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RAID 1 vs RAID 5 and to the bottom of it !
Dear All,
I've got a question regarding hardware RAID 1 (R1) and RAID 5 (R5) Don't consider following issues, because they are not playing in this question: a) Minimum disk setup and setting up hot spares b) Write intensive situations or sequential reads situations c) Dynamic resizing a RAID d) Same manufacturer and same lot drive issues (this count for both setups) e) Performance issue when 1 HD goes down. Look at following diagram for better understanding : RAID 1 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Copy1 Copy2 Strip3 Strip4 Copy3 Copy4 Strip5 Strip6 Copy5 Copy6 Strip7 Strip8 Copy7 Copy8 HD1 & HD2 are duplicated on HD3 & HD4. If HD1 crashes then the copy is simply used instead. Recovery is simply; replace HD1 and copy the entire HD3 to it. RAID 5 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Strip3 Parity1-3 Strip4 Strip5 Parity4-6 Strip6 Strip7 Parity7-9 Strip8 Strip9 Parity10-12 Strip10 Strip11 Strip12 Instead of real duplicating disks like R1, R5 is creating a parity (XOR) and distributes this round robin wise on all disks. Question : ---------- I feel more secure when data is written to a R5 then it is on a R1. Why? scenario RAID 1 : Suppose that strip 1 is written on HD1 and duplicated on HD3, but there was a bad sector on HD3, so a real sync copy would never work. When HD1 fails after 1 year, and it's replaced and I restore a copy of HD3 on it, then my guess is that "Original HD1" and "restored HD1" are never identical or you have to mark bad sectors that came from HD3 to the new restored HD1 and still then there is a difference with "Original HD1" and "restored HD1". scenario RAID 5 : All this will never happen because there is no identical c opy of data. If a sector is going bad on HD1 then this will be marked and data will be written on an other sector on HD1. When HD1 is failing then removing and inserting a new one will generate automatic new data recovered from HD2, HD3 & HD4. This is very strait forward. Why this question : ------------------- I feel that RAID 1 was intended for fast realtime backup, and when HD1 is giving huge problems, you can boot from the backup HD. I don't feel that this system was made for "keeping data online without a second of interruption" If feel that RAID 5 was made for "keeping data online without a second of interruption" and must be seen in that way. So RAID 5 could be seen as a successor of RAID 1. (Please do not use points a,b,c,d & e as the BUT story) Also, a hard-disk can go bad in a heartbeat, but can also slowly give some hints that there is something wrong (sectors going bad on a certain place). And most of the time a slowly death is what he will do. Am I right that RAID 1 will not give a solution for slowly death but RAID 5 will. Am I right? Please do not take a,b,c,d & e points into consideration because they are not the basics for this questions. I want to go to the basics of RAID 1 and RAID 5 in online system interruption? I can go 1 step further and say that RAID 0 was a great solution for gaining bandwidth and with no much effort a backup system could be made and they named it RAID 1. But no much thinking was done for the backup solution if you take online data in account that cannot be interrupted. If I read some articles on RAID 1 then I read a lot of "Then you can restart from the backup disk". And this is what start me thinking and did a lot of research on it. Please, do give me your opinion. Kind regards, John. |
#2
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RAID 1 vs RAID 5 and to the bottom of it !
