RAID Primer: What's in a number?
by Dave Robinet on September 7, 2007 12:00 PM EST- Posted in
- Storage
Introduction
The majority of home users have experienced the agony of at least one hard drive failure in their lives. Power users often experience bottlenecks caused by their hard drives when they try and accomplish I/O-intensive tasks. Every IT person who has been in industry for any length of time has dealt with multiple hard drive failures. In short, hard drives have long caused the majority of support headaches in standard desktop or server configurations today, with little hope of improvement in the near term.
With the increased use of computers in the daily lives of people worldwide, the dollar value of data stored on the average computer has steadily increased. Even as MTBF figures have moved from 8000 hours in the 1980s (example: MiniScribe M2006) to the current levels of over 750,000 hours (Seagate 7200.11 series drives), this increase in data value has offset the relative decrease of hard drive failures. The increase in the value of data, and the general unwillingness of most casual users to back up their hard drive contents on a regular basis, has put increasing focus on technologies which can help users to survive a hard drive failure. RAID (Redundant Array of Inexpensive Disks) is one of these technologies.
Drawing on whitepapers produced in the late 1970s, the term RAID was coined in 1987 by researchers at the University of California, Berkley in an effort to put in practice theoretical gains in performance and redundancy which could be made by teaming multiple hard drives in a single configuration. While their paper proposed certain levels of RAID, the practical needs of the IT industry have brought several slightly differing approaches. Most common now are:
RAID 0 - Data Striping
RAID 1 - Data Mirroring
RAID 5 - Data Striping with Parity
RAID 6 - Data Striping with Redundant Parity
RAID 0+1 - Data Striping with a Mirrored Copy
Each of these RAID configurations has its own benefits and drawbacks, and is targeted for specific applications. In this article we'll go over each and discuss in which situations RAID can potentially help - or harm - you as a user.
The majority of home users have experienced the agony of at least one hard drive failure in their lives. Power users often experience bottlenecks caused by their hard drives when they try and accomplish I/O-intensive tasks. Every IT person who has been in industry for any length of time has dealt with multiple hard drive failures. In short, hard drives have long caused the majority of support headaches in standard desktop or server configurations today, with little hope of improvement in the near term.
With the increased use of computers in the daily lives of people worldwide, the dollar value of data stored on the average computer has steadily increased. Even as MTBF figures have moved from 8000 hours in the 1980s (example: MiniScribe M2006) to the current levels of over 750,000 hours (Seagate 7200.11 series drives), this increase in data value has offset the relative decrease of hard drive failures. The increase in the value of data, and the general unwillingness of most casual users to back up their hard drive contents on a regular basis, has put increasing focus on technologies which can help users to survive a hard drive failure. RAID (Redundant Array of Inexpensive Disks) is one of these technologies.
Drawing on whitepapers produced in the late 1970s, the term RAID was coined in 1987 by researchers at the University of California, Berkley in an effort to put in practice theoretical gains in performance and redundancy which could be made by teaming multiple hard drives in a single configuration. While their paper proposed certain levels of RAID, the practical needs of the IT industry have brought several slightly differing approaches. Most common now are:
RAID 0 - Data Striping
RAID 1 - Data Mirroring
RAID 5 - Data Striping with Parity
RAID 6 - Data Striping with Redundant Parity
RAID 0+1 - Data Striping with a Mirrored Copy
Each of these RAID configurations has its own benefits and drawbacks, and is targeted for specific applications. In this article we'll go over each and discuss in which situations RAID can potentially help - or harm - you as a user.
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ShadowFlash - Monday, March 2, 2009 - link
RAID 10 not as fault tolerant as RAID 5 ??? unlikely...RAID 5 is used when capacity cannot be sacrificed for the increased data protection of RAID 10. Yes, RAID 0+1 is horrible, and should be avoided as mentioned in other posts. RAID 10 sets will absolutely rebuild faster than a RAID 5 in almost all situations. With the dirt cheap pricing of modern large capacity drives, I can think of almost no situation where RAID 5 is preferable to RAID 10. The flaw is in the way hard drives die, and parity. I was going to type out a long explanation, but this link covers it well.http://miracleas.com/BAARF/RAID5_versus_RAID10.txt">http://miracleas.com/BAARF/RAID5_versus_RAID10.txt
I strongly urge any home user not to use RAID 5 ( or any other parity form of RAID ). RAID 5 is antiquated and left over from the days when cost vs capacity was a major concern. RAID 10 also dosen't require as expensive of a controller card.
And remember if you do insist on RAID 5 to never use it as a system disk. The parity overhead from the many small writes an OS performs is far too great a penalty.
I'm not trying to start a fight, just trying to educate on the flaws of parity.
Codesmith - Sunday, September 9, 2007 - link
The drives in my 2 drive RAID 1 array are 100% readable as normal drives by any SATA controller.With any other RAID configuration you are dependent on both remembering the proper settings, performing the rebuild properly and most importantly, finding a compatible controller.
Until the manufactures decide to standardize, the system you have in place to protect your data could have you waiting days to access your data.
I am planning to add a RAID 5/6 array for home theater usage, but the business documents are staying on the RAID 1 array.
Anonymous Freak - Saturday, September 8, 2007 - link
That's my acronym for it. It also describes my desire for it.RAID = Redundant Array of Independent Disks.
AIDS = Array of Independent Disks, Striped.
