Re: LVM RAID5 with missing disk?
Bob Proulx a écrit : Pascal Hambourg wrote: Bob Proulx a écrit : I favor RAID6's extra redundancy for more safety but I still use RAID1 too. RAID 1 can provide as much or more redundancy than RAID 6. RAID 1 on 3 disks provides as much redundancy as RAID 6. RAID 1 on 4 disks provides more redundancy than RAID 6 (but half the usable space). Do you have any articles or blogs or postings you have written that would summarize raid alternatives? I would enjoy reading whatever you have written on the subject. Or if you recommended other references. There is no need to write anything new about that subject. All is already available on Wikipedia, raid.wiki.kernel.org and so on. What I wrote above is simply derived from the definition of the RAID levels : A RAID 1 array of N disks provides fault tolerance for up to N-1 disks. A RAID 6 array provides fault tolerance for up to 2 disks. N-1 (RAID 1 fault tolerance) = 2 (RAID 6 fault tolerance) if N = 3 -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/54881c18.2020...@plouf.fr.eu.org
Re: LVM RAID5 with missing disk?
Pascal Hambourg wrote: Bob Proulx a écrit : Do you have any articles or blogs or postings you have written that would summarize raid alternatives? I would enjoy reading whatever you have written on the subject. Or if you recommended other references. There is no need to write anything new about that subject. All is already available on Wikipedia, raid.wiki.kernel.org and so on. What I wrote above is simply derived from the definition of the RAID levels : A RAID 1 array of N disks provides fault tolerance for up to N-1 disks. A RAID 6 array provides fault tolerance for up to 2 disks. N-1 (RAID 1 fault tolerance) = 2 (RAID 6 fault tolerance) if N = 3 The reason I nudged was because reading man pages should always have all of the details anyone would ever need. However that only works if one already knows how things work. Reference documents may have all of the details but it is nice to read tutorials and HOWTOs that guide, explain and tutor people through with overall conclusions. By the very nature of them they make judgments. Reference docs have facts but being able to put the rationale of the decision making into place around those facts is still valuable. You seem to have a good grasp of the problem space and as I said I would have enjoyed reading any of your tutorials on it. If there aren't any written then that is okay too. But in any case. All is okay. Thanks! :-) Bob signature.asc Description: Digital signature
Re: LVM RAID5 with missing disk?
Bob Proulx a écrit : I favor RAID6's extra redundancy for more safety but I still use RAID1 too. RAID 1 can provide as much or more redundancy than RAID 6. RAID 1 on 3 disks provides as much redundancy as RAID 6. RAID 1 on 4 disks provides more redundancy than RAID 6 (but half the usable space). -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/5486e90b.3020...@plouf.fr.eu.org
Re: LVM RAID5 with missing disk?
Hello Pascal, Pascal Hambourg wrote: Bob Proulx a écrit : I favor RAID6's extra redundancy for more safety but I still use RAID1 too. RAID 1 can provide as much or more redundancy than RAID 6. RAID 1 on 3 disks provides as much redundancy as RAID 6. RAID 1 on 4 disks provides more redundancy than RAID 6 (but half the usable space). Do you have any articles or blogs or postings you have written that would summarize raid alternatives? I would enjoy reading whatever you have written on the subject. Or if you recommended other references. Thanks! Bob signature.asc Description: Digital signature
Re: LVM RAID5 with missing disk?
