Raid and Online Capacity Expansion

[Jesse]

Hmmmm,

Good points.

Does this:

However, if you do use software RAID, keep in mind that the operating system _is_ the software that does the RAID. You can't load the operating system onto a RAID set that the operating system creates/manages, and shouldn't do it even if you could - how would you recover from a problem?

appy to intel application accelerator raid? If the only thing on the array is the os and apps then the only downside would be that I would have to re-construct the array if I did a clean install--right? If I have to do a clean install then I'm thinking rebuilding the array would be no big deal--no?

I would really like to eliminate downtime by having the os on a raid array (on a raid 1 array if putting it on the raid 5 array is a bad idea), but if I would have to get a second hware raid card to do it then that is just more trouble than I am willing to go to.

TIA

Jesse

[jptaz]

Wow!

I have never heard not to put the os on a RAID 5 array I personally do and I believe others do. In fact almost every server I have ever seen has this configuration.

What possible reason would you have for not simply partitioning 2 volumes on a RAID 5 drive?

I have a 30GB C: drive and 1.3TB Media drive.

I considered using a the IDE Raid controller with 2 old drives but could not get the onboard raid to work with the RAID card.

John

[src666]

The main reason you don't put the OS on a RAID 5 array is performance. While RAID 5 is very fast at reads, it is pretty slow for writes (compared to a single disk or mirrored pair). If you should lose a drive, performance absolutely tanks across the board, making recovery operations painful at best.

In most cases, the OS partition also includes the swap file, which sees a very large amount of write activity. You can correct for this by moving the swap file to a seperate disk, but very few configurations would benefit from this. Who wants to dedicate a hard drive to the swap file?

After doing a bit of research, I am seeing a lot more tollerance for putting the OS on RAID 5. It used to be a pretty hard-and-fast rule. Perhaps today's controllers make it more reasonable. But in practice (20+ years of experience), I have never seen a professional server set up with the OS on a RAID 5 array.

The configuration I learned, and see in use regularly, is the OS on RAID 1, the data on RAID 5. Another option is to put the OS on a standalone drive, and make regular images of that drive. But I, personally, would never put the OS on RAID 5.

Honestly, I've been reconsidering my use of RAID 5 in my media server. RAID 5 performance degrades rapidly in a write-heavy environment. With 5 tuners already and more to come later, not to mention HD down the road, I'm concerned about the possibility of getting boxed in by write performance.

I'm considering going back to just using the individual drives, instead of any type of RAID array. I lose the fault tollerance, but a single drive failure only costs me the data on that drive. A dual drive failure costs the data on 2 drives. With RAID 5, a dual drive failure costs the entire array, and I just don't have a backup solution for 1+ TB of data.

[stanger89]

The other thing is that while RAID is fast for reads, it's only large sequential reads (streaming). It's pretty bad for small random reads/writes, which is what the OS does a lot of.

[jptaz]

Well so far I have not seen any problems with writing 2 HD Streams and 5 SD streams Simultaneously. Also I am watching 2 HD shows not being recorded and 2 SD not being recorded.

The company I work for has 10+ servers all running on a RAID 5 drives. we have 10+ customers who run our software on servers purely RAID 5. Maybe 5 to 10 years ago RAID 5 controllers couldn't handle the load, but a decent caching RAID 5 controller can keep up just fine.

The theory is sound behind a seperate OS Disk that is RAID 1, but the practical loads you think the OS is putting on the disk is far from the point of causing problems. Now if you have a bare bones just meeting RAID 5 configuration of 3 disks you will have problems, but with at least 5 disks there is no issue. Also seperate disks for swap files have been common place in the unix/linux world and is actually a wise choice since it prevents fragmentation of the swap file.

But as far as single drives as opposed to a RAID 5 I think I will take the risk of 2 drive failures since those odds are low and they really have to happen before the rebuild to my hot spare and if that big a failure occurs I am looking at some down time to figure out what could cause such a failure.

John