Re: Home desktop/server RAID upgrade

[David Brown <david.brown@hesbynett.no>]

Hi Mark,

What would be really useful here is a description of what you actually 
/want/.  What do you want to do with these drives?  What sort of files 
are they - big or small?  Do you need fast access for large files?  Do 
you need fast access for many files in parallel?  How important is the 
data?  How important is uptime?  What sort of backups do you have?  What 
will the future be like - are you making one big system to last for the 
foreseeable future, or do you need something that can easily be 
expanded?  Are you looking for "fun, interesting and modern" or "boring 
but well-tested" solutions?

Then you need to make a list of the hardware you have, or the budget for 
new hardware.

Without know at least roughly what you are looking for, it's easy to end 
up with expensive SSDs because they are "cool", even though you might 
get more speed for your money with a couple of slow rust disks and a bit 
more ram in your system.  It may be that there is no need for any sort 
of raid at all - perhaps one big main disk is fine, and the rest of the 
money spent on a backup disk (possibly external) with rsync'd copies of 
your data.  This would mean longer downtime if your main disk failed - 
but it also gives some protection against user error.

And perhaps btrfs with raid1 would be the best choice.

A raid10,f2 is often the best choice for desktops or workstations with 2 
or 3 hard disks, but it is not necessarily /the/ best choice.

mvh.,

David


On 01/06/14 16:25, Mark Knecht wrote:
> Hi Craig,
>     Responding to both you and David Brown. Thanks for your ideas.
>
> - Mark
>
> On Sat, May 31, 2014 at 9:40 AM, Craig Curtin <craigc@prosis.com.au> wrote:
>> It sounds like the op has additional data ports on his MOBO - wouldn't he be
>> better off looking at a couple of SSDs in raid 1 for his OS, swap etc and
>> his VMs and then leave the rest for data as raid5 - By moving the things
>> from the existing drives he gets back space and only purchases a couple of
>> good sized fast SSDs now
>>
>
> It's a possibility. I can get 240GB SSDs in the $120 range so that's
> $240 for RAID1. If I take the five existing 500GB drives and
> reconfigure for RAID5 that's 2TB. Overall it's not bad going from
> 1.4TB to about 2.2TB but being it's not all one big disk I'll likely
> never use it all as efficiently. Still, it's an option.
>
> I do in fact have extra ports:
>
> c2RAID6 ~ # lspci | grep SATA
> 00:1f.2 IDE interface: Intel Corporation 82801JI (ICH10 Family) 4 port
> SATA IDE Controller #1
> 00:1f.5 IDE interface: Intel Corporation 82801JI (ICH10 Family) 2 port
> SATA IDE Controller #2
> 03:00.0 SATA controller: Marvell Technology Group Ltd. 88SE9123 PCIe
> SATA 6.0 Gb/s controller (rev 11)
> 06:00.0 SATA controller: JMicron Technology Corp. JMB363 SATA/IDE
> Controller (rev 03)
> 06:00.1 IDE interface: JMicron Technology Corp. JMB363 SATA/IDE
> Controller (rev 03)
> c2RAID6 ~ #
>
> Currently my 5-drive RAID6 uses 5 of the Intel ports. The 6th port
> goes to the CD/DVD drive. Some time ago I bought the SATA3 Marvell
> card and a smaller (120GB) SSD. I put Gentoo on it and played around a
> bit but I've never really used it day-to-day. Part of my 2-drive RAID1
> thinking was that I could build the new RAID1 on the SATA3 controller
> not even touch the existing RAID6. If it works reliably on that
> controller I'd be done and have 3TB.
>
> I think David's RAID10 3-drive solution could possibly work if I buy 3
> of the lower cost new WD drives. I'll need to think about that. Not
> sure.
>
> Thanks,
> Mark
>
>
> On Sat, May 31, 2014 at 9:40 AM, Craig Curtin <craigc@prosis.com.au> wrote:
>> It sounds like the op has additional data ports on his MOBO - wouldn't he be
>> better off looking at a couple of SSDs in raid 1 for his OS, swap etc and
>> his VMs and then leave the rest for data as raid5 - By moving the things
>> from the existing drives he gets back space and only purchases a couple of
>> good sized fast SSDs now
>>
>>
>> Sent from my Samsung tablet
>>
>> .
>>
>>
>> -------- Original message --------
>> From: David Brown
>> Date:31/05/2014 21:01 (GMT+10:00)
>> To: Mark Knecht ,"L.M.J"
>> Cc: Linux-RAID
>> Subject: Re: Home desktop/server RAID upgrade
>>
>> On 30/05/14 22:14, Mark Knecht 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.
>>
>> With 3 drives, you have several possibilities.
>>
>> Raid5 makes "stripes" across the three drives, with 2 parts holding data
>> and one part holding parity to provide redundancy.
>>
>> Raid1 is commonly called "mirroring", because you get the same data on
>> each disk.  md raid has no problem making a 3-way mirror, so that each
>> disk is identical.  This gives you excellent redundancy, and you can
>> make three different reads in parallel - but writes have to go to each
>> disk, which can be a little slower than using 2 disks.  It's not often
>> that people need that level of redundancy.
>>
>> Another option with md raid is the raid10 setups.  For many uses, the
>> fastest arrangement is raid10,f2.  This means there is two copies of all
>> your data (f3 would be three copies), with a "far" layout.
>>
>> <http://en.wikipedia.org/wiki/Linux_MD_RAID_10#LINUX-MD-RAID-10>
>>
>> With this arrangement, reads are striped across all three disks, which
>> is fast for large reads.  Small reads can be handled in parallel.  Most
>> reads while be handled from the outer half of the disk, which is faster
>> and needs less head movement - so reading is on average faster than a
>> raid0 on the same disks.  Small writes are fast, but large writes
>> require quite a bit of head movement to get everything written twice to
>> different parts of the disks.
>>
>> The "best" option always depends on your needs - how you want to access
>> your files.  A layout geared to fast striped reads of large files will
>> be poorer for parallel small writes, and vice versa.  raid10,f2 is often
>> the best choice for a desktop or small system - but it is not very
>> flexible if you later want to add new disks or replace the disks with
>> bigger ones.
>>
>> md raid is flexible enough that it will even let you make a 3 disk raid6
>> array if you want - but a 3-way raid1 mirror will give you the same disk
>> space and much better performance.
>>

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