RAID questions/advise

Hi guys,

I've had a couple of RAID boxes ticking away in the corner for years now without a problem. But now our needs have expanded, and I'm looking to build replacements. Big replacements. And I consider myself to be anything but an expert in the field, especially where mdadm is concerned. So I have a few questions to ask in hopes that someone out there can help me out.

How large of an array can mdadm handle?

If I use my hardware RAID cards in JBOD mode, how does the kernel handle naming drives when there's more than 26 drives on the system? (i.e. what does it do when it reaches /dev/sdz and there are drives left to be named?)

From what I hear, ext3 can handle filesystems up to 32TB in size, but has anyone actually done this? Can anyone attest to how well it works? Or is there another filesystem type that's better suited to large (12TB - 32TB) filesystems?

Finally, according to the mdadm FAQ, when a drive goes down:
19. What should I do if a disk fails?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Replace it as soon as possible:
mdadm --remove /dev/md0 /dev/sda1
halt
<replace disk and start the machine>
mdadm --add /dev/md0 /dev/sda1

Since my OS drive will not be a part of the RAID (it'll have a mirrored RAID of it's own), I presume that the halt command won't be necessary. I assume that it would be perfectly reasonable of me to remove the drive and replace it while the system is running? I.e. mdadm can handle running in degraded mode for the duration of the replacement/rebuild process? (This is a deal-breaker question -- if mdadm can't, then I'll have to pursue other measures. Hotswap drives will be up and running at all times though, so I presume I can configure mdadm to make use of them immediately upon detecting a drive failure.)

My other option is to get a SAN/NAS of some type, but building machines like this has proven to be very effective and cheap compared to SAN/NAS'es, at least in the past. I haven't checked recently though, but any advice is welcome.

Pondering my options,

Michael


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