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RAIDfail: Don't use RAID 5 on small arrays

By | May 19, 2009, 11:14am PDT

Big storage companies stopped recommending RAID 5 a couple of years ago. But I still see small 4-drive arrays touting RAID 5 for home and small office use.

Big mistake. You want to save money, but you also want to keep your data. RAID 5 isn’t worth it.

What’s the problem?
The problem is that RAID 5 only protects against a single disk failure. But SATA drives are spec’d at one Unrecoverable Read Error (URE) every ~12.5 TB.

Let’s do the math.

In a small 4 drive array using 2 TB disks, if you lose a disk you have 6 TB - 3 drives - of remaining capacity. That includes the parity data used to reconstruct the data lost on the failed drive.

Reading through that 6 TB you have a better than 40% chance of encountering an URE - and at that point the disk rebuild will stop since the RAID controller doesn’t have the information it needs to reconstruct your data.

Then you pull out your backup copies. You have backups, right?

How to use a small RAID array.
4-drive arrays have lots of advantages: cost; performance (with FireWire or eSATA) fast enough for HD video editing; and portability.

But if you care about your data, RAID 5 is too big a threat. And if you don’t mind risking your data - as in performance driven apps like video editing where the data copies are on tape or another disk - RAID 0 (striping) is cheaper and faster.

Most small arrays come with a RAID 1 (mirroring) option that copies your data to 2 different disks. Lose 1 and the other should have it - subject to the occasional URE.

If you want availability and better performance use RAID 1+0 - often abbreviated RAID 10 - which combines mirroring and striping to provide 2 complete copies of your data with the performance of 2 striped drives.

The Storage Bits take
The attraction of RAID 5 is that it gives you 3 drives worth of capacity on a 4 drive array - but at the cost of having to use backups if an URE is encountered. Better to use RAID 1 and get 2/3rds the capacity of RAID 5 with a much lower chance of data loss.

The biggest storage mistake consumers make is to believe that any storage device is 100% safe. It isn’t.

Maintain at least 2 copies of any data you value. If the data is vital, make that 3 copies. And if thinking about RAID levels makes your teeth ache, consider a Drobo or the new Drobo Pro.

Storage is cheap. Use lots.

Comments welcome, of course. Check out an earlier post Why RAID 5 stops working in 2009 for more details on the RAID 5 problem.

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Topics

Data, RAID 5, RAID, Storage, Hardware, Robin Harris

Robin Harris has been messing with computers for over 30 years and selling and marketing data storage for over 20 in companies large and small.

Disclosure

Robin Harris

Robin Harris is a president of TechnoQWAN, a consulting and analyst firm in northern Arizona. He also writes StorageMojo.com, a blog which accepts advertising from companies in the storage industry, and has a 25 year history with IT vendors. He has many industry contacts, many of whom are friends and all of whom he has opinions about. Robin has relationships with many companies in the technology industry. Every company he writes about may have sought to influence his opinion through carefully-crafted marketing messages and self-serving white papers, gifts ranging from desk calendars, t-shirts, lunches and trips as well as analyst or consulting assignments. He also invests in some technology companies. He may accept payment for services in stock as well. Robin discloses financial investments in or client relationships with companies named in Storage Bits. To help readers sort out the gold from the dross in his writings, Robin tries to communicate his reasons as clearly as he can. If you agree, you are intelligent and discerning. If you disagree, well, you disagree. In all cases, Robin encourages readers to subject everything they read, see or hear on the internet or from politicians to some simple questions: * What assumptions are implicit in the world view and judgments of the author? * What, if any, is the factual basis for the opinions the author expresses? * Is it reasonable, logical and clear? Your critical faculties: use ‘em or lose ‘em!

Biography

Robin Harris

Harris has been messing with computers for over 30 years and selling and marketing data storage for over 20 in companies large and small. He introduced a couple of multi-billion dollar storage products (DLT, the first Fibre Channel array) to market, as well as a many smaller ones. Earlier he spent 10 years marketing servers and networks. After leaving corporate life he founded TechnoQWAN, a consulting and analyst firm. He also developed StorageMojo into one of the top storage industry blogs.

