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Re: GRUB2, GPT, mdadm & LVM

From: Chris Murphy
Subject: Re: GRUB2, GPT, mdadm & LVM
Date: Fri, 19 Jul 2013 16:10:46 -0600

On Jul 19, 2013, at 12:03 PM, Sander Smeenk <address@hidden> wrote:

> Quoting Chris Murphy (address@hidden):
>>>> So the first think to check is how many physical disks are visible at
>>>> grub prompt.
>>> Only the first 8 disk (on the first controller?) show up in grub:
>>> There should be another 8 disks. 
>> Wait. Another 8? I'm counting 8 disks are found. The grub-install
>> --debug lists 11. So it seems only 3 are missing.
> No, it lists 16. Really. :-)

Oh you're right, p is the 18th letter of the alphabet. I just couldn't count 
fingers apparently.

Anyway, that seems fragile. If you were to one day get three disk that spin up 
just a bit slow, but within tolerance, before grub comes up, then what happens? 
You can't boot? Not worth it. I'd regress the setup to put /boot on a USB flash 
drive; or put /boot and / on a small SSD, and make the RAID6 LVM for /home and 
/data and whatever else you need.

I'd be thinking about how you want to be booting this system when one of 16 
disks dies, which is basically a certainty with so many disks in one array. I 
really hope you've checked the SCSI layer time out setting and the SCT ERC 
setting of all the drives or it's simply a disaster waiting to happen. 
Performance wise and size wise I'm skeptical of the setup. I'd post your use 
case on the linux md raid list, and see if there are alternative layouts that 
will better meet your requirements with the hardware you have.

Chris Murphy

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