Re: Home desktop/server RAID upgrade

[Roberto Spadim <rspadim@gmail.com>]

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@gmail.com>:
> On Fri, May 30, 2014 at 1:14 PM, Mark Knecht <markknecht@gmail.com> wrote:
>> On Fri, May 30, 2014 at 12:29 PM, L.M.J <linuxmasterjedi@free.fr> wrote:
>>> Le Fri, 30 May 2014 12:04:07 -0700,
>>> Mark Knecht <markknecht@gmail.com> 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 #
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-- 
Roberto Spadim
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