RAID configuration for resilience

[David WE Roberts]

What RAID configurations are people using?

I am due to populate my PC with long-term storage for video and music and
when getting towards a TB of data resilience would be good.

Options look to be:
RAID 1 (mirroring): 50% of gross capacity but only requires 2 discs
RAID 5 (striping + parity): at least 66.6% of gross capacity but requires at
least 3 discs. Better 'bangs per buck' if you use 4 or more discs (if there
is room).

Also, it seems logical to buy a spare (to be unused) disc unless I plan to
scrap the RAID array if a disc fails after 2 years or more and upgrade to
newer larger discs.
The assumption being that my chosen drives will only be commercially
available for the next 2 years if not less.

All in all a potentially expensive process - and I have so far scraped by
without resilience - but backup is now requiring very large discs anyway.

Of course you should still back up RAID arrays in case a second disc fails
before you can replace the first failed disc.

Oh, and if I decide to go the NAS route, is it O.K. to build a NAS box out
of an older spec PC?

Cheers

Dave R

--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

(\__/)
(='.'=)
(")_(")

[Jaimie Vandenbergh]

On Sun, 30 Dec 2012 17:14:56 -0000, "David WE Roberts"
<nos...@btinternet.com> wrote:

>What RAID configurations are people using?

RAIDZ1 over four 2Tb disks. It's the ZFS equivalent of RAID5.

>I am due to populate my PC with long-term storage for video and music and
>when getting towards a TB of data resilience would be good.

Not as good as backups.

Resilience just keeps you going when a disk dies. Backups protect you
against everything else - disk controller death, filesystem
corruption, lightning strikes, accidental deletion, fire and flood...

>Options look to be:
>RAID 1 (mirroring): 50% of gross capacity but only requires 2 discs
>RAID 5 (striping + parity): at least 66.6% of gross capacity but requires at
>least 3 discs. Better 'bangs per buck' if you use 4 or more discs (if there
>is room).

Yep. I'd commend the idea of putting them into a NAS, since keeping
them away from cups of coffee and cats also helps keep them alive.
Unless you need better than 100meg/second out of your disks (gigE),
it's by far the best plan. Tuck it in the cellar or a cool cupboard
somewhere out of earshot!

>Also, it seems logical to buy a spare (to be unused) disc unless I plan to
>scrap the RAID array if a disc fails after 2 years or more and upgrade to
>newer larger discs.
>The assumption being that my chosen drives will only be commercially
>available for the next 2 years if not less.

Shouldn't worry about it: buying a replacement disk later will be
cheaper anyway, and you don't need matched sets: the new one does need
to be not-smaller than the others, but over 1Tb all the mfr's seem to
use exactly the same number of available blocks per disk of a given
size, so that's easy. I've never bothered to keep a stock on hand.

>All in all a potentially expensive process - and I have so far scraped by
>without resilience - but backup is now requiring very large discs anyway.
>
>Of course you should still back up RAID arrays in case a second disc fails
>before you can replace the first failed disc.

You can also use arrays with two disk redundancy - RAID6 (or RAIDZ2 in
my case). This is generally better than having a spare disk kicking
around, too.

>Oh, and if I decide to go the NAS route, is it O.K. to build a NAS box out
>of an older spec PC?

Yep. My NAS is an HP Microserver N36L (N40L is the update, both now
discontinued but available still), which has four caddy SATA slots
plus space for more in a 5.25" bay, and eSATA. It uses an AMD dual
1.3GHz cpu, pretty slow.

I run FreeNAS8, which is not an excessively user-friendly NAS distro,
but it works really well once you've got your head around it. I chose
it because it was the easier way to get ZFS based storage.

NAS4Free may be better these days, I really need to have a play with
it.

I've had various dedicated NAS chassis over the years, favourite was a
Netgear NV+ but apparently they're shit nowadays - mine was from
before Netgear bought out the company. Unfortunately it just became
too slow with much larger disks than existed when it was bought! So I
looked into various options, went with the HP.

And once I'd built it, I bought another HP and put the old 4x1Tb disk
array in that as a stripe set (JBOD, no redundancy) to backup the new
NAS onto.

Cheers - Jaimie
--
Power corrupts, but intermittent power corrupts absolutely
-- Jeff Bell, asr

[Jaimie Vandenbergh]

On Sun, 30 Dec 2012 17:14:56 -0000, "David WE Roberts"
<nos...@btinternet.com> wrote:

>What RAID configurations are people using?
>
>I am due to populate my PC with long-term storage for video and music

I should have commented on this: Unless you're the only person in the
house and only have one computer, keeping your bulk media on a NAS is
a much better location - you can share it (readonly if needed!) to all
devices, quite possibly including your telly and hifi, as many are
bright enough these days to read DLNA streams or even straight Windows
shares.

And you don't need your main PC on to do it, which is often nice from
a power usage point of view (the little HP's use about 30W compared to
a 'proper' PC's 90-150W plus).