Robin writes, consults, coaches and lives among the mountains of northern Arizona.

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RE: RAIDfail: Don't use RAID 5 on small arrays
FAULKNE 13th Oct
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0 Votes
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Drobo
mcfaul@... 19th May 2009
So.. the article says not use use RAID 5 - then
recommends that the consumer buys a box (Drobo)
- which essentially just operates as a RAID 5
array when its full???? (put in 4 x 1 TB disks
and get 3 TB of usable space).

Seems incredibly stupid

at least the drobopro has dual disk redundancy
which should minimise the effect of and UREs
0 Votes
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Drobo isn't a striped RAID array
Robin Harris 19th May 2009
That's why it can use disks of different sizes and give you more capacity
than the smallest disk.

That's why its better than a RAID 5 array - it isn't one.

HTH,

Robin
0 Votes
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yes it is
mcfaul@... 19th May 2009
if its empty enough it will mirror, but as you
fill it up it HAS to use parity

give it :

1TB
1TB
750GB
500GB

you get 2.25TB of usable space (check
drobulator)


the ONLY way to do this is
mirror the "top" 250GB across the two 1TB
raid 3/5 on the next 250GB of the two 1TB
drives and the top 250GB of the 750GB drive
then Raid 3/5 across the last 500GB of all four
disks

this ***IS*** exactly as dangerous as the raid
5 array described in the article (mainly
because it is doing it for the majority of the
data)

if you can see some way of getting 2.25TB of
data onto an array of 1000/1000/750/500 (3.25TB
total space) disks WITHOUT using parity (which
gives the dangers described in the article)
then i would be fascinated to hear it

I've owned a drobo, i've owned a drobo v2, and
now i own a droboPro, i'm very active on
drobospace.com - when they are full - they operate essentially as proprietary raid 5
arrays - with all the risks and problems that
come with that
0 Votes
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2 copies.
thewelshboy 21st May 2009
Which is why Robin suggested buying two Drobos to ensure you always have two copies. But then you run into the management/risk situation of managing that yourself..

http://www.matrixstore.net/2009/05/21/diy-good-or-bad-short-video/
0 Votes
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Two drobos are not cost practical
georgeou 26th May 2009
Two drobos are not cost practical. It's far
more practical and cost effective to get one
RAID-6 array.
0 Votes
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Drobo uses a mixure of RAID technologies including RAID-5
georgeou 19th May 2009
Drobo uses a mixure of RAID technologies
including RAID-5
http://blogs.zdnet.com/Ou/?p=508
0 Votes
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I don't get it
tryonQc 19th May 2009
at the end of the reference drobo article it says "Note that the 4 slot Drobo won?t handle 2 drive failures at once."

What's the difference between drobo/raid5 then?

Also It would have been worth mentioning (copied/past from the linked post): raid5 problem: Normal arrays don?t know which blocks have data so a 2nd URE kills the array.
So, raid5 does have to read the entire 6TB remaining (out of 8TB) instead of just the 2TB needed.

finally I'd like to say thanks for these articles on raid5 as I was thinking of building my own ... I'm going for WHS instaed :P
0 Votes
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Robin mistakenly believes the Drobo doesn't use RAID-5
georgeou 19th May 2009
You're not the one with the confusion. Robin
mistakenly believes the Drobo doesn't use RAID-
5 when in fact it does.
0 Votes
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RE: RAIDfail: Don't use RAID 5 on small arrays
mcfaul@... 19th May 2009
drobo can use raid 1/5 internally so it can be part
mirrored and part parity - basically as long as your two
largest drives are the same size - you can use the
maximum amount of protected space available - you dont
need identical sized drives as you do with traditional
raid 5 arrays.
0 Votes
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RE: RAIDfail: Don't use RAID 5 on small arrays
tryonQc 19th May 2009
The only difference I see then is that it knows where the recovery bytes are so it doesn't need to read all the data from the other disks which reduce the risk of failrues (we had 40% f or 6TB, if we only have to read 2TB out of those 6 we cut the 40% by 3)
0 Votes
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So long as two disks don't fail at once, you're good to go
georgeou 19th May 2009
So long as two disks don't fail at once, you're good
to go with RAID-5. People will only have problem if
they neglect to swap out a bad drive.

