Ubuntu - OK to raid0 and raid1 on same two drives?

Question

I'm building a household Ubuntu 14.04 server and just installed two 3TB drives for the files and media. The box (HP DC7900 SFF) uses Intel Matrix RAID which, according to wikipedia is neither hardware nor software RAID but firmware RAID. I'm thinking of installing two partitions on each drive (four total) and mirroring (RAID1, ~2.5TB) the files with one partition from each drive and striping (RAID0, ~1TB) the media with the other two partitions, for a total of ~3.5TB.

The other option is to mirror/RAID1 both drives, so a lesser total of ~3TB.

If we're streaming a movie, listening to music, and reading/writing files, would the first RAID0 & RAID1 configuration give us the best performance or would just going straight RAID1 be faster? I understand if each partition was on its own disk it would be faster, but don't know if there are performance issues with the configuration I'm proposing. I like the extra 0.5TB we'd get from the first configuration but would be happy to give it up for better performance. I need the redundancy, so going all RAID0 isn't an option.

Answer 1

Well, using both RAID1 and RAID0 (RAID10) would be faster then just RAID1, but that requires a minimum of 4 drives. Unless you wish to purchase more drives. Stick with RAID1 which gives mirroring but no performance boost.

Answer 2

Although what you proposed would work, it doesnt make a lot of sense.

First off, I will propose a better solution: Use three 3TB drives and do a RAID5. RAID5 gives you the redundancy and speed. It allows for the loss of any single drive, while still functioning during its degraded state. RAID5 also gives a performance boost, as data is written sequentially to each drive. This gives you 6TB of faster and redundant data.

Now, lets talk about your solution. Since you didnt mention any hardware, I can only assume you are using software RAID. Software RAIDs do provide redundancy, however the performance boost they give is minimal, as the computer is performing the RAID calculations. On top of that, you are creating two volumes across two disks. Again, your method would work, but if you decided to watch a movie and use files at the same time, you would be thrashing your disks and lose a lot of performance.

Answer 3

Since you are talking about RAID on partitions, I'm assuming you must be talking about software RAID (mdadm on Ubuntu).

Typically media files tends to be large and don't lend themselves well to caching at OS level. So to be able to use your media, you need to be able to read as much data without undue head movement. But you would be reading from both disks. (RAID0).

Should you now start reading or writing to the data area, your disk heads will be moving between the two partitions of one disk (on which the media file is). You would have gotten a speed benefit most likely from you mirroring for data, but if the media file portion you are reading happens to be on that disk, you disk head will have moved away from the data partition and needs to move back.

So your disks are going to be working hard to keep up if you use both partitions simultaneously.

Big disk caches may help, but media files tend to be big. So it will be a race to read media files vs read/writing data while trying to keep two disparate actions happy.

Since you require 1TB unprotected, you may as well rather use a fourth 1TB disk if possible.

The other option is to mirror/RAID1 both drives, so a lesser total of ~3TB.

This would probably be a better option. If you use two directories, say /disk2/data and /disk2/media, you would share the free space. (Typically people tend to overspec.)

Additionally you could put your RAID1 disk in LVM and get the benefit of future painless expansion. It will also allow for flexible volume growth/shrinkage and data migration while maintaining two separate mount points for data and media if you really need to, all with one RAID1-volume.