RAID Primer: What's in a number?
by Dave Robinet on September 7, 2007 12:00 PM EST- Posted in
- Storage
RAID 1+0 / 0+1
RAID 1+0 (10) or 0+1 attempts to get the best of all worlds: It generally provides the best read and write performance, as well as offering a level of redundancy for its data when compared to RAID 0.
Both RAID 0+1 and RAID 1+0 are considered "nested" solutions, which is to say they use RAID 0's data striping and RAID 1's mirroring capabilities. The difference between the two is that RAID 1+0 (10) creates a striped set from a series of mirrored drives while RAID 0+1 creates a second striped set to mirror the primary striped set.
In practice, the only reason an administrator would choose either RAID 0+1 or 1+0 (10) is in extremely I/O intensive operations which would bottleneck a RAID 5 or RAID 6 array, and where drive cost is not a major concern. The redundancy provided is in reality very low although RAID 1+0 offers better fault tolerance and rebuild capabilities than 0+1.
In an RAID 1+0 array all but one drive from each RAID 1 set could fail without damaging the mirrored data. However, if the failed drive or drives is not replaced, the last working drive in the set then becomes a single point of failure for the entire array. So if that single hard drive fails, all data stored in the entire array is then lost.
The RAID 0+1 array can operate if one or more drives (greater than 4 drives utilized) fail in the same mirror set, However, if two or more drives fail on either side of the mirroring set, then data on the entire array is lost. Also, once a failed drive is replaced, in order to rebuild its data all the disks in the array must participate in the rebuild. In the case of RAID 1+0, it only has to re-mirror the lost drive so the rebuild process is substantially faster.
Pros:
In the IT world, some level of RAID is virtually guaranteed to be employed on any production server due to the relatively high failure rate of hard disks compared with most other components in the system. For end-users, though, the picture becomes far murkier. Most home computers occupy large amounts of time seeking from small file to small file, with the resulting speed limitation imposed by the physical mechanisms of the drive itself (rotational speed, etc). These limitations are not overcome even by the top-performing RAID 0. The only benefits, therefore, that users can seek in RAID are to increase overall capacity of their single drive, add a level of redundancy for their system, or to improve large-file performance.
The attraction of RAID for users seeking a large single drive is diminishing by the day, due to the massive single drive sizes on the market today. When a capacity conscious user can get a full terabyte of space in a single physical package, the argument becomes one of backing up said data, rather than seeing a 2TB drive on their system.
In the case of redundancy, there is most certainly an argument for taking advantage of the RAID 1 feature found on many motherboards (and even in most operating systems). As stated previously, most users have experienced a hard drive failure at one point in their lives, and as more of our daily work shifts to a computing platform, data integrity is becoming increasingly important. More to the point, however: Should users be more worried about backing up their data to removable media on a periodic basis to protect against the accidental deletion or corruption of data, or in keeping their machine up and running when a complete failure occurs?
This type of question can only be answered by the individual user themselves, and depends on the nature of data being stored on the system. We recently provided a first look at Windows Home Server, which may prove to be a far more compelling backup solution than any form of RAID. That does require the use of an entire computer, but the user-controlled data mirroring, volume shadow copy, and the ability to support multiple systems certainly make it a viable alternative in households with multiple computers.
It also bears mention that redundant storage of data using RAID really isn't a sufficient backup strategy for most businesses, and some form of off-site storage of backups should also be considered. RAID can be useful in making sure that systems remain operational in the event of a hard drive failure, but other catastrophes -- flooding, fire, theft, etc. -- can still claim all of the data on a RAID storage device. If the data is truly important, saving periodic backups to a different medium and storing it at a separate location should be considered.
Large-file performance is likely the most compelling reason to adopt RAID in a home system. For video editing operations, bandwidth in write operations is an absolute must, and RAID 0 fills this need very well. Increasingly, however, hard drives are finding their way into new areas of the home - home theater PCs, PVRs, and home video archival systems are but a few of the "read-often, write-less but always needed" systems which could benefit from a solution like RAID 5 or even the more performance oriented RAID 5+1.
At the end of the day, anyone looking into a more elaborate storage solution owes it to themselves to consider the practical implication of the decisions they make. One size most definitely does not fit all in the world of hard drive storage and RAID, and the wrong choice can certainly be more harmful than helpful in this regard.
We would like to thank Adaptec for providing the charts utilized in our article today.