A 4-drive RAID-5 volume is beginning to make sense
because it offers you 75% of total raw capacity
whereas mirroring only offers you 50% capacity. RAID-
5 makes even more sense when you have a 6-drive volume
and you're getting 83.3% of total capacity.
0 Votes
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That's exactly the problem
rosanlo 20th May 2009
Because the drives are put into the system at the same time, environmental characteristics
that cause 1 drive to fail can easily cause
> 1 drive to fail simultaneously, or in short
order. Some organizations without daily
on-site IT help aren't aware that 1 drive has
already failed.

Also, the probability of error or drive failure
is not constant. It increases as the drives
age. So the risk of a > 1 drive failure are
much higher as the years roll by. In fact,
it may be cheap insurance to simply replace
all the drives in an array after, say, 3
years, given the steep decline in drive prices
every year.

Additionally, if the 2nd drive were to fail during the rebuild process, the organization
would also be toast.

From the outset, the array has to be engineered
to sustain a 2 drive failure.
0 Votes
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Tine is not the only enemy
techr@... 20th May 2009
I had a drive that was only six months old blow a chip (literally) on the controller board. An identical drive ran well past it's prime, so I have to chalk that failure up to a power surge. I've also seen brand new drives with manufacturing defects; any one remember the IBM 270Mb drives that you had to take out and spin, to get the platters to rotate after they stalled? Based on that, I would say that it would be better to use different manufacturers and replace drives at different intervals, so not all drives are the same age or manufacturer lots.
I believe either Western Digital or Seagate had issues with their 1 and 1.5 Tb drives around Christmas. This was a firmware issue, which put every drive at risk of failure to work within parameters.
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That's exactly the problem? (RAID 6)
Yndoendo 20th May 2009
RAID 5 is already designed to allow for 2 drives to fail it is called RAID 6.

So far in the last 10 years I have never had a RAID 5 where 2 drives failed at the same time. His logical is also flawed because you should always have a backup system that you can take off site, what happens if there is a fire your RAID is worthless. The only servers I have installed that don't have a backup system are Terminal Servers.

What statistics are is calculations based off of? Server hard drives are different desktop hard drives. I have never just took hard drives off the shelves at your local Best Buy and used them in a server.

Plus his idea that the business might not even know that a drive has fail means that it is a bad solution. You always want to use a sever with error / malfunction messaging / e-mail that HP or DELL has.

Is he talking about white box servers? I don't know anyone in their right mind that would recommend a white box server for a company. Most of all if that is the company's only server. You have no true business support with a white box. HP and DELL have parts shipped next day built into the server cost versions paying $50 to have a hard drive shipped over night from Newegg because Best Buy does not have the same one in stock.

And the biggest thing is he didn't even state that the RAID level should be selected for what kind of job the server will be doing. Database, File Servers, and Application servers all have improved or reduced performance based on the RAID level.
0 Votes
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I never said RAID equals backup
georgeou 26th May 2009
I never said RAID equals backup. I never said
you don't need backup on top of RAID. I know
what RAID 6 is and I've been teaching this
stuff a long time. What I said was that for
most smaller jobs (where you already have
backup), RAID-5 is sufficient.

