RE: Spares and partitioning huge disks

My warning about user error was not targeted at you!  :)
Sorry if it seemed so.

And the order does not matter!

A:
Remove the failed disk.
Fail the spare
System is degraded.
Add the failed/repaired disk.
Rebuild starts.

B:
Remove the failed disk.
Add the failed/repaired disk.
Fail the spare
System is degraded.
Rebuild starts.

Both A and B above require the array to go degraded until the repaired disk
is rebuilt.  But with A, the longer you delay adding the repaired disk, the
longer you are degraded.  In my case, that would be less than 1 minute.  I
do fail the spare last, but not really much of an issue.  No toast anyway!

It would be cool if the rebuild to the repaired disk could be done before
the spare was failed or removed.  Then the array would not be degraded at
all.

If I ever re-build my system, or build a new system, I hope to use RAID6.

The Seagate test is on-line.  Before I started using the Seagate tool, I
used dd.

My disks claim to be able to re-locate bad blocks on read error.  But I am
not sure if this is correctable errors or not.  If not correctable errors
are re-located, what data does the drive return?  Since I don't know, I
don't use this option.  I did use this option for awhile, but after
re-reading about it, I got concerned and turned it off.

This is from the readme file:
Automatic Read Reallocation Enable (ARRE)
        -Marreon/off  enable/disable ARRE bit
           On, drive automatically relocates bad blocks detected
           during read operations.  Off, drive creates Check condition
           status with sense key of Medium Error if bad blocks are
           detected during read operations.

Guy

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of maarten
Sent: Saturday, January 08, 2005 12:17 PM
To: linux-raid@vger.kernel.org
Subject: Re: Spares and partitioning huge disks

On Saturday 08 January 2005 17:32, Guy wrote:

I don't recall having 2 disks with read errors at the same time.  But
others on this list have.  Correctable read errors is my most common
problem with my 14 disk array.  I think this partitioning approach will
help.  But as you say, it is more complicated, which adds some risk, I
believe.  But you can compute the level of reduced risk, but you can't
compute the level of increased risk.

true.  Especially since LVM is completely new to me.

Some added risk:
	More complicated setup, increases user errors.

I have confidence in myself (knock, knock).  I triplecheck every action I do

with the output of 'cat /proc/mdadm' before hitting [enter] so as to not
make 
thinking errors like using hdf5 instead of hde6, and similar mistakes.
I'm paranoid by nature, so that helps, too ;-)

	Example:  Maarten plans to have 2 spare partitions on an extra disk.
Once he corrects the read error on the failed partition, he needs to

remove

the failed partition, fail the spare and add the original partition back

to

the correct array.  He has a 6 times increased risk of choosing the wrong

You must mean in the other order. If I fail the spare first, I'm toast! ;-)

partition to fail or remove.  Is that 36 time increased risk of user

error?

Of course, the level of error may be negligible, depending on who the user
is.  But it is still an increase of risk.

First of all you need to make everything as uniform as possible, meaning all

disks belonging to array md3 are numbered hdX6, all of md4 are hdX7, etc.
I suppose this goes without saying for most people here, but it helps a LOT.

than 6.  Is there a sweet spot?

Heh. Somewhere between 1 and 36 I'd bet. :)

Also, Neil has an item on his wish list to handle bad blocks.  Once this

is

built into md, the 6 partition idea is useless.

I know but I'm not going to wait for that.  For now I have limited options.
Mine has not only the benefits outlined, but also the benefit of being able
to 
use an older disk as a spare. I guess having this with a spare beats having 
one huge array without a spare.  Or else I'd need to buy yet another 250GB 
drive, and they're not really 'dirt cheap' if you know what I mean.  

I test my disks every night with a tool from Seagate.  I don't think I

have

had a bad block since I started using this tool each night.  The tool is
free, it is called "SeaTools Enterprise Edition".  I assume it only works
with Seagate disks.

That's interesting.  Is that an _online_ test, or do you stop the array
every 
night ?  The latter would seem quite error-prone by itself already, and the 
former... well I don't suppose Seagate supports linux, really.

Maarten


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