On Mon, 08 Jun 2009 11:38 -0500, "Boyd Stephen Smith Jr." <b...@iguanasuicide.net> wrote: > In <1244343455.32414.1319142...@webmail.messagingengine.com>, > whollyg...@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 -- whollyg...@letterboxes.org -- http://www.fastmail.fm - Access all of your messages and folders wherever you are -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