If two disks fail, you have to fall back on a
backup restore and they only thing you've lost
is some time. Now time is money but you have
to figure in the additional cost of the drives
and hardware. If you determine that you can't
afford to go down period (in the case of an e-
commerce server or critical company database),
you choose RAID-6 for better hardware uptime in
addition to backups. If the server is for some
less time sensitive application where it's not
so bad if you're down a few hours, RAID-5 plus
backup is good enough.
0 Votes
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The solution to this is obvious
theshowmecanuck@... 30th May 2009
You just need to build your server over the space of several years, adding a drive every six months or so. That way you have better odds that the mtbf won't be reached on all drives at once. But if you're *really* impatient then i guess you'll just have to chance it, and build it all at once.
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The REAL problem here ...
George Mitchell 19th May 2009
Is that people are using cheapo software RAID that, while often as fast or even faster than hardware RAID, does not have the disk maintenance capabilities of hardware RAID. RAID is about more than just speed. It is about reliability and when it comes to reliability software RAID comes up short. It doesn't provide a dedicated system to actively manage hard drives and neither due the typical file systems that operate on top of it. What is emerging as the best bet for reliability is true hardware RAID with active bad block checking PLUS a next gen filesystem like ZFS or btrfs with active checksumming. Aside from that, if you want reliability USE RAID 1 because with either hardware or software RAID, you can almost always pull out a drive if necessary and just hook it directly to your computer and read it. You can't do that with other RAID levels because the RAID has to mess with your data format. With RAID 1 most RAID systems these days keep their superblock at the end of the drive so as not to prevent using the drive in emergency WITHOUT the RAID controller. With Drobo, I tend to agree with George Ou. Robin seems to be accepting Drobo marketing hype at face value. I'm not sure thats possible.
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Another perspective
Richard Flude 19th May 2009
http://subnetmask255x4.wordpress.com/2008/10/28/sata-
unrecoverable-errors-and-how-that-impacts-raid/

Plus:
RAID is not a substitute for a backup strategy.
SATA RAID-1 has consistently performed well for me.

0 Votes
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Doesn't make sense
sanjaydhar 19th May 2009
How can an Unrecoverbale Read Error make the entire RAID set unusable?

If my RAID volume is in a degraded state (one failed drive) and I hit a URE on a block of data, from an application perspective I can see that I may not be able to read that file. How does this trash the whole RAID volume?
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re:Doesn't make sense
subnetmask255x4 20th May 2009
(A)bort, (R)etry, (I)gnore, (F)ail...

How many people remember that from using floppies with DOS?

Well, URE is not the same thing. If you are reading data from a drive and it runs into a corrupt/bad sector, it maps around it, and if it had data in it, then you end up with a corrupted file. However, you are not reading from the drive directly, you are reading from a RAID volume, and the RAID controller is then reading from the drive. It is then up to your RAID controller, be it hardware or software, to decide what to do next.

Your RAID controller can do one of two things:
1. try to act like this is a file corrupt error (less likely)
2. assume the drive is going bad, and mark it as such (more likely)

If you had a drive go bad, and your RAID controller was in the process of building data onto the hot spare, then if you run into a URE, you get 2 bad drives at once.
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Drive maintenance ...
George Mitchell 20th May 2009
The problem with this scenario is that it assumes the controller in question is NOT doing drive maintenance in the run up to the event. Most quality hardware RAID controllers are continuously scrubbing the disks in the background actively looking for bad blocks. Robin's scenario assumes that there are often bad blocks just lying around waiting for disaster to strike. But this is ONLY the case with software RAID. Data blocks don't usually just expire instantaneously. They decay over time. If your RAID controller is constantly running bad block routines and constantly monitoring SMART tools, this whole mess is almost not possible to happen. Those of us who come from a system admin background know this. But todays generation is hooked on cheap software RAID that masquerades as hardware RAID. And, yes, with that kind of architecture all sorts of nasty things can happen.
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soft vs. hard raid
emilp 21st May 2009
While I mostly agree, it is worth clarifying on
the SW vs. HW statement.

If you by software raid mean the all-to-common
bios aided raid junk, you are most likely
correct (I've never considered using them
enough to find out). On the other hand
software raid as implemented in e.g. linux MD
_does_ allow you to scrub your disks.
0 Votes
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it's not about bad blocks
subnetmask255x4 21st May 2009
URE does not mean that the drive hits a bad block. A bad block is when the data that is read from the drive, does not match the CRC checksums. This can be anywhere from "there was this one checksum error this one time," to the "data on the drive looks like line noise." In this case, the drive will try to read the data, send it while setting an error register, and tell the system "here is your data, it might be bad, and this block is no longer valid, I have moved what I could to this other block."

