RAID Levels

RAID (Redundant Array of Independent Disks) is a set of methods (or levels) for storing data on a set of disks to improve performance, reliability, or both.

RAID levels 0, 1 and 5 are most common. RAID is most commonly implemented in hardware controllers. A RAID controller appears to a host computer just as any other storage device would. Behind the hardware RAID controller is a group of disk drives. Depending on which RAID level the controller is configured for, it will store data on the disks in different ways.

The common RAID levels have the following characteristics:

• RAID Level 0 -- "striping/concatenation". RAID Level 0 is not redundant, hence does not truly fit the "RAID" acronym. In level 0, data is split across drives, resulting in higher data throughput. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in data loss.  RAID level 0, has the lowest cost of any RAID organization because it does not employ redundancy at all. This scheme offers good performance since it never needs to update redundant information. Surprisingly, it does not have the best performance in either read or write operations. Redundancy schemes that duplicate data, such as mirroring, can perform better on reads by selectively scheduling requests on the disk with the shortest expected seek and rotational delays. Without, redundancy, any single disk failure will result in data-loss. Non-redundant disk arrays are widely used in environments where performance and capacity, rather than reliability, are the primary concerns. The size of a data block, which is known as the "stripe width", varies with the implementation. When it comes time to read back data stored in RAID level 0, all disks can be read in parallel. In this case the overall performance is the combination of the performance of each disk. Striping also reduces the chance that one disk will be subject to much more load than others (a ``hot-spot''). There is no reliability or redundancy added by this level.
   
• RAID Level 1 -- "mirroring". Mirroring remains in enterprise environment popular due to its simplicity and high level of data availability. It uses twice as many disks as a non-redundant disk array.  Those two disk care called submirrors (it can be also more then two submirros, but this implementation is used very infrequently).   Whenever data is written to a submirror A the same data is also written to submirror B, so that there are always two copies of the information. When data is read, it can be retrieved from the disk with the shorter seek and rotational delays. If a disk fails, the other copy is used to service requests. Mirroring is frequently used in database applications where availability and transaction time are more important than storage efficiency. The cost of disk storage per megabyte doubles in RAIL level 1: The total usable storage space is half the physical storage capacity because half the disks are mirrored on the other half. Reliability is maximized because every disk has a duplicate. If a disk fails, its mirrored copy can be synced to a spare or replacement disk. The device continues operating during recovery. If implemented in hardware, RAID 1  improves disk performance too. 
• RAID Level 5 -- provides fault tolerance by distributing parity information across some or all of an array's member disk drives. RAID-5 is a good choice in multi-user environments which are not write performance sensitive. However, at least three, and more typically five drives are required for RAID-5 arrays. Reads substantially outperforms small writes. Level 5 is often used with write-back caching to reduce the asymmetry. The block-interleaved distributed-parity eliminates the parity disk bottleneck by distributing the parity information uniformly over all of the disks. Another advantage to distributing the parity information is that it also distributes data over all of the disks rather than over all but one. This allows all disks to participate in servicing read operations in contrast to redundancy schemes with dedicated parity disks in which the parity disk cannot participate in servicing read requests. Block-interleaved distributed-parity disk array have the best small read, large write performance of any redundancy disk array. Small write requests are inefficient compared with  mirroring due the need to perform read-modify-write operations to update parity. This is the major performance weakness of RAID level 5 disk arrays.
  
  RAID-5 increases reliability allowing any one disk to fail without losing information. The space overhead is low. Exclusive OR or ``parity'' values are calculated for each block stripe within the disk group. Blocks containing the parity values are interleaved among all the disks. If one disk fails, the data which was on that disk can be reconstructed using the data and parity blocks from the operational disks.
  
  Follow the 20-percent rule when creating a RAID5 volume: because of the complexity of parity calculations, volumes with greater than about 20 percent writes should probably not be RAID5 volumes. If data redundancy is needed, consider mirroring.

Software RAID vs Hardware RAID

The RAID levels can also be implemented in the host's software on any collection of individual disks. RAID-0 (striping) and RAID-1 (mirroring) are the simplest to implement in a host driver and there are many "software RAID" implementations.

Recovery procedures are the most difficult aspect of software RAID. Performance of software RAID may be slower than hardware RAID for a couple reasons. Software RAID levels one and higher often require more data to be transfered between hosts and storage than would be required for hardware RAID. For example, the host must make two writes to separate disks when maintaining mirrors whereas the data would be written only once to a hardware RAID device. In addition to the extra I/O to maintain parity, RAID-5 XOR calculations require extra CPU cycles.