Cheers - Jaimie
--
"First they came for the verbs, and I said nothing because verbing weirds
language. Then they arrival for the nouns, and I speech nothing because
I no verbs." - Quoted by Peter Ellis, afp

[Tired]

Jaimie Vandenbergh wrote:
:: On Sun, 30 Dec 2012 17:14:56 -0000, "David WE Roberts"

:: <nos...@btinternet.com> wrote:
::
::: What RAID configurations are people using?
::
:: RAIDZ1 over four 2Tb disks. It's the ZFS equivalent of RAID5.
::
::: I am due to populate my PC with long-term storage for video and
::: music and when getting towards a TB of data resilience would be
::: good.
::
:: Not as good as backups.
::
:: Resilience just keeps you going when a disk dies. Backups protect you
:: against everything else - disk controller death, filesystem
:: corruption, lightning strikes, accidental deletion, fire and flood...

::

OT:

I had a motherboard with an nvidia raid option. It was enabled with raid 0,
two drives stripped. The motherboard battery ran out, the mb defaulted to a
non raid sata setup, machine attempted to boot from one of the disks that
was part of the raid. Obvious fail to boot message. Battery replaced, bios
set backup for nvidia raid.

Raid now degraded to the point that the nvidia raid bios no longer recogises
one of the drives as being part of the raid.

Quite possibly the worst implementation of raid. Well done nvidia.

[Jaimie Vandenbergh]

On Sun, 30 Dec 2012 20:13:33 -0000, "Tired" <n...@no.com> wrote:

>OT:
>
>I had a motherboard with an nvidia raid option. It was enabled with raid 0,
>two drives stripped. The motherboard battery ran out, the mb defaulted to a
>non raid sata setup, machine attempted to boot from one of the disks that
>was part of the raid. Obvious fail to boot message. Battery replaced, bios
>set backup for nvidia raid.
>
>Raid now degraded to the point that the nvidia raid bios no longer recogises
>one of the drives as being part of the raid.
>
>Quite possibly the worst implementation of raid. Well done nvidia.

My word, that's impressively piss poor!

I've always avoided using any mobo RAID, since the mobo is fairly
likely to be the part that dies and the disk sets are clearly not very
portable.

And I've avoided using RAID cards unless I have a spare one of them,
too.

(One of the very many nice things about ZFS is that it's OS
independent so anything that'll run ZFS can mount the set if your host
pops. And one of the nice things about FreeNAS is that it lives on a
USB stick. Take the stick and drives to any random spare computer, get
them all plugged in, instant franken-NAS!)

These days with all the spare CPU power floating around running
software RAID is not slower - and is often faster - than dedicated
RAID cards.

Cheers - Jaimie
--
It's all fun and games until someone loses an eye. Then it's a scavenger hunt.

[Daniel James]

In article <akbb4i...@mid.individual.net>, David WE Roberts wrote:
> What RAID configurations are people using?

I'm currently only using RAID1 on anything. I may use something beefier
if/when I need more storage and/or disk prices fall through the floor.

I'm not sure that RAID5 is still considered worthwhile ... there are
some performance trade-offs that don't seem so wonderful now that disks
are fast and (relatively) cheap.

Google "say no to raid 5" for a variety of articles ...

> Also, it seems logical to buy a spare (to be unused) disc ...

You won't have difficulty sourcing a spare disk when you need one, and
(barring more floods in Thailand) the price will probably be lower than
it is now.

OTOH having a spare disk that you can swap in as soon as a drive in your
RAID array fails does mean that you can minimize the time that you run
without redundancy. You really *don't* want to find that a second drive
fails before you have replaced the first and rebuilt the array.

It depends how important your data are, and how well you have them
backed up elsewhere.

> Oh, and if I decide to go the NAS route, is it O.K. to build a NAS
> box out of an older spec PC?

I'd certainly recommend putting your RAID array in a server of some kind
-- be it a NAS or a full fileserver -- especially if the RAID array is
to be shared.

Commercially available NAS boxes can be very economical for one or two
drives, larger boxes tend to be disproportionately expensive. The HP
Microserver (still available, but has the �100 cashback now finally
ended?) has 4 bays -- 5 if you use the optical drive bay -- and is
cheaper (even without the cashbask) than any 4-bay NAS I've seen.

I run Debian Server in mine, but packaged NAS solutions like FreeNAS are
also popular.

Another possibility is a Drobo box.

http://www.drobo.co.uk/

I think they're cheekily expensive for what you get, but they do make
the management of a redundant array of mismatched disks very easy.

[Jaimie Vandenbergh]

On Mon, 31 Dec 2012 19:24:16 -0000, Daniel James <dan...@me.invalid>
wrote:

>In article <akbb4i...@mid.individual.net>, David WE Roberts wrote:
>> What RAID configurations are people using?
>
>I'm currently only using RAID1 on anything. I may use something beefier
>if/when I need more storage and/or disk prices fall through the floor.

I cunningly filled up my NAS just before the floods.

>I'm not sure that RAID5 is still considered worthwhile ... there are
>some performance trade-offs that don't seem so wonderful now that disks
>are fast and (relatively) cheap.

It certainly is still viable, and with all the spare CPU cycles
kicking around these days is no noticeable drag at all in a modern PC.
Saves power over using multiple RAID1's too.