Gary Dale wrote: Mart van de Wege wrote: The problem is not that RAID5 does not provide resilience against a single disk failure. The problem is that with modern disk capacities, the chances of *another* disk failing while the array is rebuilding have significantly risen. Especially when all the disks came out of the the same batch, they tend to fail at similar times. I know Best Practice is to mix disks in RAID arrays, but who actually practices that, instead of just taking the risk of failure and covering it with a higher RAID level, like RAID6 in this case? The chances of two disks failing within hours of one another is very small even for disks from the same batch. Statistics are a wonderful thing. If statistically the odds are one out of a thousand that sounds very unlikely. But if so then out of the 2,500+ subscribers to this mailing list there should be 2.5 of us who have experienced this failure personally. In isolation it is unlikely that it will happen to any one individual. But in the population it is simply the statistics that it will happen. Twice now over twenty years and many systems I have had a double disk failure where both drives in a RAID 1 mirror failed within hours of each other. Both were when the drives were from the same vendor batch and had been purchased together for RAID and had been running an identical number of hours. Within hours in one case was seven days later between the two drives. Within hours in another case was within 36 hours of each other. Spinning devices don't last forever. The moral to this story? I always mix disks in RAID arrays to try to decouple age failure modes of drives due to this experience. When I see a single disk raid failure as quickly as practical I jump on getting the drive replaced and the degraded RAID sync'd again. I ensure that there are good backups. These are good and safe recommendations that I think everyone can agree are good Best Practices. I favor RAID6's extra redundancy for more safety but I still use RAID1 too. Bob signature.asc Description: Digital signature
Re: LVM RAID5 with missing disk?
Gary Dale a écrit : On 05/12/14 03:35 PM, Pascal Hambourg wrote: You can think of the RAID algorithms as parity checks. A mirror is even parity. This point of view is a bit twisted, but I can understand and won't argue. While the disks are not physically assigned to be data or parity, you can recreate a failed RAID 5 disk by recalculating the parity based on the surviving disks. Not only the parity but also the missing data. Linux can use a special RAID 10 mode (mirror+stripe) with two or three disks. If you have different sized disks, yes. Why ? Linux RAID, as most RAID types, uses disks of the same size. The more usual case is to use similar disks. If one disk is not striped, you lose some of the performance improvement. RAID 10 with two disks makes little sense. Linux RAID 10 allows striping even on two disks (far mode). with 6 disks, RAID 6 will give you double the capacity of 4 disks or get you immunity to 3 disks failing. RAID 6 can survive 2 disk failures regarless of the number of disks in the array. You misread the sentence. You can run RAID with any number of parity disks by tweaking the algorithms. Most people don't bother using more than 2 parity disks but there is no theoretical reason why you couldn't, to get as much safety as desired. Misread ? You mentionned RAID6. RAID 6 is a standard RAID level consisting of block-level striping with double distributed parity, providing fault tolerance up to 2 failed drives. Not 3. Of course you can define any algorithm you like, but if your custom RAID has triple parity, then it's not RAID 6. Prior to being able to boot to an mdadm RAID 5 array, I regularly had 3 to 5 disk RAID 1 /boot partitions - why not use the disks since they are already there and it keeps the partitioning the same across the drives. Of course. RAID 1 as a standard level is not limited to 2 mirrored disks. It can have any number. RAID 6 can be considered a tweaked RAID 5 and RAID 5 can be considered a tweaked RAID 1. Going this way, anything can be considered a tweaked anything else, if you tweak enough... -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/5484419e.5050...@plouf.fr.eu.org
Re: LVM RAID5 with missing disk?
Gary Dale extremegroundmai...@gmail.com writes: On 04/12/14 12:51 PM, Dan Ritter wrote: On Thu, Dec 04, 2014 at 02:13:59PM +0100, mad wrote: Hi! I wanted to create a RAID5 with lvm. The basic setup is something like lvcreate --type raid5 -i 2 -L 1G -n my_lv my_vg which would mean 3 physical drives would be used in this RAID5. But can I specify that one drive is missing as it is possible with mdadm? I don't think so, no. You can create your RAID with mdadm and put LVM on top of that. In general I strongly recommend against using RAID5. RAID1, 10, or 6 are all better options if your data's availability is important to you. -dsr- Sorry, but there are good reasons to use RAID 5 and better reasons to NOT use RAID 10. RAID 1 and RAID 5 are both immune to single disk failures in their most common configurations (1 or more data disks with 1 parity disk). The problem is not that RAID5 does not provide resilience against a single disk failure. The problem is that with modern disk capacities, the chances of *another* disk failing while the array is rebuilding have significantly risen. Especially when all the disks came out of the the same batch, they tend to fail at similar times. I know Best Practice is to mix disks in RAID arrays, but who actually practices that, instead of just taking the risk of failure and covering it with a higher RAID level, like RAID6 in this case? -- We will need a longer wall when the revolution comes. --- AJS, quoting an uncertain source. -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/86y4qm76vi@gaheris.avalon.lan
Re: LVM RAID5 with missing disk?