When the drive hits an URE, the drive can not read the data for some reason. This could be for any reason, but is not "the block is bad because it didn't pass CRC check."

This requires the drive to set either the ERR bit or the DF (Device Fault) bit. Note, the SATA specification does not indicate that the SError register to be set. This causes the drive to be in either an undefined error state, or a device fault state.

While the SATA spec does not define how such a state is required to be handled, the ATA spec does allow for the DF fault to be "only cleared by power cycling the drive."

http://www.t13.org/Documents/UploadedDocuments/docs2005/e05127r2-DF_Bit_Recovery.pdf

While the undefined ERR status is considered a persistent error, and can be handled by either the Freeze or Abort/Fail response. Note, this Abort/Fail is the same as the "drive is not physically present" error response.

Therefore, if a drive encounters a URE, the RAID controller, or software RAID, is supposed to consider it no longer functioning properly, and only re-connect to it, after a drive reset, if at all.
0 Votes
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Sans RAID ici
johnfenjackson@... Updated - 20th May 2009
Thanks for the discussions - they have helped me solidify my new storage strategy for implementation in 2010.

1. RAID 0 can give a performance boost suitable for work in progress. However I like the look of the OCZ Vertex SSD's where speed is key. [I note AK Hughes eliminated this immediately in his design of a performance PC: deciding to spend his money on the latest i920 CPU and mobo. instead. Faster disk and more memory would suit me better.]

2. The small volume of precious files I have will be backed up to the web, USB stick and ...

3. The convenience of DROBO or WINDOWS HOME SERVER is tempting but I am crossing them off for two reasons. Firstly cost: a DROBO 4-bay machine is almost 400 pounds Sterling without disks in the UK. Similarly the HP MediaSmart Server with 500GB. These prices, for a dedicated function, are greater than those for a general purpose computer. [I expect a decent OS on a decent OEM PC to have a decent file system, hot-swap drive trays and full backup capability now that we are in the 21st century - but that's for my M$ and OEM bashing posts.]

Secondly I think we should kick the RAID technology of DROBO and the proprietary scheme of WHS into touch. I am persuaded by George Ou's post that DROBO is in fact 'dynamic RAID 5', with a potentially dangerous rebuild activated not only by disk errors but also capacity increases. Nor do I wish to be locked in to any vendor's special treatment.

If the HP MediaSmart box is the best OEM's can do then I build my own ATOM-based file server ... or if I am a busy professional subcontract this job to my IT department or external IT shop. Moreover since the I in RAID stood for Inexpensive - which disks have now become - I build two servers and simply synchronise data between them, dispensing with RAID altogether, or effectively implementing RAID 1 in a slightly different form if you wish. One of these could function as a media centre PC, as per http://formortals.com/Home/tabid/36/EntryID/183/Default.aspx

4. Other half-baked thoughts (in addition to the half-baked ones above):

- with USB3 just around the corner at 10x the transfer rate I'm hoping DROBO3 will have a different architecture. Instead of the wimpy, monetising HP MediaSmart, something like ... http://www.theregister.co.uk/2009/05/15/dell_does_via_nano/
but for data storage. Of course there are only two chances of a properly-engineered, cost-effective, consumer-friendly design appearing: fat chance and slim chance.

- I wonder whether iSCSI to a dedicated SOLARIS box (for free ZFS) will be a decent option [given partly that M$ and OEM's lock up the iSCSI target in expensive Storage Server Solutions and that ZFS is more secure]; suggested the same to Jason Perlow to try out, with no feedback to date. Heaven forbid iSCSI to a MAC with ZFS wink

- CUCKU looks worthy of investigation. Indeed one of my neighbours might be delighted if I replaced his old PC with a new machine, so delighted that he might even share some of the cost which I was about to spend on data replication anyway. Invite with free beer coming soon happy
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RE: RAIDfail: Don't use RAID 5 on small arrays
snoop0x7b 19th Jul
@johnfenjackson@...

iSCSI to a ZFS volume would certainly be better than xsan. I'm of the opinion that anything BUT xsan is a good solution.
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Day of reckoning
wkulecz 20th May 2009
Let's see, one URE every 12.5TB, and after three more doubling of capacities every drive will be defective out of the box!

