Take the 2-minute tour ×
Ask Ubuntu is a question and answer site for Ubuntu users and developers. It's 100% free, no registration required.

I have a new machine with one 128GB SSD and two 1TB HDD. On the SSD is the OS and my initial thought was to put the two HDD in RAID 1 for user data.

After some more thought I came up with two other setups and now I'm in doubt :) Can someone advise what would be the best setup?

1: single SSD and HDD in RAID 1 (original thought)

2: Create 2 partitions on the HDD (128GB and 872GB). Put the two 872GB in RAID 1 and create another RAID 1 with the SSD and one 128GB HDD partition.

3: Create 2 partitions on the HDD (750/250), put the 705GB in RAID 1 and use the 2 250GB as backup and make automatic snapshots of the SSD to (one of) these partitions.

I think the 2 main questions are:

Is it advisable to create a raid array with only part of a drive and actively use the other part of that drive or should you always use the full disk?

Is it advisable to create a raid 1 array with a SSD and HDD or will that blow the whole speed advantage of the SSD?

share|improve this question
add comment

3 Answers

up vote 2 down vote accepted

Is it advisable to create a raid array with only part of a drive and actively use the other part of that drive or should you always use the full disk?

The advice is always to use as much similar devices as you can. I'm not sure how much of that advice is superstition though, but lets look at this first:

Is it advisable to create a raid 1 array with a SSD and HDD or will that blow the whole speed advantage of the SSD?

No. Don't do that, you will blow the speed. Apart from the fact that they are different devices etc, the one thing I am quite sure of is that your speeds will drop, as you are going to be working at the speed of the slowest disk -> your ssd-hdd combo will be way slower then your SSD :

You should NOT do that to your SSD. Your first option is the best one. Just give yourself safe userspace, and speedy system disk. Backup that disk. (remember, RAID is not a backup strategy)

share|improve this answer
    
The advice is always to use as much similar devices as you can. -- The more the drives are identical and used identically, it tends to fail on identical times too. My suggestion is to find similar but not identical drives of the same size, yet different manufacturing date or brand. –  gertvdijk Nov 15 '12 at 10:06
    
I agree completely. –  Nanne Nov 15 '12 at 10:08
add comment

First question

For performance reasons, don't do this. It will degrade performance when I/O is performed on the other partition. However, if performance is not of your concern, but redundancy is, you can consider this.

Second question

No, don't do this. The chain is as strong as the weakest link here. Your array will perform more like a HDD and you'll waste an expensive SSD.

Instead, use...

dm-cache or flashcache (quite advanced and experimental). It enables you to use your SSD as cache (read and write) for your slower hard drives. Huge performance gain and tweakable to find balance in durability and performance.

See also: How do I install and use flashcache/bcache to cache HDD to SSD?

share|improve this answer
    
I'm not familiar with those cache solutions, but wouldn't you (just like swap) want to minimize those kind of things on your SSD, as they would wear out your device rather sooner then you'd want? –  Nanne Nov 15 '12 at 10:05
    
@Nanne No, you should just use your SSD for its speed. If you really worry about wear on your drive then disable write caching and use read caching only. I also suggest you to read more about write endurance and why it will probably not affect you here. –  gertvdijk Nov 15 '12 at 10:11
    
I agree you should use the SSD for speed, that's obvious. I'm just not knowledgable about the inner workings of the SSD enough to know if the worries people have are indeed valid: are they 'bs' for the whole "no swap!" thing as well? About read caching: that wouldn't really matter for the device used for storing the cache, would it? –  Nanne Nov 15 '12 at 10:19
    
@Nanne Read operations are very light for an SSD. You should indeed avoid unnecessary heavy write operations, but stay realistic. In this Intel PDF document in section 2.3 mentions 15TB as total write endurance for the 120GB drive. You may decide to turn off the flashcache for a while when transferring the bulk of stuff when starting. –  gertvdijk Nov 15 '12 at 10:28
    
What I meant was that using the disk as read-cache wouldn't neccessairily mean you would write less, would it? The cached data still needs to be written to the SSD, so the fact that you are caching only "read" stuff from the HDD, doesn't really matter, does it? (to be clear: not saying you're not right, just trying to wrap my head around it :) ) –  Nanne Nov 15 '12 at 10:33
show 1 more comment

The first option is not completely stupid. "Hybrid RAID1" does work if configured properly. I have ran this setup myself until my SSD started acting weird and moved out of the box.

See these articles for guidance:

http://tansi.info/hybrid/

http://www.vinsec.net/2012/05/hybrid-hdd-ssd-raid1.html

http://superuser.com/questions/293144/combining-ssd-and-hard-disk-in-software-raid1

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.