RAID 0 or 5?

[Bleek]

I'm purchasing or building a mission critical server that will be servicing moderate I/O requests, nothing to strenious. I wanted to incorporate fault tolerance but wasn't sure which would be my best option.

RAID 5 is only software right, or can you buy RAID 5 cards? I understand the overheads wouldn't be that bad on a modern system anyway. Theres definately a stronger reason to run RAID 5 because of its read speed and theres less disc wastage.

Does anyone run RAID 5, prefferably in a critical position, and what is the performance and reliability like, any problems?

[nichomach]

Originally Posted by Bleek

I'm purchasing or building a mission critical server that will be servicing moderate I/O requests, nothing to strenious. I wanted to incorporate fault tolerance but wasn't sure which would be my best option.

RAID 5 is only software right, or can you buy RAID 5 cards? I understand the overheads wouldn't be that bad on a modern system anyway. Theres definately a stronger reason to run RAID 5 because of its read speed and theres less disc wastage.

Does anyone run RAID 5, prefferably in a critical position, and what is the performance and reliability like, any problems?

RAID5 can be done in software OR hardware. I assume you would be using SCSI drives, right? I run two servers with RAID5 arrays in and reliability has been excellent - I HAVE had a drive fail, hot-swapped it, and rebuilt with no interruption of service. Both the machines are Dell Poweredges. The key thing is to actually have the monitoring software properly installed and working with notifications working; you don't want to lose 1 drive, not get notified and then lose another. You won't have seen a service interruption by losing one drive, so if a second goes...bye bye partition, bye bye data, hello rebuild .

Performance has also been good. If it's REALLY that critical, you might want to look at mirrored RAID5 arrays. Highest reliability is provided by two separate controllers running two separate arrays, but it doubles the cost of your storage.

[Bleek]

Actually I'll be looking at Seagate 10k SATA drives...

[Stoo]

raid 0 is striping only so it has no reliability advantage at all, in fact it's a disadvange, as once one of a r0 set goes, that's pretty much buggered the lot.

raid 5 is the best bet, and as you're wanting SATA, you'll be wanting something like 3ware hardware raid cards - http://www.3ware.com/products/serial_ata.asp

[Butuz]

Hi m8

I assume you mean Raid 1 (mirroring) as opposed to Raid 0? Raid 0 = striping without parity, which in real terms means if you lose one drive, you lose the data of all drives in the array, which is not what you want in a mision critical server.

Raid 5 (or Raid 50, which is 2 raid 5 arrays in raid 0 for want of a better description) is the only way to go for mission critical, and a hardware raid 5 controller is the only way to go for raid 5 - as raid 5 needs to calcualate using XOR calculations which are pretty damn procesor intensive, thus software raid 5 uses considerably more CPU power, and is slower than hardware raid 5 - the speed difference is even more exagerated because hardware raid 5 controllers generally come with 128MB odd of cache which increases average read speed considerably.

The capacity of the array will be N drive's - 1 so if you have 6 x 36GB drives then your capacity will be 180GB (216GB - 36GB) but if you have 3 x 72GB drives then your capacity would be only 144GB (216GB - 72GB) so your better of using higher numbers of smaller drives as you "lose" less data to parity, and you enhd up with a better performing array.

You have to have 3 or more drives, and the total number of drives is limited to the number of ports/connnetors your raid card can support. As your talking Serial ATA raid 5 (good choice with 10k drives, but make sure the drives have 5 year warranty a'la WD Raptor) you can get Adaptec 4 port cards, or 3Ware Escalade 4 / 8 / 12 port cards.

4 port cards with raid 5 and 64MB cache start at £200 -£250 and go skyward for 8 or 12. The other thing worth considering is that if your using raid 5 and SATA drives, there are a few cases around that support multiple hot swap SATA drives. Ive seen supermicro ones that inherantly support 4 or 5 drives, and i think if you muck about with supermicro's additional 5 bay SATA backplanes you could fit quite a few drives in a full tower. (theres little point in going raid 5 without hot swappability IMHO, the chances of you b0rking the server as you neel under your desk in the dark with a screwdriver are far higher than just uncliping a drive and then cliping another in).