Previously John wrote:
Dear All, I've got a question regarding hardware RAID 1 (R1) and RAID 5 (R5) Don't consider following issues, because they are not playing in this question: a) Minimum disk setup and setting up hot spares b) Write intensive situations or sequential reads situations c) Dynamic resizing a RAID d) Same manufacturer and same lot drive issues (this count for both setups) e) Performance issue when 1 HD goes down. Look at following diagram for better understanding : RAID 1 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Copy1 Copy2 Strip3 Strip4 Copy3 Copy4 Strip5 Strip6 Copy5 Copy6 Strip7 Strip8 Copy7 Copy8 HD1 & HD2 are duplicated on HD3 & HD4. If HD1 crashes then the copy is simply used instead. Recovery is simply; replace HD1 and copy the entire HD3 to it. RAID 5 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Strip3 Parity1-3 Strip4 Strip5 Parity4-6 Strip6 Strip7 Parity7-9 Strip8 Strip9 Parity10-12 Strip10 Strip11 Strip12 Instead of real duplicating disks like R1, R5 is creating a parity (XOR) and distributes this round robin wise on all disks. Question : ---------- I feel more secure when data is written to a R5 then it is on a R1. Why? scenario RAID 1 : Suppose that strip 1 is written on HD1 and duplicated on HD3, but there was a bad sector on HD3, so a real sync copy would never work. When HD1 fails after 1 year, and it's replaced and I restore a copy of HD3 on it, then my guess is that "Original HD1" and "restored HD1" are never identical or you have to mark bad sectors that came from HD3 to the new restored HD1 and still then there is a difference with "Original HD1" and "restored HD1". scenario RAID 5 : All this will never happen because there is no identical c opy of data. If a sector is going bad on HD1 then this will be marked and data will be written on an other sector on HD1. When HD1 is failing then removing and inserting a new one will generate automatic new data recovered from HD2, HD3 & HD4. This is very strait forward. Forget about defect management. It is a very very rarely used machanism, most sectors are fine. Why this question : ------------------- I feel that RAID 1 was intended for fast realtime backup, and when HD1 is giving huge problems, you can boot from the backup HD. I don't feel that this system was made for "keeping data online without a second of interruption" If feel that RAID 5 was made for "keeping data online without a second of interruption" and must be seen in that way. So RAID 5 could be seen as a successor of RAID 1. (Please do not use points a,b,c,d & e as the BUT story) Also, a hard-disk can go bad in a heartbeat, but can also slowly give some hints that there is something wrong (sectors going bad on a certain place). And most of the time a slowly death is what he will do. Am I right that RAID 1 will not give a solution for slowly death but RAID 5 will. Am I right? Please do not take a,b,c,d & e points into consideration because they are not the basics for this questions. I want to go to the basics of RAID 1 and RAID 5 in online system interruption? I can go 1 step further and say that RAID 0 was a great solution for gaining bandwidth and with no much effort a backup system could be made and they named it RAID 1. But no much thinking was done for the backup solution if you take online data in account that cannot be interrupted. If I read some articles on RAID 1 then I read a lot of "Then you can restart from the backup disk". And this is what start me thinking and did a lot of research on it. Please, do give me your opinion. Actually RAID1 and RAID5 are the same if the RAID1 uses just 2 disks. If you do RAID1 with 3 disks and RAID5 with 3 disks, you get different redundancy. An n disk RAID1 can tolerate n-1 disk faulres. An n disk RAID5 can tolerate a 1 disk failure. Arno |
#3
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RAID 1 vs RAID 5 and to the bottom of it !
Actually RAID1 and RAID5 are the same if the RAID1 uses just 2 disks.
If you do RAID1 with 3 disks and RAID5 with 3 disks, you get different redundancy. An n disk RAID1 can tolerate n-1 disk faulres. An n disk RAID5 can tolerate a 1 disk failure. Dear Arno, I do not agree when you said that an n disk RAID1 can tolerate n-1 disk failures. Look to the chart : RAID 1 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Copy1 Copy2 Strip3 Strip4 Copy3 Copy4 Strip5 Strip6 Copy5 Copy6 Strip7 Strip8 Copy7 Copy8 If HD1 and HD3 (which is a strip copy of HD1) goes down then your RAID 1 is broken! This is not the case when HD1 and HD2 goes down in a RAID 1 setup. Further, RAID1 is based on strip duplication where RAID5 is based on strip's parity calculation. And at last, I can predict in most cases a HD-Crash if you look into the system log (Win2K) and see the read/write disk errors showing up in none RAID system. At that moment I replace disks. And in most of the cases this is what is going to happen. If the OS want to write data to an hardware RAID it will never see bad sectors because if the OS wants to write a sector then in the background there will be more sectors written and verified by the RAID controler, so error handling sits on the RAID controler and must be transparent to the OS (In a RAID 5 setup there will be also reading of other sectors to recalculate the parity and writing the new value, it is this calculation that makes RAID 5 slower in writing data. This is also the why question that RAID 1 and RAID 5 is totaly different and are NOT the same!). So, in my opinion the original post and questions are unchanged. Please do give me your opinions. Kind regards, John. |
#4
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RAID 1 vs RAID 5 and to the bottom of it !