"RAID" 0 has very few legitimate uses. If you value the data stored at all, and have any care at all about uptime, it's inappropriate. If all you want is an ultra-fast 'scratch' disk, it is appropriate. Before ultra-large drives, I used a RAID-0 of 9 GB, 10k RPM SCSI drives as my capture and edit partition for video editing, and that's about it. Once the editing was done, I wrote the finished file back out to DV tape, and transcoded to something more manageable for computer use, and storage on my main ATA hard drive.
MadAd - Saturday, September 8, 2007 - link
[quote]"Higher quality RAID 1 controllers can outperform single drive implementations by making both drives active for read operations. This can in theory reduce file access times (requests are sent to whichever drive is closer to the desired data) as well as potentially doubling data throughput on reads"[/quote]Its not the best place to post here I know, but as a home user with a 1tb 0+1 pata array on a promise fastrack (on a budget) I was thinking of looking on ebay for a reliable replacement controller with the above characteristics, but dont know what series cards are both inexpensive for a second user now and fit an x32 pci.
Thanks a lot
tynopik - Saturday, September 8, 2007 - link
> but as a home user with a 1tb 0+1 pata array on a promise fastrack (on a budget) I was thinking of looking on ebay for a reliable replacement controller with the above characteristics, but dont know what series cards are both inexpensive for a second user now and fit an x32 pci.saying you currently have a 0+1 array i assume you have at least 4 drives, probably 4 500gb drives
since 0+1 provides the speed of raid0 with the mirroring of raid1 i'm not sure what you're looking for. if you went for a straight raid1 solution your system would see 2 500gb volumes instead of 1 1tb volume.
and not sure what you mean by x32 pci, just a regular pci slot? if you're talking about PCIe they only go to x16 and can't say i'm aware of any 'reasonable' card that uses more than x8
MadAd - Sunday, September 9, 2007 - link
urm....no4x250 and im wondering what enterprise class controller is cheap on ebay that uses pata drives, an x32 pci slot
(not pcie, see- http://en.wikipedia.org/wiki/Peripheral_Component_...">http://en.wikipedia.org/wiki/Peripheral_Component_...
and performs as quoted from the article, because my promise controller is good but still a home class controller. Just i dont know the enterprise segment at all and I thought some of these guys would.
tynopik - Sunday, September 9, 2007 - link
> 4x250then you aren't using raid0+1, just raid0
> (not pcie, see- http://en.wikipedia.org/wiki/Peripheral_Component_...">http://en.wikipedia.org/wiki/Peripheral_Component_...
it's the x32 that is confusing
if you search that page you will see x32 doesn't show up anywhere on it
i'm going to assume you just mean 32-bit PCI which is standard which is what practically every motherboard manufactured today has at least one of
but still i can't answer your question about which PCI (no need to say 32-bit, it's assumed) raid controllers support IDE drives with enhanced read speed, sorry
MadAd - Sunday, September 9, 2007 - link
http://en.wikipedia.org/wiki/Peripheral_Component_...">http://en.wikipedia.org/wiki/Peripheral_Component_...Zak - Saturday, September 8, 2007 - link
I gave up on RAID "for protection" a long time ago. I tried everything from software RAID to on-board 1 and 5, and to $300 cards with controllers and on-board RAM and 5 hard drives. It is not worth the hassle for home use or even small business, period. I absolutely agree with that article from Pudget computers. I had more problems due to raid controllers acting up than hard drive failures. RAID will nor protect you against directory corruption, accidental deletion and infections - things that happen A LOT more often than hard drive failures. RAID adds level of complexity that involves extra maintenance and extra cost.My current solution is two external FW or USB drivers. I run two redundant Retrospect backups every 12 hours, one right after another that backs up my storage drive plus one at night that mirrors the drive to another. It's probably an overkill but I'll take it over any RAID5 any time: three separate drives, three separate file systems - total protection against file deletion, directory corruption and infections (the external drives are dismounted between backups. I do the same on Macs and PCs.
I still may use RAID0 for scratch and system disks for speed, but my files are kept on a separate single drive that gets triple backup love.
Zak
Sudder - Saturday, September 8, 2007 - link
Hi,can anybody point me in the right direction?
I want to switch from backing up my porn (lets call it data ;-) ) on DVD to saving my data on HDs (since cost/gig arn't that far appart anymore and bue-ray will IMHO not catch up fast enough in regards of cost/gig to be a good alternative for me).
But since loosing 1 HD (which can always happen) puts one back a couple of 100gigs at once I want some redundance.
Going for RAID 5 (I'm not willing to spend the extra money on RAID 6) has the huge disatvantage (for _my_ scenario) that if I loose 2 HDs (which also might happen since I plan to store the discs "offline" most of the time) _all_ my data is gone.
So I'm looking for a solution which stores my data in a "normal" way on the discs + one extra disk with the parity (somewhat like RAID 3 but without the striping).
I don't care about read/write speed too much, I just want the redundance and the cost effectiveness of RAID 5 (RAID 1 would also be too expensive for me) but without the danger of loosing all if more than 1 disc is gone*. Also, if I just want to read some data this way it should be sufficent to plug in just one disc instead of the whole array with RAID 5.
So, does anyone know if such a Sollution is allready implemented somewhere? (it also should be able to calculate the parity "on the fly" so that if I change one single file on one of the discs I don't have to wait until the parity of the whole array is recalculated but just for the corresponding sectors that have actually changed)
* this solution isn't that much better than RAID 5 with small arrays, but the more discs there are in the array, the more data will survive if 2 (or even more) discs die - with RAID 5 all is lost (and going for multiple 3 disc RAID 5 arrays isn't verry cost effective)