Hardware RAID

The hardware based system manages the RAID subsystem independently from the host and presents to the host only a single disk per RAID array. This way the host doesn't have to be aware of the RAID subsystems(s).

• The controller based hardware solution. DPT's SCSI controllers are a good example for a controller based RAID solution. The intelligent controller manages the RAID subsystem independently from the host. The advantage over an external SCSI---SCSI RAID subsystem is that the contoller is able to span the RAID subsystem over multiple SCSI channels and and by this remove the limiting factor external RAID solutions have: The transfer rate over the SCSI bus.
  
• The external hardware solution (SCSI/Fiber channel RAID). An external RAID box moves all RAID handling "intelligence" into a controller that is sitting in the external disk subsystem. The whole subsystem is connected to the host via a normal SCSI controller and appears to the host as a single or multiple disks. This solution has drawbacks compared to the controller based solution: The single SCSI channel used in this solution creates a bottleneck. Newer technologies like Fiber Channel can ease this problem, especially if they allow to trunk multiple channels into a Storage Area Network.
  4 SCSI drives can already completely flood a parallel SCSI bus, since the average transfer size is around 4KB and the command transfer overhead - which is even in Ultra SCSI still done asynchronously - takes most of the bus time.

Software RAID

Special and pretty complex driver is needed to implement software RAID solution. This is more error prone and less compatible then hardware based solutions, especially Fiber Channel based, but it is cheaper.

Just like any other application, software-based arrays occupy host system memory, consume CPU cycles and are operating system dependent. By contending with other applications that are running concurrently for host CPU cycles and memory, software-based arrays degrade overall server performance. Also, unlike hardware-based arrays, the performance of a software-based array is directly dependent on server CPU performance and load.

Except for the array functionality, hardware-based RAID schemes have very little in common with software-based implementations. Since the host CPU can execute user applications while the array adapter's processor simultaneously executes the array functions, the result is true hardware multi-tasking. Hardware arrays also do not occupy any host system memory, nor are they operating system dependent.

Hardware arrays are also highly fault tolerant. Since the array logic is based in hardware, software is NOT required to boot. Some software arrays, however, will fail to boot if the boot drive in the array fails. For example, an array implemented in software can only be functional when the array software has been read from the disks and is memory-resident. What happens if the server can't load the array software because the disk that contains the fault tolerant software has failed? Software-based implementations commonly require a separate boot drive, which may be included or not in the array.

NEWS CONTENTS

• 20191102 : Raid-5 is obsolete if you use large drives , such as 2TB or 3TB disks. You should instead use raid-6 ( two disks can fail) ( Oct 03, 2019 , hardforum.com )

Old News ;-)

Andre Molyneux's Weblog

• RAID0 is striping, in which data is spread across multiple spindles (disks). Data written to the RAID0 is broken up into chunks of a specified size (interlace value) and spread across the disks:

  +--------+     +--------+
  | Chunk 1|     |Chunk 2 |
  +--------+     +--------+
  | Chunk 3|     |Chunk 4 |
  +--------+     +--------+
  | Chunk 5|     |Chunk 6 |
  +--------+     +--------+
    Disk A         Disk B
  |                       |
  +--------Stripe---------+
  

  In this case the term RAID is a misnomer as no redundancy is gained. The address space is 'interlaced' across the disks, improving performance for I/Os bigger than the interlace/chunk size as a single I/O will spread across multiple disks. In the example above, for an interlace size of 16k, the first 16k of data would reside on Disk A, the second 16k on Disk B, the third 16k on Drive A, and so on. The interlace size needs to be chosen when the logical device is created and can't be changed later without recreating the logical device from scratch.
  

• RAID1: RAID1 is mirroring. Every byte of data written to the mirror is duplicated on both disks:

  +--------+     +--------+
  | Copy A | <-> | Copy B |
  +--------+     +--------+
    Disk A         Disk B
  |                       |
  +--------Mirror---------+
  

  The advantages are that you can lose either disk and the data will still be accessible, and reads can be alternated between the two disks to improve performance. The drawbacks are that you've doubled your storage costs and incurred additional overhead by having to generate two writes to physical devices for every one that's done to the logical mirror device.