Previously John wrote:
Actually RAID1 and RAID5 are the same if the RAID1 uses just 2 disks. If you do RAID1 with 3 disks and RAID5 with 3 disks, you get different redundancy. An n disk RAID1 can tolerate n-1 disk faulres. An n disk RAID5 can tolerate a 1 disk failure. Dear Arno, I do not agree when you said that an n disk RAID1 can tolerate n-1 disk failures. Look to the chart : RAID 1 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Copy1 Copy2 Strip3 Strip4 Copy3 Copy4 Strip5 Strip6 Copy5 Copy6 Strip7 Strip8 Copy7 Copy8 If HD1 and HD3 (which is a strip copy of HD1) goes down then your RAID 1 is broken! This is not the case when HD1 and HD2 goes down in a RAID 1 setup. Further, RAID1 is based on strip duplication where RAID5 is based on strip's parity calculation. Oh, but that is not a 4 disk RAID1. It is a 2+2 disk RAID10 or RAID0+1. A 4 disk RAID1 looks like this: HD1 HD2 HD3 HD4 Data1 Copy1 Copy2 Copy3 And at last, I can predict in most cases a HD-Crash if you look into the system log (Win2K) and see the read/write disk errors showing up in none RAID system. At that moment I replace disks. And in most of the cases this is what is going to happen. If the OS want to write data to an hardware RAID it will never see bad sectors because if the OS wants to write a sector then in the background there will be more sectors written and verified by the RAID controler, so error handling sits on the RAID controler and must be transparent to the OS (In a RAID 5 setup there will be also reading of other sectors to recalculate the parity and writing the new value, it is this calculation that makes RAID 5 slower in writing data. This is also the why question that RAID 1 and RAID 5 is totaly different and are NOT the same!). So, in my opinion the original post and questions are unchanged. Please do give me your opinions. RAID10 only makes sense if you need more than the speed of a RAID1. But it is very dependent on tha actual system configuratio whether you get a real speed improvement, because you have to transport twice that data and swith between more disks than RAID1. Arno |
#5
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RAID 1 vs RAID 5 and to the bottom of it !
John wrote: RAID 1 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Copy1 Copy2 Strip3 Strip4 Copy3 Copy4 Strip5 Strip6 Copy5 Copy6 Strip7 Strip8 Copy7 Copy8 HD1 & HD2 are duplicated on HD3 & HD4. If HD1 crashes then the copy is simply used instead. Recovery is simply; replace HD1 and copy the entire HD3 to it. This is not RAID 1. I'd call it RAID 10 |
#6
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RAID 1 vs RAID 5 and to the bottom of it !
"Arno Wagner" wrote in message
Previously John wrote: Dear All, I've got a question regarding hardware RAID 1 (R1) and RAID 5 (R5) Don't consider following issues, because they are not playing in this question: a) Minimum disk setup and setting up hot spares b) Write intensive situations or sequential reads situations c) Dynamic resizing a RAID d) Same manufacturer and same lot drive issues (this count for both setups) e) Performance issue when 1 HD goes down. Look at following diagram for better understanding : RAID 1 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Copy1 Copy2 Strip3 Strip4 Copy3 Copy4 Strip5 Strip6 Copy5 Copy6 Strip7 Strip8 Copy7 Copy8 HD1 & HD2 are duplicated on HD3 & HD4. If HD1 crashes then the copy is simply used instead. Recovery is simply; replace HD1 and copy the entire HD3 to it. RAID 5 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Strip3 Parity1-3 Strip4 Strip5 Parity4-6 Strip6 Strip7 Parity7-9 Strip8 Strip9 Parity10-12 Strip10 Strip11 Strip12 Instead of real duplicating disks like R1, R5 is creating a parity (XOR) and distributes this round robin wise on all disks. Question : ---------- I feel more secure when data is written to a R5 then it is on a R1. Why? scenario RAID 1 : Suppose that strip 1 is written on HD1 and duplicated on HD3, but there was a bad sector on HD3, so a real sync copy would never work. When HD1 fails after 1 year, and it's replaced and I restore a copy of HD3 on it, then my guess is that "Original HD1" and "restored HD1" are never identical or you have to mark bad sectors that came from HD3 to the new restored HD1 and still then there is a difference with "Original HD1" and "restored HD1". scenario RAID 5 : All this will never happen because there is no identical copy of data. If a sector is going bad on HD1 then this will be marked and data will be written on an other sector on HD1. When HD1 is failing then removing and inserting a new one will generate automatic new data recovered from HD2, HD3 & HD4. This is very strait forward. Forget about defect management. It is a very very rarely used machanism, most sectors are fine. Most no less, how reassuring. Why this question : ------------------- I feel that RAID 1 was intended for fast realtime backup, and when HD1 is giving huge problems, you can boot from the backup HD. I don't feel that this system was made for "keeping data online without a second of interruption" If feel that RAID 5 was made for "keeping data online without a second of interruption" and must be seen in that way. So RAID 5 could be seen as a successor of RAID 1. (Please do not use points a,b,c,d & e as the BUT story) Also, a hard-disk can go bad in a heartbeat, but can also slowly give some hints that there is something wrong (sectors going bad on a certain place). And most of the time a slowly death is what he will do. Am I right that RAID 1 will not give a solution for slowly death but RAID 5 will. Am I right? Please do not take a,b,c,d & e points into consideration because they are not the basics for this questions. I want to go to the basics of RAID 1 and RAID 5 in online system interruption? I can go 1 step further and say that RAID 0 was a great solution for gaining bandwidth and with no much effort a backup system could be made and they named it RAID 1. But no much thinking was done for the backup solution if you take online data in account that cannot be interrupted. If I read some articles on RAID 1 then I read a lot of "Then you can restart from the backup disk". And this is what start me thinking and did a lot of research on it. Please, do give me your opinion. Actually RAID1 and RAID5 are the same if the RAID1 uses just 2 disks. No ****. If you do RAID1 with 3 disks and RAID5 with 3 disks, you get different redundancy. An n disk RAID1 can tolerate n-1 disk faulres. An n disk RAID5 can tolerate a 1 disk failure. Arno |
#7
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RAID 1 vs RAID 5 and to the bottom of it !
