RAID Primer: What's in a number?
by Dave Robinet on September 7, 2007 12:00 PM EST- Posted in
- Storage
RAID 0
RAID 0 takes two or more disk drives and writes data in a "stripe" across each disk. Data is accessed by requesting the stripe from the array, resulting in the disks more or less simultaneously feeding their portion of the data back to the controller. The overall capacity of the array is equal to the sum of the formatted capacities of all drives, and disk usage is more or less spread evenly among all drives in the array.
The net result is that the system will see much faster sustained transfer rates for both read and write operations compared to a single drive. File access time, however, is not measurably improved by leveraging multiple disks in a RAID 0 set, which means that systems which require frequent access of small, non-contiguous files (as is often the case in desktop configurations) generally do not benefit from RAID 0.
RAID 0 is an excellent choice for video editing and large-scale "solving" applications, where large files need to be read and written in a continuous manner.
Perhaps the greatest drawback to RAID 0 is that the arrays are rendered inaccessible when a single drive in the array fails. In that sense, RAID 0 isn't actually RAID at all, as it lacks the "Redundant" part of the equation. Data reliability and retention is decreased exponentially as drives are added to a RAID 0 setup, so unless frequent backups are made - or if the data is not regarded as even remotely important - RAID 0 should be approached with caution.
Pros:
RAID 1 sits at the other extreme of the spectrum. It makes a continuous copy of all data from one disk (which is written to and read from by the system) onto another physical disk which is in "standby" mode. This "standby" disk is held in reserve by the controller for when a failure is detected on the first disk. At that point in time, the controller "fails over" to the second disk in the system, with all data still available to the user.
While RAID 1 usually offers no performance benefits (and indeed, it often slightly degrades performance in some situations), it does increase the uptime of the host computer by allowing it to remain online even after a disk in the system has failed. This makes it an extremely popular option for mirroring operating systems on enterprise-class servers, and for small office users without the need for massive amounts of data storage but a requirement for constant uptime.
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 (both drives can read different data simultaneously). Most consumer RAID 1 controllers do not provide this level of sophistication, however, resulting in performance that is at best slightly worse than what would be achieved with a single drive. Software RAID 1 solutions also lack support for reading from both drives in a RAID 1 set simultaneously.
Pros:
RAID 0 takes two or more disk drives and writes data in a "stripe" across each disk. Data is accessed by requesting the stripe from the array, resulting in the disks more or less simultaneously feeding their portion of the data back to the controller. The overall capacity of the array is equal to the sum of the formatted capacities of all drives, and disk usage is more or less spread evenly among all drives in the array.
The net result is that the system will see much faster sustained transfer rates for both read and write operations compared to a single drive. File access time, however, is not measurably improved by leveraging multiple disks in a RAID 0 set, which means that systems which require frequent access of small, non-contiguous files (as is often the case in desktop configurations) generally do not benefit from RAID 0.
RAID 0 is an excellent choice for video editing and large-scale "solving" applications, where large files need to be read and written in a continuous manner.
Perhaps the greatest drawback to RAID 0 is that the arrays are rendered inaccessible when a single drive in the array fails. In that sense, RAID 0 isn't actually RAID at all, as it lacks the "Redundant" part of the equation. Data reliability and retention is decreased exponentially as drives are added to a RAID 0 setup, so unless frequent backups are made - or if the data is not regarded as even remotely important - RAID 0 should be approached with caution.
Pros:
- Excellent streaming performance
- Maximum capacity available for users (sum of all disks)
- No redundancy of data
- Negligible performance benefits for many users
RAID 1 sits at the other extreme of the spectrum. It makes a continuous copy of all data from one disk (which is written to and read from by the system) onto another physical disk which is in "standby" mode. This "standby" disk is held in reserve by the controller for when a failure is detected on the first disk. At that point in time, the controller "fails over" to the second disk in the system, with all data still available to the user.
While RAID 1 usually offers no performance benefits (and indeed, it often slightly degrades performance in some situations), it does increase the uptime of the host computer by allowing it to remain online even after a disk in the system has failed. This makes it an extremely popular option for mirroring operating systems on enterprise-class servers, and for small office users without the need for massive amounts of data storage but a requirement for constant uptime.
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 (both drives can read different data simultaneously). Most consumer RAID 1 controllers do not provide this level of sophistication, however, resulting in performance that is at best slightly worse than what would be achieved with a single drive. Software RAID 1 solutions also lack support for reading from both drives in a RAID 1 set simultaneously.
Pros:
- Redundancy of data
- Lowest cost data redundancy available (one additional disk)
- Simple operations make it easy to implement solution using software only
- Poor usage of drive capacity (only 50% of purchased hard drive capacity available)
- Typically no performance benefit over a single hard disk
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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)