4 disks: RAID-6 or RAID-10 ..

4 disks: RAID-6 or RAID-10 ..

[Gordon Henderson]

I'm building a little test server and I wanted ~500GB of storage with
2-drive redundancy, so the best price vs. num. drives vs. the need for 2
drive redundancy came to 4 x 250GB drives. (And I have a mobo with 5 SATA
ports, and taking into account case power requirements, etc. 4 drives has
worked out quiet well, and cheaper than 2 x 500GB drives!)

So RAID-6, which I have have been using for a year or so now with good
results, or RAID-10, which I've never used.

I suspect RAID-10 might give me more performance, not having the parity
calculations to do, but is it stable and reliable? I've not been paying
much attention to it recently ... (Servers intended use is what I
understand is termed 'LAMP' these days - Linux, Apache, MySQL, PHP,
although it's going to be handling up to 10,000 emails a day too with spam
and virus checking)

Cheers,

Gordon

Re: 4 disks: RAID-6 or RAID-10 ..

[berk walker]

Gordon Henderson wrote:

>I'm building a little test server and I wanted ~500GB of storage with
>2-drive redundancy, so the best price vs. num. drives vs. the need for 2
>drive redundancy came to 4 x 250GB drives. (And I have a mobo with 5 SATA
>ports, and taking into account case power requirements, etc. 4 drives has
>worked out quiet well, and cheaper than 2 x 500GB drives!)
>
>So RAID-6, which I have have been using for a year or so now with good
>results, or RAID-10, which I've never used.
>
>I suspect RAID-10 might give me more performance, not having the parity
>calculations to do, but is it stable and reliable? I've not been paying
>much attention to it recently ... (Servers intended use is what I
>understand is termed 'LAMP' these days - Linux, Apache, MySQL, PHP,
>although it's going to be handling up to 10,000 emails a day too with spam
>and virus checking)
>
>Cheers,
>
>Gordon
>-
>To unsubscribe from this list: send the line "unsubscribe linux-raid" in
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>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
>  
>
RAID-6 *will* give you your required 2-drive redundancy.
b-

Re: 4 disks: RAID-6 or RAID-10 ..

[Gordon Henderson]

On Fri, 17 Feb 2006, berk walker wrote:

> RAID-6 *will* give you your required 2-drive redundancy.

Hm. I was under the impression (mistakenly?) that RAID10 (as opposed to
RAID1+0) would give me 2 disk redundancy in far mode, however maybe I need
to re-read the stuff on RAID10 again ...

Gordon

Re: 4 disks: RAID-6 or RAID-10 ..

[Francois Barre]

2006/2/17, Gordon Henderson <gordon@drogon.net>:
> On Fri, 17 Feb 2006, berk walker wrote:
>
> > RAID-6 *will* give you your required 2-drive redundancy.
>
Anyway, if you wish to resize your setup to 5 drives one day or
another, I guess raid 6 would be preferable, because one day or
another, a patch will popup and make raid6 resizing possible. Or won't
it ?

Re: 4 disks: RAID-6 or RAID-10 ..

[Gordon Henderson]

On Fri, 17 Feb 2006, Francois Barre wrote:

> 2006/2/17, Gordon Henderson <gordon@drogon.net>:
> > On Fri, 17 Feb 2006, berk walker wrote:
> >
> > > RAID-6 *will* give you your required 2-drive redundancy.
> >
> Anyway, if you wish to resize your setup to 5 drives one day or
> another, I guess raid 6 would be preferable, because one day or
> another, a patch will popup and make raid6 resizing possible. Or won't
> it ?

Resizing isn't something I really care for. This particular box will be
sent away to a data centre where it'll stay for 3 years until I replace
it.

(And if I really do need more disk space in the meantime, I'll just build
another :)

Still scratching my head, trying to work out if raid-10 can withstand
(any) 2 disks of failure though, although after reading md(4) a few times
now, I'm begining to think it can't (unless you are lucky!) So maybe I'll
just stick with Raid-6 as I know that!

Cheers,

Gordon

Re: 4 disks: RAID-6 or RAID-10 ..

[Andy Smith]

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On Fri, Feb 17, 2006 at 03:14:37PM +0000, Gordon Henderson wrote:
> Still scratching my head, trying to work out if raid-10 can withstand
> (any) 2 disks of failure though, although after reading md(4) a few times
> now, I'm begining to think it can't (unless you are lucky!) So maybe I'll
> just stick with Raid-6 as I know that!

RAID-10 cannot survive the failure of *any* two disks as if two
disks in one of the mirrors died then the whole mirror would be lost
which loses you a segment of the upper stripe.  *If* a second disk
dies, then with 4 didks total you have 50% chance of it being the
one you're relying on.

If you require to withstand the loss of *any* two disks then you
need RAID-6.

-- 
http://strugglers.net/wiki/Xen_hosting -- A Xen VPS hosting hobby
Encrypted mail welcome - keyid 0x604DE5DB

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Re: 4 disks: RAID-6 or RAID-10 ..

