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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tynopik - Saturday, September 8, 2007 - link
> 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).unRAID
http://www.lime-technology.com/">http://www.lime-technology.com/
tynopik - Saturday, September 8, 2007 - link
i should point out that1. it does NOT join your drives together into one volume, each drive is separate (this is basically necessary for what you want unless you go the WHS route)
2. it has to be run on a dedicated system that it turns into NAS (you can't run it on your main desktop for instance)
that said, i really like the idea, almost all of the advantages of the WHS mechanism but much more space efficient (in most cases, i assume the largest drive will always be 'lost' to parity data)
Dave Robinet - Saturday, September 8, 2007 - link
Really, you're looking for something that is several RAID 1 mirrors of single volumes.I can think of nothing off-the-shelf that fits all those needs, though "rolling your own" may help:
- Buy two drives. Create one large partition (say, D:) on drive 1. Mirror that.
- Buy two more drives. Create another large partition (say, E:) on drive 3. mirror that.
Etc, etc.
It's still the same volume, but if you do it using software, the two drives won't be dependent on each other in any way.
If you tear one of those drives out of your computer and slap it onto another one (USB connector, etc), then it'll come up just fine, with or without the mirror.
It's inelegant, and really not something I'd ever push on someone - but you've come up with a kind of oddball request, there. :) Might I ask what it's for? Maybe your criteria can be adjusted in some way.
tynopik - Saturday, September 8, 2007 - link
> but you've come up with a kind of oddball request, there. :) Might I ask what it's for? Maybe your criteria can be adjusted in some way.i understand what he's getting at
he wants protection from drive failure, so a 'parity drive' that can rebuild any one drive that fails is handy
but he's also concerned about losing more than 1 drive simultaneously
having just a plain filesystem on the disk is far more robust than any sort of striping system as worst comes to worst you can just yank any surviving drives and recover what's on them
- a series of raid 1 arrays (like what you described) works but isn't particularly flexible (need equal sized drives)
- WHS is more flexible and powerful but it still requires double the amount of storage (EXPENSIVE)
- this only requires 1 extra drive and allows it to backup any number of other drives
it comes from a desire for some protection but not being able or willing to spend enough for true duplication plus wanting something that fails gracefully (ie not raid5)
i would actually like something like that for my system, there's a chance of recovering everything, but if it hits the fan i'll be able to at least recover something plus it's not that expensive
don't forget there may be physical limitations. if you have 4 physical drives filled with data, you might have enough room and power connectors for a 5th drive, but not for 4 more
Sudder - Sunday, September 9, 2007 - link
tnx, this goes a big step in the right direction
since unraid uses slackware there should be (at least in theory) a possibility to do this with the linux "Logical Volume Manager" (allthough one would probably have to do some work so that the TOC is saved on every disc to still being able to access the data if some of the other discs are gone)
But even without, seperate volumes and the option to access them one by one, by mounting the ReiserFS, is good enough for me.
and that's a big downside.
When I have some time I'll probably try to run it in a virtual machine (the "use a physical disc" option in VM should reduce the perfomance-penaltys significantly), but I'm not that optimistic that this will also work with the "bigger", non-free Versions that can handle more than 3 discs (e.g. handling of the registration Key, since I allready ran into some pre-boot USB Issues with VM when I tried to test the bitlocker-feature of Vista in a VM - although it just might have been my old stick or my USB-contoller ..)
yes (that's kind of a given) - the option to use discs of different sizes is a nice bonus though, since the array can now grow more "organically" over time (you just buy the disc with the best cost/gig ratio at the moment you need it without limmiting yourself to one size like with RAID 5)
with the port-multiplier Option of SATA II (up to 15 drives per cable) and an external casing I think there are ways to cope (and if you plan in advance to have X bays/connectors avalable, you just have to start a new array if the old one is full - which might be a good idea anyway as soon as you come close to double digit disc-numbers - although, that might take some time with modern disc sizes ;-) )
Again: I don't want nessecarily to being able to access my data all the time, I just want to switch from my current "DVD-storrage" to a "HD-storrage".
So what I'm looking for now is the funktionallity of unRAID (without the limitation of the drive nuber), being able to run in a VM and for free ;-).
I allready checked freeNAS and NASlite but they all seemed to be fixed either on RAID and/or JBOD without parity .. any suggestions?
tynopik - Sunday, September 9, 2007 - link
> with the port-multiplier Option of SATA II (up to 15 drives per cable)and which consumer level products support port-multipliers?
it's an optional part of the spec and most don't implement it
if you're willing to do a lot of extra work and hassle and really want offline storage you can fill a bunch of external drives with a virtual filesystem (like truecrypt for instance) and then with them all connected run par to build a par file across all your virtual disk files
disadvantages are numerous, have to be able to connect all disks at once, if you update one little piece of one drive have to recalculate the par file across all of them, etc
Sudder - Sunday, September 9, 2007 - link
most e-sata ports support it (although some controllers like the ones using JMB36X just support "Command-based Switching", e.g. all sollutions with Sil 3132 chips (e.g. many notebook S-ata - PCMCIA adapters) even support "FSI-based Switching" which works a little like SCSI (the command is sent to one disc and then the bus is free again, so you can get "close" to the theoretical 300 MB/sec with multiple discs and the bus is not blocked by one working disk (with 4 discs connected, a test showed still 40MB/sec transfer from each of the 4 parallel working discs ..)
so take AFAIKR one of the many new gigabyte mo-boards with 2 ports that can be used as e-sata, put e.g. a "Dawicontrol DC-6510 PM" on the other end of the cable (one is about 100 bucks) add a powersupply and a housing and you are good for 10 extra discs ..
look at more recent motherboards (e-sata slowly shows up on more and more boards) and you'll find that it's supported more and more
well, I'm kind of too lazy to to the par thing each and every time I just change one little file - or to be more practical, I can verry much immagine myself pushing the rebuild further and further into the future as long as I can forsee that I will add more stuff in the verry near future which then again will require a rebuild .. (if I don't find a usable sollution which does it "on the fly" I'll probably end up doing it "by hand" (evetually by adding a small RAID 5 "file-buffer" to my System to strech the write/par Intervalls) but I _really_ would prefer an automatic solution without a most likely multi-hour rebuild process (reading all discs, calculating and writing the hole par-disk) after each little change ..)
Witling - Saturday, September 8, 2007 - link
Something I don't usually see in articles on Raid is the complete lack of protection from failure due to a virus or installation of a bad driver. Both disks get corrupted.I am a home user of Raid 1 through a controller built in to the motherboard using a popular Redmond Washington operating system.
Dave Robinet - Saturday, September 8, 2007 - link
Yep - I touched briefly on this in the last part of the article.Users need to look closely at if an ARCHIVAL system (tape, etc) is better for their needs than RAID 1.
Let's face it - RAID 1 is for "(almost) always on / critically needed to be working when powered up" configurations ONLY. How many home computers fall into this category... really?
kobymu - Saturday, September 8, 2007 - link
In certain cases RAID 1 will give you better read performance.