• Concatenation: One additional type of logical device that's involved when combining stripes and mirrors in SVM is a concatenation. There is no RAID nomencalture associated with a 'concat':

  +--------+
  | Disk A |
  +--------+
  | Disk B |
  +--------+
  | Disk C |
  +--------+
    Concat
  

  A concatenation aggregates several smaller physical devices into one large logical device. Unlike a stripe, the address space isn't interlaced across the underlying devices. This means there's no performance gain from using a concatenated device.

Since RAID0 improves performance, and RAID1 provides redundancy, someone came up with the idea to combine them. Fast and reliable. Two great tastes that taste great together!

When combining these two types of 'logical' devices there's a choice to be made -- do you mirror two stripes, or do you stripe across multiple mirrors? There are pros and cons to each approach:

• RAID 0+1: In RAID 0+1, the stripes are created first, then are mirrored together. Logically, the resulting device looks like:

  +------------------------------------+     +------------------------------------+
  | +--------+  +--------+  +--------+ |     | +--------+  +--------+  +--------+ |
  | | Chunk 1|  |Chunk 2 |  |Chunk 3 | |     | | Chunk 1|  |Chunk 2 |  |Chunk 3 | |
  | +--------+  +--------+  +--------+ |     | +--------+  +--------+  +--------+ |
  | | Chunk 4|  |Chunk 5 |  |Chunk 6 | |     | | Chunk 4|  |Chunk 5 |  |Chunk 6 | |
  | +--------+  +--------+  +--------+ |<--->| +--------+  +--------+  +--------+ |
  | | Chunk 7|  |Chunk 8 |  |Chunk 9 | |     | | Chunk 7|  |Chunk 8 |  |Chunk 9 | |
  | +--------+  +--------+  +--------+ |     | +--------+  +--------+  +--------+ |
  |   Disk A      Disk B      Disk C   |     |    Disk D     Disk E      Disk F   |
  +------------------------------------+     +------------------------------------+
           Stripe 1                                        Stripe 2
  |                                                                               |
  +-----------------------------------Mirror--------------------------------------+
  

  Advantage: Simple administrative model. Issue one command to create the first stripe, a second command to create the second stripe, and a third command to mirror them. Three commands and you're done, regardless of the number of disks in the configuration.
  Disadvantage: An error on any one of the disks kills redundancy for all disks. For instance, a failure on Disk B above 'breaks' the Stripe 1 side of the mirror. As a result, should disk D, E, or F fail as well, the entire mirror becomes unusable.

• RAID 1+0: In RAID 1+0 the opposite approach is taken. The disks are mirrored first, then the mirrors are combined together into a stripe:

  +-----------------+  +-----------------+  +-----------------+
  |    Chunk 1      |  |     Chunk 2     |  |     Chunk 3     |
  +-----------------+  +-----------------+  +-----------------+
  |    Chunk 4      |  |     Chunk 5     |  |     Chunk 6     |
  +-----------------+  +-----------------+  +-----------------+
  |    Chunk 7      |  |     Chunk 8     |  |     Chunk 9     |
  +-----------------+  +-----------------+  +-----------------+
  Disk A <---> Disk B  Disk C <---> Disk D  Disk E <---> Disk F
        Mirror 1             Mirror 2             Mirror 3
  |                                                           |
  +-----------------------------------------------------------+
                            Stripe
  

  Advantage: A failure on one disk only impacts redundancy for the chunks of the stripe that are located on that disk. For instance, a failure on Disk B above only loses redundancy for every third chunk (1, 4, 7, etc.) Redundancy for the other stripe chunks is unaffected, so a second disk failure could be tolerated as long as the second failure wasn't on Disk A.
  Disadvantage: More complicated from an administrative standpoint. The administrator needs to issue one creation command per mirror, then a command to stripe across the mirrors. The six-disk example above would require four commands to create, while a twelve disk configuration would require seven commands.

[Nov 02, 2019] Raid-5 is obsolete if you use large drives , such as 2TB or 3TB disks. You should instead use raid-6 ( two disks can fail)

Notable quotes:

"... RAID5 can survive a single drive failure. However, once you replace that drive, it has to be initialized. Depending on the controller and other things, this can take anywhere from 5-18 hours. During this time, all drives will be in constant use to re-create the failed drive. It is during this time that people worry that the rebuild would cause another drive near death to die, causing a complete array failure. ..."