Cheers - Jaimie
--
The cure for boredom is curiosity. There is no cure for curiosity.
-- Dorothy Parker

[Daniel James]

In article <a0q3e8l67j0keucnb...@4ax.com>, Jaimie

Vandenbergh wrote:
> I cunningly filled up my NAS just before the floods.

I bought 4 Hitachi 2GB drives at around £45+VAT just before the prices
shot up ... I wish I'd bought more! I've got two Microservers with only
two drives in, and I wish I'd upgraded my ReadyNAS Duo from 2x1GB at the
same time.

>> I'm not sure that RAID5 is still considered worthwhile ... there
>> are some performance trade-offs that don't seem so wonderful now
>> that disks are fast and (relatively) cheap.
>

> It certainly is still viable ...

"viable" is not quite the same as "worthwhile" ...

One of the arguments that's being banded about is that as single drives
are now more reliable than was once the case (are they?) and are
available in larger capacities it just isn't worth messing around with
RAID5 and that RAID1 (or even an un-RAIDed drive) will do. I must say
that apart from IBM Ultrastars (the SCSI equivalent of the old
Deathstar) I've had very few drive failures.

Another argument concerns the likelihood of a second disk failure during
a rebuild after replacing a disk, and how that likelihood increases as
the array grows in size, and as the individual disks grow in size. With
individual drives reaching multi-terabyte sizes, and multiple drives in
an array, the chance of a second failure ceases to be insignificant.

The advice given is generally to use RAID6 or some other arrangement
that offers more redundancy, especially when drive prices are low
(there's still the extra cost of powering another drive, and of
obtaining a larger enclosure, of course).

Sourcing drives from different batches/vendors, and other precautions of
that kind, designed to reduce the likelihood of two disks failing in the
same time frame help, of course, but as the amount of data involved in a
rebuild goes up so must the chance that a drive will fail before it's
complete.

Cheers,
Daniel.

[Rob]

As others have said or hinted at, probably the most critical things are:

1) How much downtime can you tolerate?
2) What degree of resiliency is good enough?

Motherboard-based RAID5 is bad for (2), unless you have a spare
identical (or possibly the same RAID chipset) board, as if the mobo
dies and you can't replace it, you won't be able to recover your data
without great difficulty, if at all.

A commercial NAS box is equally bad if you can't replace the NAS box
easily, should that die on you. That is compounded if the NAS uses some
weird proprietary variant of a filing system so you can't use other
hardware to pull the data off. Sometimes even a replacement NAS box
won't do the trick without tedious fiddling with the firmware version.

Most versions of RAID1 (simple mirroring) I've seen allow you to
easily get the data off on another non-RAID system, should the mobo
fail, so I generally only use RAID1 now, despite the fact that
it doubles my storage costs on critical systems.

I do use RAID 10 on a 4TB (4 x 2TB drives) eSATA-connected box on
a number-crunching system where I need maximum drive access speed,
but even data on that box is automatically backed-up to other network
drives when it's idle. It used to be RAID-5, but the 36H rebuild
time annoyed the hell out of me once too many times (ie the first
time!), and it was only caused by a minor issue with the array,
but reduced access speed to 25% of normal for all of that time.

All in all, I'd say having two or more simple backups *on different
hardware* is the main thing. Implement that backup storage in a
way that suits your needs (ie if fast access is needed and you
use a NAS, make sure you use gigabit and the NAS can actually
serve files at gigabit speeds - many can't even get close.)

HTH
--
Rob

[David WE Roberts]

"Rob" <no...@nowhere.noway.con> wrote in message
news:ako5h4...@mid.individual.net...

> On 30/12/2012 17:14, David WE Roberts wrote:
>> What RAID configurations are people using?
>>
>> I am due to populate my PC with long-term storage for video and music
>> and when getting towards a TB of data resilience would be good.
>>
>> Options look to be:
>> RAID 1 (mirroring): 50% of gross capacity but only requires 2 discs
>> RAID 5 (striping + parity): at least 66.6% of gross capacity but
>> requires at least 3 discs. Better 'bangs per buck' if you use 4 or more
>> discs (if there is room).
>>
>> Also, it seems logical to buy a spare (to be unused) disc unless I plan
>> to scrap the RAID array if a disc fails after 2 years or more and
>> upgrade to newer larger discs.
>> The assumption being that my chosen drives will only be commercially
>> available for the next 2 years if not less.
>>
>> All in all a potentially expensive process - and I have so far scraped
>> by without resilience - but backup is now requiring very large discs
>> anyway.
>>
>> Of course you should still back up RAID arrays in case a second disc
>> fails before you can replace the first failed disc.
>>
>> Oh, and if I decide to go the NAS route, is it O.K. to build a NAS box
>> out of an older spec PC?
>

Thanks to all contributors.
Looking like a sensible backup strategy may be the best way to go - although
since backup will be to an HDD you still have the problem of the backup
medium failing unless you run two seperate backup drives.
Still, two systems each with a dedicated backup drive which also back each
other up might be a solution.