Re: mdadm on lenny64, why two spares?
On Mon, 08 Jun 2009 11:38 -0500, "Boyd Stephen Smith Jr."
<bss@iguanasuicide.net> wrote:
> In <[🔎] 1244343455.32414.1319142213@webmail.messagingengine.com>,
> whollygoat@letterboxes.org wrote:
> >Can anyone tell me why the following command
> >creates 2 spares instead of just one?
> >
> ># mdadm -C /dev/md0 -v -e1 -l5 -b internal \
> > -n3 /dev/hde1 /dev/hdg1 /dev/hdi1 \
> > -x1 /dev/hdk1 --name FileServ -a yes
>
> It's mdadm being "smart".
>
> Evidently, something about the raid5 personality
> makes it faster to sync a single disk rather than
> to sync across the whole array.[1] So, unless given
> the "-f" flag, mdadm creates all RAID 5 arrays as
> "X-1" synced disks and one spare. Once anything is
> written to the array, it will start "recovery" of
> the spare.
>
> You can either let mdadm create it it with one more
> spare than you expect, or you can force the behavior
> with "-f". The former will be faster. IIRC, this is
> all documented on in the mdadm man page. It's a long
> read, but I'm pretty sure it is in there.
[snip]
> [1] I'm not sure why this is. When doing the initial
> RAID 5 sync you should be able to spread writes across
> all the disks, and sync very quickly. However, the way
> mdadm does it, all the writes go to a single disk and
> it's sustained write speed becomes a bottleneck.
It is a long read. I've read it a few times and there
is still lots I don't understand. For example, I never
really understood the --force switch in the create
context. I think I get it now.
Nevertheless, this doesn't seem to be the behaviour I
remember from an Etch system. I have to say though
that I was plagued with h/w problems on that (now trashed)
system, so I don't know what normal behaviour looks
like. I'm now having another stab at mdadm with a
new mobo and a new DIMM.
I'm still certain of the ide expansion cards though. I've
been putting them in the machine one at a time. I was
alarmed when I saw two spares because it seems to me
that on the last system I saw one spare and all the rest
resyncing. I want to be sure of the system before I add
another ide daughter card. So, I'll give it a little
more time and see if the extra spare doesn't get
integrated into the array.
thanks,
will
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