In any event, my real world experience is hard drives are far more reliable than any of the alternatives -- optical or tape.

Most cost effective for me remains system on RAID-1 (to minimize down time) and RAID-5 with backups of both to external drives.

Backing up the data is easy, but Windows makes it near impossible to back up all the work invested in installing and configuring the machine in the event of MB failure, each new version seems to make the task more daunting.

My development system remains Windows 2000 because I know the hacks needed to move the system to a bigger hard drive or a new motherboard.

But I'm moving to Linux wherever possible because of the ease of moving a system disk to another motherboard and booting it with only a minor amount (compared to Windows) of arcane knowledge required (mostly grub issues and Linux kernel chipset issues as to if the first disk should be /dev/hda or /dev/sda). At least it not malicious like Windows actively refusing to try any other disk drivers than the ones it installed with.
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ADG? SAS?
tdasherjr 20th May 2009
What a useless commentary with no mention of RAID 6/ADG or SAS drives. We do a lot of small RAID 5/6 but 95%+ is SAS which is a different world.
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RE: RAIDfail: Don't use RAID 5 on small arrays
cerving 20th May 2009
REAL servers using Xeon processors and midrange systems like the IBM iSeries routinely have RAID-5 arrays of 8 or more drives. The difference is that they aren't the cheap high-capacity SATA drives, they're the much more expensive SAS or SCSI-U320 drives that have lower capacity but much higher reliability.

What the author fails to recognize is that the chance of a double-drive read error is 1 in 12.5 TB SQUARED, so it's actually 1/1.56 ** 26, which is an incredibly small number. Single-drive errors do happen, but they are corrected before the data is ever presented.

Seagate's latest enterprise drives have a FAILURE rate of about .6% per year, so that means that 1 out of every 160 drives will fail each year. The reality is that they are a lot more reliable than that, but prudence dictates keeping a spare on-hand, or reserving a hot-swap drive in the array.

RAID-10's main advantage over RAID-5 is not reliability, it's performance when there's a lot of random drive reads going on. If you're not pegging your drive I/O, you're just as well off with RAID-5 and you save 50% on your drive costs.
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Chances of failure
cburkitt2 20th May 2009
You're right about the chances of a double read failure. However, in Robin's scenario, one drive has already failed. He is interested in the chances of a URE during the rebuild of the failed drive. A URE during the rebuild causes the entire rebuild process to fail. I'm not sure why. This seems to be a fault in the design of the RAID controller rather than an inherent flaw in RAID 5. Perhaps it's because the rebuild process knows nothing about the file system; it just looks at raw bits. If so, it seems like building additional intelligence into the rebuild process could fix the issue.
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Has this person been a sysadmin?
someitguy79 20th May 2009
Does anyone at ZDnet actually work in this industry? I have been a Sys Admin over a decade I have never had a URE in any form of RAID stop a rebuild permanently. I have them stop, alert require me to "sign off" on a rebuild anyway option. Notifying me of some basic corruption. Never had more than a file or two go bad. That is what backups are for. RAID is control the outage not replace the backup. With a hot-swap you can do without on outage at all.

Each form of RAID has its benefits and downfalls. I have even been somewhere which RAID was foregone because the drives couldn't handle the constant load of the poorly written app. Without RAID the system would run for months. With it 6 - 8 weeks. Yes tried different RAIDs and drives. We simply monitored SMART and replace the drive periodically. Mind you extremely rarely crazy situation.

Each disk in a RAID-5 contains a piece of parity stripe. Unlike RAID-3 where only one drive had it. Although I agree on the small RAIDs a one is better on economics and simplicity. But, not less reliable. At least w/ RAID 5 I get a checksum at that lower-level.