RAID 1+0 (10) or 0+1 attempts to get the best of all worlds: It generally provides the best read and write performance, as well as offering a level of redundancy for its data when compared to RAID 0.
Both RAID 0+1 and RAID 1+0 are considered "nested" solutions, which is to say they use RAID 0's data striping and RAID 1's mirroring capabilities. The difference between the two is that RAID 1+0 (10) creates a striped set from a series of mirrored drives while RAID 0+1 creates a second striped set to mirror the primary striped set.
In practice, the only reason an administrator would choose either RAID 0+1 or 1+0 (10) is in extremely I/O intensive operations which would bottleneck a RAID 5 or RAID 6 array, and where drive cost is not a major concern. The redundancy provided is in reality very low although RAID 1+0 offers better fault tolerance and rebuild capabilities than 0+1.
In an RAID 1+0 array all but one drive from each RAID 1 set could fail without damaging the mirrored data. However, if the failed drive or drives is not replaced, the last working drive in the set then becomes a single point of failure for the entire array. So if that single hard drive fails, all data stored in the entire array is then lost.
The RAID 0+1 array can operate if one or more drives (greater than 4 drives utilized) fail in the same mirror set, However, if two or more drives fail on either side of the mirroring set, then data on the entire array is lost. Also, once a failed drive is replaced, in order to rebuild its data all the disks in the array must participate in the rebuild. In the case of RAID 1+0, it only has to re-mirror the lost drive so the rebuild process is substantially faster.
Pros:
- Best performance available, as the system disk is essentially a RAID 0 array.
- Expensive in terms of drives.
- Usable storage space is only half of the total drive capacity.
- Only minimally fault tolerant.
In the IT world, some level of RAID is virtually guaranteed to be employed on any production server due to the relatively high failure rate of hard disks compared with most other components in the system. For end-users, though, the picture becomes far murkier. Most home computers occupy large amounts of time seeking from small file to small file, with the resulting speed limitation imposed by the physical mechanisms of the drive itself (rotational speed, etc). These limitations are not overcome even by the top-performing RAID 0. The only benefits, therefore, that users can seek in RAID are to increase overall capacity of their single drive, add a level of redundancy for their system, or to improve large-file performance.
The attraction of RAID for users seeking a large single drive is diminishing by the day, due to the massive single drive sizes on the market today. When a capacity conscious user can get a full terabyte of space in a single physical package, the argument becomes one of backing up said data, rather than seeing a 2TB drive on their system.
In the case of redundancy, there is most certainly an argument for taking advantage of the RAID 1 feature found on many motherboards (and even in most operating systems). As stated previously, most users have experienced a hard drive failure at one point in their lives, and as more of our daily work shifts to a computing platform, data integrity is becoming increasingly important. More to the point, however: Should users be more worried about backing up their data to removable media on a periodic basis to protect against the accidental deletion or corruption of data, or in keeping their machine up and running when a complete failure occurs?
This type of question can only be answered by the individual user themselves, and depends on the nature of data being stored on the system. We recently provided a first look at Windows Home Server, which may prove to be a far more compelling backup solution than any form of RAID. That does require the use of an entire computer, but the user-controlled data mirroring, volume shadow copy, and the ability to support multiple systems certainly make it a viable alternative in households with multiple computers.
It also bears mention that redundant storage of data using RAID really isn't a sufficient backup strategy for most businesses, and some form of off-site storage of backups should also be considered. RAID can be useful in making sure that systems remain operational in the event of a hard drive failure, but other catastrophes -- flooding, fire, theft, etc. -- can still claim all of the data on a RAID storage device. If the data is truly important, saving periodic backups to a different medium and storing it at a separate location should be considered.
Large-file performance is likely the most compelling reason to adopt RAID in a home system. For video editing operations, bandwidth in write operations is an absolute must, and RAID 0 fills this need very well. Increasingly, however, hard drives are finding their way into new areas of the home - home theater PCs, PVRs, and home video archival systems are but a few of the "read-often, write-less but always needed" systems which could benefit from a solution like RAID 5 or even the more performance oriented RAID 5+1.
At the end of the day, anyone looking into a more elaborate storage solution owes it to themselves to consider the practical implication of the decisions they make. One size most definitely does not fit all in the world of hard drive storage and RAID, and the wrong choice can certainly be more harmful than helpful in this regard.
We would like to thank Adaptec for providing the charts utilized in our article today.
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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)