"... If during a rebuild one of the remaining disks experiences BER, your rebuild stops and you may have headaches recovering from such a situation, depending on controller design and user interaction. ..."

"... RAID5 + a GOOD backup is something to consider, though. ..."

"... Raid-5 is obsolete if you use large drives , such as 2TB or 3TB disks. You should instead use raid-6 ..."

"... RAID 6 offers more redundancy than RAID 5 (which is absolutely essential, RAID 5 is a walking disaster) at the cost of multiple parity writes per data write. This means the performance will be typically worse (although it's not theoretically much worse, since the parity operations are in parallel). ..."

Oct 03, 2019 | hardforum.com

RAID5 can survive a single drive failure. However, once you replace that drive, it has to be initialized. Depending on the controller and other things, this can take anywhere from 5-18 hours. During this time, all drives will be in constant use to re-create the failed drive. It is during this time that people worry that the rebuild would cause another drive near death to die, causing a complete array failure.

This isn't the only danger. The problem with 2TB disks, especially if they are not 4K sector disks, is that they have relative high BER rate for their capacity, so the likelihood of BER actually occurring and translating into an unreadable sector is something to worry about.

If during a rebuild one of the remaining disks experiences BER, your rebuild stops and you may have headaches recovering from such a situation, depending on controller design and user interaction.

So i would say with modern high-BER drives you should say:

• RAID5: 0 complete disk failures, BER covered
• RAID6: 1 complete disk failure, BER covered

So essentially you'll lose one parity disk alone for the BER issue. Not everyone will agree with my analysis, but considering RAID5 with today's high-capacity drives 'safe' is open for debate.

RAID5 + a GOOD backup is something to consider, though.

1. So you're saying BER is the error count that 'escapes' the ECC correction? I do not believe that is correct, but i'm open to good arguments or links.

   As i understand, the BER is what prompt bad sectors, where the number of errors exceed that of the ECC error correcting ability; and you will have an unrecoverable sector (Current Pending Sector in SMART output).

   Also these links are interesting in this context:

   http://blog.econtech.selfip.org/200...s-not-fully-readable-a-lawsuit-in-the-making/

   The short story first: Your consumer level 1TB SATA drive has a 44% chance that it can be completely read without any error. If you run a RAID setup, this is really bad news because it may prevent rebuilding an array in the case of disk failure, making your RAID not so Redundant. Click to expand...

   Not sure on the numbers the article comes up with, though.

   Also this one is interesting:
   http://lefthandnetworks.typepad.com/virtual_view/2008/02/what-does-data.html

   BER simply means that while reading your data from the disk drive you will get an average of one non-recoverable error in so many bits read, as specified by the manufacturer. Click to expand...

   Rebuilding the data on a replacement drive with most RAID algorithms requires that all the other data on the other drives be pristine and error free. If there is a single error in a single sector, then the data for the corresponding sector on the replacement drive cannot be reconstructed, and therefore the RAID rebuild fails and data is lost. The frequency of this disastrous occurrence is derived from the BER. Simple calculations will show that the chance of data loss due to BER is much greater than all other reasons combined. Click to expand...

   These links do suggest that BER works to produce un-recoverable sectors, and not 'escape' them as 'undetected' bad sectors, if i understood you correctly.

1. parityOCP said: ↑

   That's guy's a bit of a scaremonger to be honest. He may have a point with consumer drives, but the article is sensationalised to a certain degree. However, there are still a few outfits that won't go past 500GB/drive in an array (even with enterprise drives), simply to reduce the failure window during a rebuild. Click to expand...

   Why is he a scaremonger? He is correct. Have you read his article? In fact, he has copied his argument from Adam Leventhal(?) that was one of the ZFS developers I believe.

   Adam's argument goes likes this:
   Disks are getting larger all the time, in fact, the storage increases exponentially. At the same time, the bandwidth is increasing not that fast - we are still at 100MB/sek even after decades. So, bandwidth has increased maybe 20x after decades. While storage has increased from 10MB to 3TB = 300.000 times.

   The trend is clear. In the future when we have 10TB drives, they will not be much faster than today. This means, to repair an raid with 3TB disks today, will take several days, maybe even one week. With 10TB drives, it will take several weeks, maybe a month.

   Repairing a raid stresses the other disks much, which means they can break too. Experienced sysadmins reports that this happens quite often during a repair. Maybe because those disks come from the same batch, they have the same weakness. Some sysadmins therefore mix disks from different vendors and batches.