0 Votes
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someitguy79 contact me about RAID RECOVERY PLEASE
djzoey 20th May 2009
I have some questions about RAID recovery. You seem very knowledgable about RAID. Please email me at djzoeyphat@netscape.net. PLEASE.
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What about RAID-1 ??
websquad 20th May 2009
I have a pair of 10K RPM drives in RAID-1 (system + programs) and 7.2K RPM drives in RAID-1 (date).

Does that make sense for an individual workstation?
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The key advantage of RAID 1 is ...
George Mitchell 20th May 2009
With RAID 1 if the whole thing collapses and you can't restore (both drives dead), you STILL have a chance to recover most of your data. 1) You can try to copy data from each drive individually. 2) If that fails, you get two chances to image each drive off onto a new drive of the same size and THEN repair the file structure. You can actually do this with both drives and unless both failed at the same spot (or one is completely toast), you get ALL your data back. RAID 1 is ALWAYS the safer option with small arrays.
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potentally
someitguy79 21st May 2009
Then again I have seen corruption occur (caused by flaking disk) on one disk promptly get replicated to the good disk. The system tries to figure out which is "good" data. RAID-5 that would not happen the parity would fail checksum.

I have used both RAID's and fought with disk failure and corruption. Simply put RAID-1 is cheaper, faster and simpler. If a disk fails outright then with 1 you will notice it less. For most purposes esp. in a small business I use RAID-1.

However, in more 24/7 environments dealing with critical MS-SQL servers I would use RAID-5. Superior against corruption due to hardware malfunction. Its slower and more conservative also better at getting the corruption of data issue in less than total failure right.
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RE: RAIDfail: Don't use RAID 5 on small arrays
dave.schutz@... 20th May 2009
I've used RAID 5 for years, never seen more than one drive fail and was always able to replace that drive without data loss. I'm not saying the worst can't happen and I'd have multiple drives fail at once, but that's what backups are for.
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RE: RAIDfail: Don't use RAID 5 on small arrays
monkeyman1140@... 20th May 2009
If you really want added reliability don't use consumer grade drives in your array!
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Bingo!
snafu_77 25th May 2009
If the scsi drives used in servers had the failure rate of the consumer trash on the market any activity involving servers would be crippled.
I just had a 1tb sata drive die on me after 3 months. Nothing was on it that wasn't backed up or replaceable. I'm irritated but not mad. That drive wouldn't have lasted a week in a busy server in a business.


Backups, backups, backups. Backups are for ensuring the safety of data. RAID is for hardware failover. People should use the hardware failover option that best suits their needs, knowledge and budget.
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RAID 6 anyone
James Schroer 20th May 2009
So what about RAID 6? Take a bit more of a controller but you will be able to take two hits before going down. And your still getting better then the 50% usage in a RAID 1+0.
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Don't you love percentages?
terrykuntz 20th May 2009
I always love it when people play with percentages to make a point. In this case "Better to use RAID 1 and get 2/3rds the capacity of RAID 5" Do the math, 2/3 of 3/4 is 1/2. 2/3 the capacity of RAID5 sounds better than 1/2 the capacity of your drives.
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always use scrubing for raid5
emilp 21st May 2009
When you have one disk failed (completely, i.e.
fallen of the array) and you hit an URE, you
basically have two failed diska at that
possistion. RAID5 with two faulty disk is bad.
So to avoid the UREs you should turn on disk
scrubing, in order to catch these errors before
one disk fails completely.
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RE: RAIDfail: Don't use RAID 5 on small arrays
zdneter0000 21st May 2009
I use ZFS instead, like any thinking man.
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I've given up on RAID
LarryPTL Updated - 21st May 2009
There are 26 letters in the alphabet, that's plenty to implement a very simple backup scheme using single disk drives. At the end of the day I use a DOS batch file with the XCOPY command to copy all new and modified files from one disk drive to another. When I feel like it I'll turn on a USB drive and copy to both my internal backup drive and an external drive. The batch file has a command line in it for every letter in the alphabet starting with drive G (drives C through F are my boot partition, working data drive, DVD ROM, and DVD writable drives, respectively). I keep my boot partition separate from working data files so if I have to reload windows (I've done it a few times after viruses or malware clobbered my OS) I don't overwrite my data in the process.