On 05/12/14 05:01 AM, Mart van de Wege wrote: Gary Dale extremegroundmai...@gmail.com writes: On 04/12/14 12:51 PM, Dan Ritter wrote: On Thu, Dec 04, 2014 at 02:13:59PM +0100, mad wrote: Hi! I wanted to create a RAID5 with lvm. The basic setup is something like lvcreate --type raid5 -i 2 -L 1G -n my_lv my_vg which would mean 3 physical drives would be used in this RAID5. But can I specify that one drive is missing as it is possible with mdadm? I don't think so, no. You can create your RAID with mdadm and put LVM on top of that. In general I strongly recommend against using RAID5. RAID1, 10, or 6 are all better options if your data's availability is important to you. -dsr- Sorry, but there are good reasons to use RAID 5 and better reasons to NOT use RAID 10. RAID 1 and RAID 5 are both immune to single disk failures in their most common configurations (1 or more data disks with 1 parity disk). The problem is not that RAID5 does not provide resilience against a single disk failure. The problem is that with modern disk capacities, the chances of *another* disk failing while the array is rebuilding have significantly risen. Especially when all the disks came out of the the same batch, they tend to fail at similar times. I know Best Practice is to mix disks in RAID arrays, but who actually practices that, instead of just taking the risk of failure and covering it with a higher RAID level, like RAID6 in this case? The chances of two disks failing within hours of one another is very small even for disks from the same batch. RAID 6 is preferable when you have large arrays as each additional disk raises the odds. It's also good when the location is remote so it may take a while to replace a failed disk. But for more typical arrays RAID 5 is good enough considering that RAID 6 uses more power, costs more and has reduced performance compared with RAID 5. Again, this is talking about software RAID. With high-performance hardware RAID controllers, RAID 6 is often as fast as RAID 5. -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/5481d279.7000...@torfree.net
Re: LVM RAID5 with missing disk?
Hello, Some mistakes in what you wrote. Gary Dale a écrit : RAID 1 and RAID 5 are both immune to single disk failures in their most common configurations (1 or more data disks with 1 parity disk). RAID 10 is also immune to single disk failure but uses half the disks for parity. RAID 1 and 10 are just mirrors, they have no parity. I guess you mean redundancy. RAID 5 does not use data disks and parity disks. Data and parity are distributed among all disks in the array. RAID 1 with N disks can survive N-1 disk failures. If you are concerned about availability, with 4 disks (the simplest RAID 10 configuration) Linux can use a special RAID 10 mode (mirror+stripe) with two or three disks. with 6 disks, RAID 6 will give you double the capacity of 4 disks or get you immunity to 3 disks failing. RAID 6 can survive 2 disk failures regarless of the number of disks in the array. -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/54821714.3020...@plouf.fr.eu.org
Re: LVM RAID5 with missing disk?
On 12/05/2014 03:35 PM, Pascal Hambourg wrote: Linux can use a special RAID 10 mode (mirror+stripe) with two or three disks. with 6 disks, RAID 6 will give you double the capacity of 4 disks or get you immunity to 3 disks failing. RAID 6 can survive 2 disk failures regarless of the number of disks in the array. Good to know, thanks! I'm starring this one! :) Ric -- My father, Victor Moore (Vic) used to say: There are two Great Sins in the world... ..the Sin of Ignorance, and the Sin of Stupidity. Only the former may be overcome. R.I.P. Dad. Linux user# 44256 -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/54821cd1.9050...@gmail.com
Re: LVM RAID5 with missing disk?