On Wed, 20 Sep 2006 01:39:21 +0200, "John"
wrote: Dear All, I've got a question regarding hardware RAID 1 (R1) and RAID 5 (R5) Don't consider following issues, because they are not playing in this question: a) Minimum disk setup and setting up hot spares b) Write intensive situations or sequential reads situations c) Dynamic resizing a RAID d) Same manufacturer and same lot drive issues (this count for both setups) e) Performance issue when 1 HD goes down. Look at following diagram for better understanding : RAID 1 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Copy1 Copy2 Strip3 Strip4 Copy3 Copy4 Strip5 Strip6 Copy5 Copy6 Strip7 Strip8 Copy7 Copy8 HD1 & HD2 are duplicated on HD3 & HD4. If HD1 crashes then the copy is simply used instead. Recovery is simply; replace HD1 and copy the entire HD3 to it. As others pointed out that isn't RAID1. RAID1 involves exactly 2 mirrored disks. This is RAID 0+1, a mirror or stripes. RAID 5 : -------- HD1 HD2 HD3 HD4 ------------------------------------------------ Strip1 Strip2 Strip3 Parity1-3 Strip4 Strip5 Parity4-6 Strip6 Strip7 Parity7-9 Strip8 Strip9 Parity10-12 Strip10 Strip11 Strip12 Instead of real duplicating disks like R1, R5 is creating a parity (XOR) and distributes this round robin wise on all disks. Question : ---------- I feel more secure when data is written to a R5 then it is on a R1. Why? scenario RAID 1 : Suppose that strip 1 is written on HD1 and duplicated on HD3, but there was a bad sector on HD3, so a real sync copy would never work. When HD1 fails after 1 year, and it's replaced and I restore a copy of HD3 on it, then my guess is that "Original HD1" and "restored HD1" are never identical or you have to mark bad sectors that came from HD3 to the new restored HD1 and still then there is a difference with "Original HD1" and "restored HD1". That's why better implementations employ protection mechanisms which should be used. The simplest is to regularly read the disk and replace data in bad sectors with a good mirrored copy in a different part of the disk. There are various trade names for this strategy. IBM, for example, recommends that this be scheduled by their management software to be done once a week. Some controllers can do this continuously in the background. scenario RAID 5 : All this will never happen because there is no identical c opy of data. If a sector is going bad on HD1 then this will be marked and data will be written on an other sector on HD1. When HD1 is failing then removing and inserting a new one will generate automatic new data recovered from HD2, HD3 & HD4. This is very strait forward. That is a skewed comparison. With RAID 0+1 if one of the copies is bad it is simply ignored and regenerated from the good mirror. With RAID 5 bad data can also be regenerated by reading the parity block when a CRC error occurs. But when either 0+1 or 5 are in degraded state they loose the ability to recover from such errors. In the case of 0+1 it is because it lost its mirror. In the case of RAID 5 it is because although the location of the parity blocks are distributed, each one only protects a specific stripe. It is not "distributed" protection in the sense of Usenet par files because the system doesn't and can't wait for the entire volume to be written before it starts generating parity. Parity is not inherently safer than mirroring. If there is a system or log failure, unsynchronized or stale parity is possible. Furthermore sometimes an uncorrectable I/O error during a write to the disk just prior to failure can result in lost/inaccessible data. Finally if there is a write error, the presence of parity alone will not necessarily tell you later on whether the data or ecc data is suspect. Why this question : ------------------- I feel that RAID 1 was intended for fast realtime backup, and when HD1 is giving huge problems, you can boot from the backup HD. I don't feel that this system was made for "keeping data online without a second of interruption" That may be your feeling but it isn't the reality. RAID 1 isn't a backup. For one thing as soon as the original file is deleted, corrupted, or infected the mirror is altered simultaneously. "Backup" provides a means of recovery from these problems, RAID cannot. In addition mirrored RAID levels are true availability/fault tolerant solutions. In fact many ppl see mirrored RAID levels as better for uptime as they can sustain failure of a maximum of 50% of the disks in the array (if they are the right disks). It is also simpler, which has its plusses. RAID 5 can only afford to loose one disk at a time. It simply cannot "keep data online without a second of interruption" if a second disk fails before a rebuild completes. RAID 0+1 sometimes can. If feel that RAID 5 was made for "keeping data online without a second of interruption" and must be seen in that