The other thing with raid 5 - you can configure a hot spare drive as well, which is that out of your total number of drives, you configure one just to wait alone and store no data - then if a drive in the array fails, the software automatically shuts down the broken drive and brings the spare drive online, then your free to unplug the dead drive and replace at your leasure. Raid 5 with hot spare means that 2 drives can fail in the array without you losing any data. Of course, you lose another drives worth of data (6 x 36GB - 2 x 36GB) and thus can only be done with 4 drives or more.

As your server is mission crytical - go for dual hot swappable redundant power supply's in your case too - so if one fails you can just pull it out and push another in.

I use Dell Poweredge 2600 servers with dual 700watt hotswap PSU's and 6 x 36GB hotswap SCSI drives. They are superb fast servers and come with nice on site warranty, so take them into consideration before building your own.

Hope this Helps

Butuz

[Steve]

Butuz has it summed up there.

RAID 5 takes a lot of crunching to work out the data parity - therefore a dedicated board and plenty of onboard write cache are needed otherwise write performance takes a huge hit.

[Bleek]

Originally Posted by Butuz

The other thing with raid 5 - you can configure a hot spare drive as well, which is that out of your total number of drives, you configure one just to wait alone and store no data - then if a drive in the array fails, the software automatically shuts down the broken drive and brings the spare drive online, then your free to unplug the dead drive and replace at your leasure. Raid 5 with hot spare means that 2 drives can fail in the array without you losing any data. Of course, you lose another drives worth of data (6 x 36GB - 2 x 36GB) and thus can only be done with 4 drives or more.

I was definately going hotswap with this anyway, make my life easier.

A question, these hot spares are a feature on all hardware RAID controllers right? 6x32GB, hot swap on an 8 port hw controller sounds like a good plan to me.

So in theory there would be no need for tape backup with a solid UPS stopping an suges and brown outs? Or would it be easy enough to impliment a backup of the RAID array anyway, I take it its just a volume spanned accross several drives so the backup software (Veritas Exec here) would just work as usual.

[nichomach]

Originally Posted by Bleek

I was definately going hotswap with this anyway, make my life easier.

A question, these hot spares are a feature on all hardware RAID controllers right? 6x32GB, hot swap on an 8 port hw controller sounds like a good plan to me.

So in theory there would be no need for tape backup with a solid UPS stopping an suges and brown outs? Or would it be easy enough to impliment a backup of the RAID array anyway, I take it its just a volume spanned accross several drives so the backup software (Veritas Exec here) would just work as usual.

NEVER skimp on backups; they are the sine qua non of any sort of business critical environment, and they're not just there for disk failures or power outages. RAID is good, UPS's are good, but they're not foolproof.

The RAID array will appear to the OS (and thus to Backup Exec) as just another hard disk. If implemented via a hardware controller, the OS will have no idea that it's anything other than a single disk. So yes, BE would work just as usual. In fact, it works as usual even if the array were managed via software.

[Moby-Dick]

RAID is no substitute for backups - always back everything up , and then TEST THE BACKUPS. You have no idea how daft you'll look saying "its ok, we have a tape backup" , then realising that the tape wasn't formatted correctly




I'm not speaking from experience here.....honest

[nichomach]

Originally Posted by Moby-Dick

I'm not speaking from experience here.....honest

Ah yes, we both appear to have been to the same place and come back with the same T-Shirt...

[Bleek]

Haha, fair enougth. Theres also the benefit of having tape backup which can be stored away from the machine in the event of fire or even natural disasters.

How easy is it to configure a RAID 5 array?

[Moby-Dick]

very easy - you build the array from the built in control program ( access at boot like a bios controll program )


most of the cards shoudl also come with an appropriate windows controller when you can define , build , and repair arrays.

[Moby-Dick]

Originally Posted by nichomach

Ah yes, we both appear to have been to the same place and come back with the same T-Shirt...

Indeed , which is why I now run an online backup service for people who realise the importance of backups , but dont like having to remeber tapes

[Bleek]

What is required for hotswap PSU's, how do the two PSU's interface with the motherboard? Is there a controller before the motherboard that branchs the single ATX connector into two etc?

[dgr]

my old skool had a raid 5 array without backups. the motherboard went taking the raid5 controller with it.

goodbye array . goodbye over 1000 peopes work...

IT departmnet was less than popular.

oh as a rule of thumb, and raid system with an even number of discs = danger!

[Bleek]

Originally Posted by dgr

oh as a rule of thumb, and raid system with an even number of discs = danger!

Why?

I'm considering 4 SATA drives, 3 running RAID 5 and a hot spare.