   Hence, I would not want to run a raid with 3TB disks and only use raid-5. During those days, if only another disk crashes you have lost all your data.

   Hence, that article is correct, and he is not a scaremonger. Raid-5 is obsolete if you use large drives, such as 2TB or 3TB disks. You should instead use raid-6 (two disks can fail). That is the conclusion of the article: use raid-6 with large disks, forget raid-5. This is true, and not scaremongery.

   In fact, ZFS has therefore something called raidz3 - which means that three disks can fail without problems. To the OT: no raid-5 is not safe. Neither is raid-6, because neither of them can not always repair nor detect corrupted data. There are cases when they dont even notice that you got corrupted bits. See my other thread for more information about this. That is the reason people are switching to ZFS - which always CAN detect and repair those corrupted bits. I suggest, sell your hardware raid card, and use ZFS which requires no hardware. ZFS just uses JBOD.

   Here are research papers on raid-5, raid-6 and ZFS and corruption:
   http://hardforum.com/showpost.php?p=1036404173&postcount=73

1. brutalizer said: ↑

   The trend is clear. In the future when we have 10TB drives, they will not be much faster than today. This means, to repair an raid with 3TB disks today, will take several days, maybe even one week. With 10TB drives, it will take several weeks, maybe a month. Click to expand...

   While I agree with the general claim that the larger HDDs (1.5, 2, 3TBs) are best used in RAID 6, your claim about rebuild times is way off.

   I think it is not unreasonable to assume that the 10TB drives will be able to read and write at 200 MB/s or more. We already have 2TB drives with 150MB/s sequential speeds, so 200 MB/s is actually a conservative estimate.

   10e12/200e6 = 50000 secs = 13.9 hours. Even if there is 100% overhead (half the throughput), that is less than 28 hours to do the rebuild. It is a long time, but it is no where near a month! Try to back your claims in reality.

   And you have again made the false claim that "ZFS - which always CAN detect and repair those corrupted bits". ZFS can usually detect corrupted bits, and can usually correct them if you have duplication or parity, but nothing can always detect and repair. ZFS is safer than many alternatives, but nothing is perfectly safe. Corruption can and has happened with ZFS, and it will happen again.

Is RAID5 safe with Five 2TB Hard Drives ? | [H]ard|Forum Your browser indicates if you've visited this link

https://hardforum.com /threads/is-raid5-safe-with-five-2tb-hard-drives.1560198/

Hence, that article is correct, and he is not a scaremonger. Raid-5 is obsolete if you use large drives , such as 2TB or 3TB disks. You should instead use raid-6 ( two disks can fail). That is the conclusion of the article: use raid-6 with large disks, forget raid-5 . This is true, and not scaremongery.

RAID 5 Data Recovery How to Rebuild a Failed RAID 5 - YouTube

RAID 5 vs RAID 10: Recommended RAID For Safety and ... Your browser indicates if you've visited this link

https://www.cyberciti.biz /tips/raid5-vs-raid-10-safety-performance.html

RAID 6 offers more redundancy than RAID 5 (which is absolutely essential, RAID 5 is a walking disaster) at the cost of multiple parity writes per data write. This means the performance will be typically worse (although it's not theoretically much worse, since the parity operations are in parallel).

SVM specifics

So, does SVM do RAID 0+1 or RAID 1+0? The answer is, "Yes." So it gives you a choice between the two? The answer is "No."

Obviously further explanation is necessary...

In SVM, mirror devices cannot be created from "bare" disks. You are required to create the mirror on top of another type of SVM metadevice, known as a concat/stripe*. SVM combines concatenations and stripes into a single metadevice type, in which one or more stripes are concatenated together. When used to build a mirror these concat/stripe logical devices are known as submirrors. If you want to expand the size of a mirror device you can do so by concatenating additional stripe(s) onto the concat/stripe devices that are serving as submirrors.

So, in SVM, you are always required to set up a stripe (concat/stripe) in order to create a mirror. On the surface this makes it appear that SVM does RAID 0+1. However, once you understand a bit about the SVM mirror code, you'll find RAID 1+0 lurking under the covers.

SVM mirrors are logically divided up into regions. The state of each mirror region is recorded in state database replicas* stored on disk. By individually recording the state of each region in the mirror, SVM can be smart about how it performs a resync. Following a disk failure or an unusual event (e.g. a power failure occurs after the first side of a mirror has been written to but before the matching write to the second side can be accomplished), SVM can determine which regions are out-of-sync and only synchronize them, not the entire mirror. This is known as an optimized resync.

