no problem using 3 hdd with raid1 about speed diference: write speed is the same (slowest disk give the write speed of raid array) read speed is the same (each disk have a mb/s rate) but ... number of hdd heads is diferent, in other words, if you read, part 1%-20% 40%-60% 90%-100% with 3 threads, you will end faster with 3 disks than with 2 disks raid1 with many disks give you a better parallel read speed (more disks = more heads = more read threads), but write speed is "the same" as a single disk 2014-05-30 17:36 GMT-03:00 Mark Knecht <markknecht@xxxxxxxxx>: > On Fri, May 30, 2014 at 1:14 PM, Mark Knecht <markknecht@xxxxxxxxx> wrote: >> On Fri, May 30, 2014 at 12:29 PM, L.M.J <linuxmasterjedi@xxxxxxx> wrote: >>> Le Fri, 30 May 2014 12:04:07 -0700, >>> Mark Knecht <markknecht@xxxxxxxxx> a écrit : >>> >>>> In a RAID1 would a 3-drive Red >>>> RAID1 possibly be faster than the 2-drive Se RAID1 and at the same >>>> time give me more safety? >>> >>> Just a question inside the question : how do you manager a RAID1 with 3 drives ? Maybe you're talking about >>> RAID5 then ? >> >> OK, I'm no RAID expert but RAID1 is just drives in parallel right. 2 >> drives, 3 drives, 4 drives, all holding exactly the same data. In the >> case of a 3-drive RAID1 - if there is such a beast - I could safely >> lose 2 drives. You ask a reasonable question though as maybe the way >> this is actually done is 2 drives + a hot spare in the box that gets >> sync'ed if and only if one drive fails. Not sure and maybe I'm totally >> wrong about that. >> >> A 3-drive RAID5 would be 2 drives in series - in this case making 6TB >> - and then the 3rd drive being the redundancy. In the case of a >> 3-drive RAID5 I could safely lose 1 drive. >> >> In my case I don't need more than 3TB, so an option would be a 3-drive >> RAID5 made out of 2TB drives which would give me 4TB but I don't need >> the space as much as I want the redundancy and I think RAID5 is slower >> than RAID1. Additionally some more mdadm RAID knowledgeable people on >> other lists say Linux mdadm RAID1 would be faster as it will get data >> from more than one drive at a time. (Or possibly get data from which >> ever drive returns it the fastest. Not sure.) >> >> I believe one good option if I wanted 4 physical drives would be >> RAID10 but that's getting more complicated again which I didn't really >> want to do. >> >> So maybe it is just 2 drives and the 3 drive version isn't even a >> possibility? Could be. > > Using the instructions here: > > http://sempike.blogspot.com/2012/06/linux-software-raid-mdadm-with-virtual.html > > I just built a 3 device RAID1 using loop devices and it seems to have > worked. Below md50 did not exist, I created it as a 3 device RAID1 and > then mdadm shows it's there. I have no idea if it's a good thing to do > but mdadm doesn't stop me. I would need to test other real things like > putting a file system on it, mounting it, etc., to be more confident > but this much seems to work fine. > > Great question and good experience for me doing this. Thanks! > > Cheers, > Mark > > > c2RAID6 loopraid # cat /proc/mdstat > Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] > md3 : active raid6 sdb3[9] sdf3[5] sde3[6] sdd3[7] sdc3[8] > 1452264480 blocks super 1.2 level 6, 16k chunk, algorithm 2 [5/5] [UUUUU] > > unused devices: <none> > c2RAID6 loopraid # mdadm --create /dev/md50 --level=1 --raid-devices=3 > /dev/loop[012] > mdadm: Note: this array has metadata at the start and > may not be suitable as a boot device. If you plan to > store '/boot' on this device please ensure that > your boot-loader understands md/v1.x metadata, or use > --metadata=0.90 > Continue creating array? y > mdadm: Defaulting to version 1.2 metadata > mdadm: array /dev/md50 started. > c2RAID6 loopraid # cat /proc/mdstat > Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] > md50 : active raid1 loop2[2] loop1[1] loop0[0] > 20416 blocks super 1.2 [3/3] [UUU] > > md3 : active raid6 sdb3[9] sdf3[5] sde3[6] sdd3[7] sdc3[8] > 1452264480 blocks super 1.2 level 6, 16k chunk, algorithm 2 [5/5] [UUUUU] > > unused devices: <none> > c2RAID6 loopraid # > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Roberto Spadim -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
