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-   -   Should I use LVM on top of 3ware Hardware RAID? (

bfoddy 09-22-2007 02:05 PM

Should I use LVM on top of 3ware Hardware RAID?
I have new parts, a 3ware 9650se-8, and 2x1000GB sataII drives. Over time I expect to add more drives. Starting with RAID0 striped for 2tb, then probably add 2 more drives and move to RAID5; beyond that who knows. The 3ware software seems to have good support for rebuilding raid units on-the-fly, tho its very slow (~40 hours to migrate from a 1tb single to a 2 drive raid0). Since I'm still waiting for this first test to complete, I'll take a leap-of-faith that I can then expand the fs when its done.

Primary usage will be mythtv video storage, plus various backup spaces and other misc space hog needs. I'd like the flexibility for multiple filesystems if I need. Most likely use XFS for the big video filesystem. The rest of the system is a pretty standard Mandriva Opteron server starting with 2GB RAM.

My question is, should I layer LVM on top of the raid? And if so, more specifically:

1a. If I understand the LVM model, this raid device (/dev/sdb) would be the physical volume (PV). Most LVM guides I see talk about adding additional PVs to a LV to expand a filesystem. Haven't found one talking about Expanding a PV, which is what would happen if I add more drives to the same 3ware unit, which defaults to 2TB in size. Can / should I do this?

.. OR ..

1b. The other approach I could see is limit the 3ware to say 1TB (or less) unit sizes, now each drive I add would show up to the OS as a new /dev/sdx device, allowing me to make each a PV, and therefore following most LVM howto guides. Perhaps a bit more flexibility? If this theory is good, what are the drawbacks/pitfalls to doing this?

2. I seems like most suggest I not partition the 3ware device with fdisk, but give LVM the whole device? Correct?

I guess I struggle with questions 1a/1b the most. LVM's advantage seems based on the assumption you will add physical drives, but less instruction on the advantages on top of an expandable hardware raid. Any thoughts or good detailed references people know on this subject?

Many thanks,

uselpa 09-22-2007 03:18 PM

1a/1b: You don't expand a PV, you just add a new PV to a Volume Group. A PV is a partition of a hard disk, be it an existing one or a new one.

2: my advice is to always partition a hard disk, even if you just create one partition which covers the whole disk; too many programs rely on the fact that there is a partition table.

This explains it quite well.

bfoddy 09-22-2007 04:06 PM

This article helps... I'm starting to form a better picture of the layout now.

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