The optimized resync mechanisms allow SVM to gain the redundancy benefits of RAID 1+0 while keeping the administrative benefits of RAID 0+1. If one of the drives in a concat/stripe device fails, only those mirror regions that correspond to data stored on the failed drive will lose redundancy. The SVM mirror code understands the layout of the concat/stripe submirrors and can therefore determine which resync regions reside on which underlying devices. For all regions of the mirror not affected by the failure, SVM will continue to provide redundancy, so a second disk failure won't necessarily prove fatal.

So, in a nutshell, SVM provides a RAID 0+1 style administrative interface but effectively implements RAID 1+0 functionality. Administrators get the best of each type, the relatively simple administration of RAID 0+1 plus the greater resilience of RAID 1+0 in the case of multiple device failures.

---

* concat/stripe logical devices (metadevices)

The following example shows a concat/stripe metadevice that's serving as a submirror to a mirror metadevice. Note that the metadevice is a concatenation of three separate stripes:

• Stripe 0 is a 1-way stripe (so not really striped at all) on disk slice c1t11d0s0.
• Stripe 1 is a 1-way stripe on disk slice c1t12d0s0.
• Stripe 2 is a 2-way stripe with an interlace size of 32 blocks on disk slices c1t13d0s1 and c1t14d0s2.

d1: Submirror of d0
    State: Okay
    Size: 78003 blocks (38 MB)
    Stripe 0:
        Device      Start Block  Dbase        State Reloc Hot Spare
        c1t11d0s0          0     No            Okay   Yes
    Stripe 1:
        Device      Start Block  Dbase        State Reloc Hot Spare
        c1t12d0s0          0     No            Okay   Yes
    Stripe 2: (interlace: 32 blocks)
        Device      Start Block  Dbase        State Reloc Hot Spare
        c1t13d0s1          0     No            Okay   Yes
        c1t14d0s2          0     No            Okay   Yes

** State database replicas

SVM stores configuration and state information in a 'state database' in memory. Copies of this state database are stored on disk, where they are referred to as state database replicas. The primary purpose of the state database replicas is to provide non-volatile copies of the state database so that the SVM configuration is persistant across reboots. A secondary purpose of the replicas is to provide a 'scratch pad' to keep track of mirror region states.

Recommended Links

RAID - Wikipedia, the free encyclopedia

How To Set Up Software RAID1 On A Running LVM System (Incl. GRUB Configuration) (Fedora 8) HowtoForge - Linux Howtos and Tutorials

Redundant Arrays of Independent Disks - Computerworld

Sys Admin v12, i06 Introduction to RAID

Reference

Raid Recovery Comparison Chart and Raid Types

RAID Level	Min. Num of Drives	Description	Strengths	Weaknesses
Raid 0	2	Data striping without redundancy	Highest performance	No data protection; One drive fails, all data is lost
Raid 1	2	Disk mirroring	Very high performance; Very high data protection; Very minimal penalty on write performance	High redundancy cost overhead; Because all data is duplicated, twice the storage capacity is required
Raid 2	Not Used In LAN	No practical use	Previously used for RAM error environments correction (known as Hamming Code ) and in disk drives before the use of embedded error correction	No practical use; Same performance can be achieved by RAID 3 at lower cost
Raid 3	3	Byte-level data striping with dedicated parity drive	Excellent performance for large, sequential data requests	Not well-suited for transaction-oriented network applications; Single parity drive does not support multiple, simultaneous read and write requests
Raid 4	3 (not widely used	Block-level data striping with dedicated parity drive	Data striping supports multiple simultaneous read requests	Write requests suffer from same single parity-drive bottleneck as RAID 3; RAID 5 offers equal data protection and better performance at same cost
Raid 5	3	Block-level data striping with distributed parity	Best cost/performance for transaction-oriented networks; Very high performance, very high data protection; Supports multiple simultaneous reads and writes; Can also be optimized for large, sequential requests	Write performance is slower than RAID 0 or RAID 1
Raid 0/1	4	Combination of RAID 0 (data striping) and RAID 1 (mirroring)	Highest performance, highest data protection (can tolerate multiple drive failures)	High redundancy cost overhead; Because all data is duplicated, twice the storage capacity is required; Requires minimum of four drives

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Last modified: February 19, 2020