Re: Need urgent help in fixing raid5 array

[Mike Myers <mikesm559@yahoo.com>]

Neil, the devices have moved around a bit since my post on the --examine results of each drive as an attempt to bypass a possibly bad sata backplane and known bad sata controller.  So I have to recheck the array positions of the drives.  A few questions about how md acts:

1) When I grow an array, do the existing members of the array maintain their slot positions?  So if I had 4 drives sda1 sdb1 sdc1 and sdd1 as part of a RAID5 array, and then add sde1, would sde1 take slot 4 or the array, or do the array slots get reset during a reshape operation?  

The reason I ask is that from the smartctl -a data for each drive, I can get the total powered on hours for the drive.  If the drive has a lot of hours on it, it will have been added earlier than another drive, and if the array positiosn are constant, then I can kind of reconstruct the array order on an array that's been built incrementally by look at drive power on times.

2) If a drive goes bad an is replaced by a spare, does the spare take the orginal array slot of the faulty drive?

3) It appears the slot number -1 is the member number?  That is, if I do an examine on /dev/sdc1, it tells me it's got slot 5 of md1.  But when I do an assemble operation with the --verbose flag, it says /dev/sdc1 "as added as 4".  The reason I ask if that's true, what would a slot number 0 mean in terms of what --assemble is supposed to add it as?  When I do the assemble, it's added as 0, which I don't understand if the slot number is supposed to be one higher?

4) /dev/sdf1 (the new device name) thinks it's part of md2 (when I do an examine), but can't be, because md2 is all seagate and already has 7 members in it (the right number of drives).  So it must be part of md1, which is missing a member.  When I first tried to reassemble md1, it said it only found 5 good drives, and couldn't start.  now it says it only finds 4 good drives.  So I assume sdf1 is one of the 5 good ones but got a weird superblock written to it.  Other than the drive hours trick I thought of earlier, is there any way to determine what it's slot number should have been since I am missing slots 1, 2, and 3, and have 3 candidates for slot 0?

Lasttly, it REALLY would make life a LOT easier if the devices wouldn't change evrytime they were plugged into a different controller slot, or that controller slots wouldn't change based on boot order etc...  It is a pain in rear when you have a hardware outage or a disk that isn't detected properly on boot and then hot added to have it's /dev/sdx1 label change.  I know it's not md's fault the way this works, but in a hot swap world it makes it very hard to document drive configurations and map devices under linux to physical drives.

Thanks a lot Neil.

Mike






----- Original Message ----
From: NeilBrown <neilb@suse.de>
To: Mike Myers <mikesm559@yahoo.com>
Cc: Justin Piszcz <jpiszcz@lucidpixels.com>; linux-raid@vger.kernel.org; john lists <john4lists@gmail.com>
Sent: Monday, January 5, 2009 8:00:43 PM
Subject: Re: Need urgent help in fixing raid5 array

On Tue, January 6, 2009 1:46 pm, Mike Myers wrote:
> BTW, don't I need to use the --assume-clean option in the create operation
> to have this work right?

No.  When you create a degraded raid5, it is always assumed to be clean,
because it doesn't make any sense for it to be dirty.
However it wouldn't hurt to use --assume-clean, but it won't make any
difference.

NeilBrown

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