_Tables]

Its just a standard domain controller running the usual roles, AD, DNS, DHCP, IIS, (Possibly Hyper-V), Group Policy etc etc etc. My idea was that the RAID1 array would host the OS and roles and the RAID5 would host the storage for the user areas.

So this is both local storage for servers and shared storage for users. How much space do you think you'll need in total for local server storage and all your shares? What about future growth? We don't know about the size of your environment - perhaps you could elaborate?

 

I would go for RAID 10 with a hot spare, or two RAID 1 sets, one for each purpose. IMO storage performance makes a big difference and RAID 5 or 6 wouldn't offer good enough IOPS. To be honest I can't think of any situation where I would choose RAID 5. RAID 6 would be alright in a large sequential storage scenario like a media server or backup storage, but personally I'd rather have more random IOPS performance for a scenario like yours where it's live local storage for all sorts of purposes simultaneously.

Edited September 23, 2014 by Bobby_Tables
Added explanation

[Gaz]

The server will be used for a small amount of users and they won't be storing masses of data, just their work which will be mostly word documents currently one site the students and teachers home folders combined is only 150GB. This is why I had the idea of 2 arrays, a RAID1 for the server OS and roles which would provide the necessary IOPS and also redundancy, the RAID5 would be suitable for storing users data which wont require high IOPS.

 

RAID10 with hotspare would afford some protection with speed but googing I came across this. Which then puts me off RAID10

 

MPECS Inc. Blog: HP P420i Smart Array: Adding 4 New Disks to a RAID 10 Array

I suggest dropping RAID 10 for RAID 6. We lost a server that had a single disk failure, we hot swapped the dead drive, and about two minutes later its pair died. Server Dead.

 

We would have been okay if we had a RAID 6 array.

 

Naturally everything will be backed up to a NAS drive.

 

I'm coming to the conclusion that no combination will be perfect for our needs and we will just need to get by with whatever I finally decide. All methods of raid provide some form of protection and if a drive fails it shall be replaced next day and then restored from backups.

 

Haha as I type this I just checked one of our servers thats running a RAID1 array and its got a failure predicted. xD Better get that changed pronto!

[Bobby_Tables]

...

RAID10 with hotspare would afford some protection with speed but googing I came across this. Which then puts me off RAID10

 

MPECS Inc. Blog: HP P420i Smart Array: Adding 4 New Disks to a RAID 10 Array

 

...

Yes, RAID 6 offers you slightly greater protection because it doesn't matter which two drives in the array fail, whereas with RAID10 the two failed drives would have to be from different mirrors. But in my view, that's not a reason to completely discount RAID10. It's a slight trade off of some redundancy for potentially better performance. Depending on the scenario that might be what is needed. I would suggest that with RAID10 the array repair time would be much quicker than RAID5/6, and the hot spare means the repair would start immediately. You will always hear horror stories but think about it objectively - the chances of a double failure scenario are really quite low.

[Gaz]

@Bobby_Tables I was sorta Joking about RAID10, its still a consideration, with the amount of disks I've got the I could RAID10 with a hotspare, but my other choice is my original idea of 2 arrays, RAID1 and RAID5, this would have 1 disk failure per array and also separate each of the arrays should one catastrophically fail, so I'd either lose the OS or the DATA but not both (hopefully) I could then buy another HDD down the line for a hotspare. I'm not even sure if the controller supports multiple arrays yet.

[tmcd35]

I'd come from the angle of using 600Gb SAS disks that cost around £200-£250 each. Using RAID-1 for OS is a waste. That's 1.2Tb (600Gb formatted) just for an OS that'll take up less than 60Gb, at a cost of £500. It's the same argument when looking at RAID-10, RAID-6 or Hot Spares. As you pointed out yourself the small user base doesn't warrant the IOPS boost. IMHO, Both the number (and capacity) of drives and number of users don't warrant anything more than a straight RAID-5 array. RAID-5 and a hot spare might make some sense but I almost feel given the low number of drives and their relative high capacity/expense it's a waste of £250/600Gb having it sit there doing nothing. On the other hand it could be argued £250 is good value on a £2000 (guessing) server for a bit of piece of mind. Is five drives enough for a RAID-6 array? I thought you needed more because of the double redundancy.

[Gaz]

I Guess it may seem like a waste as our current servers only use about 80GB for the OS/Software, however It could be argued a RAID5 array of 5 disks is also a waste at 2.4TB as most of that will never be used. Then there is the long rebuild times on an array with that many disks and slower performance all round, remember an array is most vulnerable during the rebuild process. RAID6 needs a minimum of 4 disks. Adaptec - Which RAID Level is Right for Me?

RAID6 is slower than RAID5 for reads and probably even worse in a degraded state, but the trade of is the redundancy. There is no best solution, thats the conclusion I've come to. If it all goes to pot then I'll just have to flatten and restore from backups.

[Bobby_Tables]

...Is five drives enough for a RAID-6 array? I thought you needed more because of the double redundancy.

Minimum of four drives for RAID6.

 

I disagree with you on your other points, but hey-ho!

[MattRVBC]

I mainly use a RAID1 mirror set for the OS (all be in not 600GB although that would leave lots and lots of disk space for heavy Microsoft patches) and RAID5 for data storage etc. allowing me to configure a hot-spare to speed up the disk replacement process. The advantage of RAID5 is that you have the distributed parity unlike RAID4 which is one of the main reasons why I use 5. Not really used RAID6 although the same concept applies but will allow for 2 disk failures from what I remember.

[DanTheLacky]

Hehe, RAID10 was something I dismissed because its impossible with 5 drives, but you are right I could do it with 4 drives plus a hot spare! 1.2TB will be plenty of space I'm sure. That would give me good speeds in both reading and writing plus data protection.

The Drives are HP drives with no tell tale signs of manufacturer.

 

Probably your best option in my opinion, RAID 5 is really frowned upon and out of date in the professional world, I know alot of the time though in schools we dont have the budget to do the most current and secure things to do.

[pcstru]

I mainly use a RAID1 mirror set for the OS (all be in not 600GB although that would leave lots and lots of disk space for heavy Microsoft patches) and RAID5 for data storage etc. allowing me to configure a hot-spare to speed up the disk replacement process.

RAID 5 is ill advised, RAID 5 with a hot spare is even worse. The problem with hot spares is that a rebuild of the array WILL fail if any element of the hardware or software is not working properly and that includes Unrecoverable Read Errors on existing drives. When a drive fails in a RAID 5, the first thing you do is back up the data. Only then should you introduce a new drive and initiate a rebuild. Rebuild failures are surprisingly common and at that point you are stuffed.

 

Good article with some numbers : Why RAID 5 stops working in 2009 | ZDNet

 

(ETA - I think his numbers are suspect (too high) but the problem is real).

Edited September 24, 2014 by pcstru