[Gordon Henderson]

On Fri, 17 Feb 2006, Andy Smith wrote:

> On Fri, Feb 17, 2006 at 03:14:37PM +0000, Gordon Henderson wrote:
> > Still scratching my head, trying to work out if raid-10 can withstand
> > (any) 2 disks of failure though, although after reading md(4) a few times
> > now, I'm begining to think it can't (unless you are lucky!) So maybe I'll
> > just stick with Raid-6 as I know that!
>
> RAID-10 cannot survive the failure of *any* two disks as if two
> disks in one of the mirrors died then the whole mirror would be lost
> which loses you a segment of the upper stripe.  *If* a second disk
> dies, then with 4 didks total you have 50% chance of it being the
> one you're relying on.

I understand the avice in a classic RAID-1+0, but Linux native RAID-10 is
somewhat different from a classic RAID-0 built in top of 2 x 2-disk
RAID-1's (AIUI). I was thinking of 3 replicas in near mode, but that
diminishes my overall disk capacity somewhat... (1.333 times a disk rather
than 2 times a disk for R6, but if performance was significantly better I
could live with it)

I pickup the hardware on Monday evening, so I'll have a few days of
playing.

Cheers,

Gordon

Re: 4 disks: RAID-6 or RAID-10 ..

[Bill Davidsen]

Gordon Henderson wrote:

>On Fri, 17 Feb 2006, Francois Barre wrote:
>
>  
>
>>2006/2/17, Gordon Henderson <gordon@drogon.net>:
>>    
>>
>>>On Fri, 17 Feb 2006, berk walker wrote:
>>>
>>>      
>>>
>>>>RAID-6 *will* give you your required 2-drive redundancy.
>>>>        
>>>>
>>Anyway, if you wish to resize your setup to 5 drives one day or
>>another, I guess raid 6 would be preferable, because one day or
>>another, a patch will popup and make raid6 resizing possible. Or won't
>>it ?
>>    
>>
>
>Resizing isn't something I really care for. This particular box will be
>sent away to a data centre where it'll stay for 3 years until I replace
>it.
>
>(And if I really do need more disk space in the meantime, I'll just build
>another :)
>
>Still scratching my head, trying to work out if raid-10 can withstand
>(any) 2 disks of failure though, although after reading md(4) a few times
>now, I'm begining to think it can't (unless you are lucky!) So maybe I'll
>just stick with Raid-6 as I know that!
>

With only four drives you can just do the possible failure cases, there 
are only six... when any one drive fails you can only survive the 
failure of two of the three remaining drives, not what you wanted. How 
reliable do you NEED here is the real question.

It isn't too hard to make the drives more reliable than the case they're 
in, how many fans and power supplies can you survive losing?

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

Re: 4 disks: RAID-6 or RAID-10 ..

[Gordon Henderson]

On Sun, 5 Mar 2006, Bill Davidsen wrote:

> >Still scratching my head, trying to work out if raid-10 can withstand
> >(any) 2 disks of failure though, although after reading md(4) a few times
> >now, I'm begining to think it can't (unless you are lucky!) So maybe I'll
> >just stick with Raid-6 as I know that!
> >
>
> With only four drives you can just do the possible failure cases, there
> are only six... when any one drive fails you can only survive the
> failure of two of the three remaining drives, not what you wanted. How
> reliable do you NEED here is the real question.

Well this was a few weeks ago and after working things out, then things
changed a little, and I decided to make a change to the server - it's now
a 5-disk RAID-6 system, and it's working just fine.

> It isn't too hard to make the drives more reliable than the case they're
> in, how many fans and power supplies can you survive losing?

This is true, but my experience of PCs over the past 10 years has been
that drives fail on average more than anything else - especially when you
have more hard drives in a box than anything else! (but I've no hard
statistical data to back this up - it's just my overvations!)

Anyway, just in-case, I do like to test things as much as I can before
taking them to the datacentre - and it appears to work with all 3 case
fans unplugged without overheating.  There's a redundant PSU, and it even
works with the processor fan turned off - slowly, but workable. I run
lm-sensors and hddtemp (although hddtemp is lying to me - it's reading 20C
too high for the drives I have in it )-: it's remotely monitored, (nagios)
and I get called & TXTd by the monitoring server should anything go wrong.

All its working set data is backed up on another server in the same place,
(rsync, overnight) and it's acting as a backup for a dozen other servers
(in the same place) so should the mobo blow up then I can recover and the
other server can assume IP addresses, etc. It's not quite "high
avalability" in that it requires human intervention to bring the backup
server online, but it's good enough for my purposes, (and my clients!),
and you have to draw the line somewhere!

(And how many single CPU, single drive RAQs out there just humming away in
datacentres all over the world.. Hmm, maybe they're onto something ;-)

Cheers,

Gordon

Re: 4 disks: RAID-6 or RAID-10 ..

[Gordon Henderson]

On Sun, 5 Mar 2006, Bill Davidsen wrote:

> I agree, but it's easier to configure to keep going with a dead drive
> than fan in many enclosures. You seem to have more heat tolerance and
> monitoring than many installations. And you have done testing on the
> heat issues, another unusual thing.

I got caught out a few years ago with a case-fan failing, and I've seen a
1U box almost catch-fire, so I'm very paranoid now! I'm also fortunate in
that the data centre I use allows me to host tower style PCs rather than
the usual 1 or 2U boxes, so I can build boxes with lots of room inside
that don't auto-roast on fan failure!

Although in a few months time I'll be looking to host some stuff
elsewhere, where space is at a premium, and therefore it's going to be 1
or 2U based servers, so it'll be an intersting diversion...

Cheers,

Gordon