My working data file is on my 2nd disk drive, I keep the internal backup of my data on the 2nd partition on the 1st disk drive. This way while windows is loading programs or page swapping on the 1st drive it can access data on the 2nd drive simultaneously using SATA's threading capabilities.

The XCOPY command is qualified by a simple 'IF' statement that looks for a pre-existing file on the drive letter. If the file exists, the XCOPY command executes; if it doesn't, it goes to the next drive letter in the alphabet.

For the USB drives, my copy of the image for drive C: is so large that it has never copied correctly, so I exclude the directory on the D: drive I store it in from the copy process. The file D:\DoNotCopyThisDirectory.txt has the name of the directory the C: drive partition image is stored in.

XCOPY D:\*.* G:\*.* /v /s /d /y /f /h /r /x /g /exclude=D:\DoNotCopyThisDirectory.txt

There's a shortcut to this batch file on my desktop that I double click. It takes less than five minutes for XCOPY to scan both drives and copy the stuff over, and about ten to fifteen minutes for each USB drive provided most of the files already exist on it.


To verify the files were correctly copied, I occasionally will run NTI Backup's file comparison utility.

If I add a new USB drive, I only have to manually install the one pre-existing file on it for the batch file to copy to it.
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RE: RAIDfail: Don't use RAID 5 on small arrays
bb_apptix 26th May 2009
> "The problem is that RAID 5 only protects against a single disk failure."

Uh... RAID 1 "only protects against a single disk failure" also...
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RAID 1 and single disk failure
subnetmask255x4 26th May 2009
Well, RAID 1 "can" protect against multiple disk failure, but it is pretty iffy.

Here is an example:
You have 4 drives, and split them into a RAID 1 with 2 sets of 2 drives (maybe even RAID 1+0). You get drives of A(1) and A(2) as the first set, and B(1) and B(2) as the mirror set. Now, you can loose both A(1) and A(2) and still have data. You can even loose A(1) and B(2) and be OK.

Simply, you can loose any two drives as long as the (#) is different. However, if you loose A(2) and B(2), or A(1) and B(1), then you are screwed.

So, you have to give up 1/2 of your space, to gain a 1/(n-1) chance of loosing your data on a second drive failure. At 4 drives, if you loose one drive, then if a second drive is lost (or a URE happens) you have a 1/3 chance of that being the drive which is the mirror of already failed drive, or a 2/3 chance of loosing a drive and everything being "A-OK."

Yeah, I like playing them kinda odds on my data. If you have up to 5 drives, then I would recommend RAID-5, odds of 2 drives being lost at once is still in the "low" range. Starting at 6 drives, you have enough space to go to RAID-6, which can loose 2 drives and keep on going.
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RE: RAIDfail: Don't use RAID 5 on small arrays
AQuyen 4th Dec 2009
Drobo is too slow for production server anyway - it's a
perfect back up device...

There is a Hardware RAID5 five drive available, no driver
needed
http://www.datoptic.com/store/hardware-raid5-five-drive-
port-multiplier-system-ebox-r5-1.html

Just plug and play
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What is he smoking?
Alex G. (DV411) Updated - 29th Jun 2010
Is RAID0 plus backup better than RAID5 plus backup - on small arrays? The article sure makes it sound like it. Then the question is, REALLY? WHY? Because sometimes two drives fail simultaneously? How does THAT make it better than RAID5?

What is he smoking?
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RE: RAIDfail: Don't use RAID 5 on small arrays
snoop0x7b 19th Jul
@Alex G. (DV411)

He's clueless. He's one of those people who thinks that if you have RAID5 you don't need to back up because the shared parity is a backup. And so instead he recommends RAID0 with backups, because clearly, all backups preserve the instantaneous state of your data at any given time! JUST LIKE PARITY!
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RE: RAIDfail: Don't use RAID 5 on small arrays
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RE: RAIDfail: Don't use RAID 5 on small arrays
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RE: RAIDfail: Don't use RAID 5 on small arrays
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