Raid, LVM and stuff ...

Stitch · 2009-06-11 03:58:18

Hi everyone,
this is my first post. Looking forward to enjoy all this.

I would like some comments on possible set up options please.

In short, I have raid5 setup with 3 disks at the moment with LVM on the top.
I am planning to add another disk (just one at the moment).
The total number of internal bays is 6 so I'll add one disk now and two later.

Question. With efficiency and speed in mind what would be the best option out of two below?

1. Add this one disk to the existing md0 so I have 4disks -> md0 -> LVM with the future setup as 6 disks -> md0 -> LVM

2. Fill the remaining 3 bays at once with 3 additional disks and create second md device so I have 3 disks -> md0 \
-> LVM
3 disks -> md1 /

I realise that with the second option I lose twice as much drive space to parity but I am interested more in speed.
With the second options I can get LVM to stripe across two md devices. Would it be speedier then the first option?

Thanks

nogoma · 2009-06-11 12:11:04

For your last option there, you probably wanted RAID 50 (a nested RAID 5+0), as described here. I don't believe LVM on top of two md devices will do the sort of striping you want; that's not really its job. So:
3 drives -> md0 RAID 5 \
+-> md2 RAID 0 -> LVM
3 drives -> md1 RAID 5/

Stitch · 2009-06-11 23:13:30

Thanks for your reply.

Lets assume that LVM does not do any striping. Do you still think that option 2 is a better idea?
I suppose I am asking the best way (in terms of performance) to feed disks to LVM, is it one dm with 6 disks or two dm's 3 disks each?

Zariel · 2009-06-11 23:21:10

Why would you need double striping? Whats more important, speed or redundancy? If your looking for speed then I would think just adding the extra drives to md0 in raid 5 or create a second raid 5 then use lvm to access it as one device if you are that way inclined.

nogoma · 2009-06-12 14:01:28

Well, as Zariel says, it comes down to which is more important, speed or reliability (and space, for that matter). Since I guess it seems like you care a bit about data reliability or you wouldn't have a RAID 5 to begin with, it sounds like RAID 50 is a good idea for you, in that you can tolerate a single drive failure in each RAID 5 array while still getting a performance benefit of the RAID 0 over the two RAID 5 devices. The wikipage I linked in my first reply does a pretty good job of describing the various nested scenarios. Personally, I have a RAID 5 array that I watch very carefully holding my code, configs, etc. on a central file server. My various machines around the house run off SSDs or RAID 0 for speed; these only ever have my system files on them (i.e. I can lose that data and I don't care; I just restore the configs/package lists from my central server and reinstall Arch). So, you could consider doing something similar - stuff like music, src files, etc. can go on your existing RAID 5 for safety, and you can simply created a separate RAID 0 for stuff you want to be fast (I then have 2 separate LVM volume groups, vg_sys on the RAID 0 and vg_data on the RAID 5). But, to bring it back to your actual question: I believe RAID 5+0 will be a good choice performance wise, assuming you still want *some* data reliability.

Stitch · 2009-06-14 15:27:26

Thank you all for the input.
I'll try 2 by 3
This is not a production machine and there is nothing on the volumes that I can't lose.
This is more to beef up my knowledge cabinet of how I can mix LVM and RAID combinations

Arch Linux

#1 2009-06-11 03:58:18

Raid, LVM and stuff ...

#2 2009-06-11 12:11:04

Re: Raid, LVM and stuff ...

#3 2009-06-11 23:13:30

Re: Raid, LVM and stuff ...

#4 2009-06-11 23:21:10

Re: Raid, LVM and stuff ...

#5 2009-06-12 14:01:28

Re: Raid, LVM and stuff ...

#6 2009-06-14 15:27:26

Re: Raid, LVM and stuff ...

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