way. So RAID 5 could be seen as a successor of RAID 1. (Please do not use points a,b,c,d & e as the BUT story) No. ALL mirrored & parity RAID levels are true availability/fault tolerant solutions Also remember there is a difference between theoretical RAID 5 and the reality of the engineering difficulties of a very complex level. Furthermore, even though you think it isn't relevant, generating parity can often be slower than mirroring. Longer rebuild = longer degraded state. When only 1 disk can be down at a time, this makes RAID 5 worse that RAID 10 or 0+1 for availability. Although it is not as straightforward as that because you need more of the same sized disks with RAID 0+1 to create the same useable volume size as RAID 5. Basically you need a very advanced RAID5 implementation to perform competitively with a more modest RAID 0+1. But you often need more disks with 0+1. So the reality is that the decision of which one to go with has to look at factors a-e, disk size, etc. Also, a hard-disk can go bad in a heartbeat, but can also slowly give some hints that there is something wrong (sectors going bad on a certain place). And most of the time a slowly death is what he will do. Correct. Often the problem is flakiness i.e. uncorrected IO errors. That is much harder to deal with. Specific RAID levels are not in themselves the solution. Am I right that RAID 1 will not give a solution for slowly death but RAID 5 will. Not really. Parity levels are not enough for better DAS controllers & especially higher-end SAN or NAS. Other advanced protection mechanisms are employed. Am I right? Please do not take a,b,c,d & e points into consideration because they are not the basics for this questions. I want to go to the basics of RAID 1 and RAID 5 in online system interruption? Not really. You are instead identifying the need for protection mechanisms that assist different RAID levels in better products. I can go 1 step further and say that RAID 0 was a great solution for gaining bandwidth and with no much effort a backup system could be made and they named it RAID 1. No. That's named RAID 0+1 But no much thinking was done for the backup solution if you take online data in account that cannot be interrupted. No. The whole point of RAID 1, 10, & 0+1 is that the storage volume is still seamlessly available during disk failure(s). The difference between them and RAID 3, 4, 5, 6 is that one group mirrors data, the other utilizes parity. Both have their plusses and minuses. Neither are foolproof. The reliability and performance of any of these levels in practice depends much more on the manufacturer, its engineers, and the sector a product is being designed for than which specific level is chosen. If I read some articles on RAID 1 then I read a lot of "Then you can restart from the backup disk". And this is what start me thinking and did a lot of research on it. Sounds like poor wording to me. A RAID 1, 0+1 or 10 system is not supposed to need to be brought down or restarted in order to recover from a disk failure/degraded state. Of course they do generally automatically rebuild on startup if there is a dropped disk. That being said some ppl do encorporate RAID 1 disks into their backup strategy. i.e. perform a backup on a RAID 1 array, remove the mirror disk & take it offsite. Rotate with other disks for subsequent backups. In event of disaster simply insert mirror disk to restore the system/rebuild the array. But, as I said before, the system need not be taken down to move the mirror disk. Please, do give me your opinion. Try a more succinct post next time Kind regards, John. Oh how polite |
#8
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RAID 1 vs RAID 5 and to the bottom of it !
On Thu, 21 Sep 2006 06:51:48 GMT, Curious George wrote:
As others pointed out that isn't RAID1. RAID1 involves exactly 2 mirrored disks. This is RAID 0+1, a mirror or stripes. Oops. I mean a mirror _of_ stripes |
#9
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RAID 1 vs RAID 5 and to the bottom of it !
George, You did it! This goes really far, and I know now that I made some
huge mistakes. Give me a day to review some of your writings and I will certainly come back to this. Thanks a lot! John. |
#10
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RAID 1 vs RAID 5 and to the bottom of it !
On Thu, 21 Sep 2006 11:53:11 +0200, "John"
wrote: George, You did it! This goes really far, and I know now that I made some huge mistakes. Give me a day to review some of your writings and I will certainly come back to this. A day! It's been 20 minutes since you posed. That's enough. Come on ricky tick! Just kidding. It's better if you take your time to look up things, digest he info, and formulate succinct questions. Thanks a lot! No prob. John. |
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