On 05/12/14 03:35 PM, Pascal Hambourg wrote: Hello, Some mistakes in what you wrote. Gary Dale a écrit : RAID 1 and RAID 5 are both immune to single disk failures in their most common configurations (1 or more data disks with 1 parity disk). RAID 10 is also immune to single disk failure but uses half the disks for parity. RAID 1 and 10 are just mirrors, they have no parity. I guess you mean redundancy. RAID 5 does not use data disks and parity disks. Data and parity are distributed among all disks in the array. RAID 1 with N disks can survive N-1 disk failures. You can think of the RAID algorithms as parity checks. A mirror is even parity. While the disks are not physically assigned to be data or parity, you can recreate a failed RAID 5 disk by recalculating the parity based on the surviving disks. If you are concerned about availability, with 4 disks (the simplest RAID 10 configuration) Linux can use a special RAID 10 mode (mirror+stripe) with two or three disks. If you have different sized disks, yes. The more usual case is to use similar disks. If one disk is not striped, you lose some of the performance improvement. RAID 10 with two disks makes little sense. with 6 disks, RAID 6 will give you double the capacity of 4 disks or get you immunity to 3 disks failing. RAID 6 can survive 2 disk failures regarless of the number of disks in the array. You misread the sentence. You can run RAID with any number of parity disks by tweaking the algorithms. Most people don't bother using more than 2 parity disks but there is no theoretical reason why you couldn't, to get as much safety as desired. Prior to being able to boot to an mdadm RAID 5 array, I regularly had 3 to 5 disk RAID 1 /boot partitions - why not use the disks since they are already there and it keeps the partitioning the same across the drives. RAID 6 can be considered a tweaked RAID 5 and RAID 5 can be considered a tweaked RAID 1. -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/548282d0.5000...@torfree.net
LVM RAID5 with missing disk?
Hi! I wanted to create a RAID5 with lvm. The basic setup is something like lvcreate --type raid5 -i 2 -L 1G -n my_lv my_vg which would mean 3 physical drives would be used in this RAID5. But can I specify that one drive is missing as it is possible with mdadm? TIA mad -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/54805e17.7040...@sharktooth.de
Re: LVM RAID5 with missing disk?
On Thu, Dec 04, 2014 at 02:13:59PM +0100, mad wrote: Hi! I wanted to create a RAID5 with lvm. The basic setup is something like lvcreate --type raid5 -i 2 -L 1G -n my_lv my_vg which would mean 3 physical drives would be used in this RAID5. But can I specify that one drive is missing as it is possible with mdadm? I don't think so, no. You can create your RAID with mdadm and put LVM on top of that. In general I strongly recommend against using RAID5. RAID1, 10, or 6 are all better options if your data's availability is important to you. -dsr- -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/20141204175144.gf30...@randomstring.org
Re: LVM RAID5 with missing disk?
On 04/12/14 12:51 PM, Dan Ritter wrote: On Thu, Dec 04, 2014 at 02:13:59PM +0100, mad wrote: Hi! I wanted to create a RAID5 with lvm. The basic setup is something like lvcreate --type raid5 -i 2 -L 1G -n my_lv my_vg which would mean 3 physical drives would be used in this RAID5. But can I specify that one drive is missing as it is possible with mdadm? I don't think so, no. You can create your RAID with mdadm and put LVM on top of that. In general I strongly recommend against using RAID5. RAID1, 10, or 6 are all better options if your data's availability is important to you. -dsr- Sorry, but there are good reasons to use RAID 5 and better reasons to NOT use RAID 10. RAID 1 and RAID 5 are both immune to single disk failures in their most common configurations (1 or more data disks with 1 parity disk). RAID 10 is also immune to single disk failure but uses half the disks for parity. If you are concerned about availability, with 4 disks (the simplest RAID 10 configuration) RAID 6 gives you the same data capacity with immunity to two disk failures and can increase capacity by 50% simply by adding another disk. You need 6 disks to get the same capacity with RAID 10 while with 6 disks, RAID 6 will give you double the capacity of 4 disks or get you immunity to 3 disks failing. Moreover, while a RAID 10 array can sometimes survive 2 disk failures, if it's the wrong 2, you are screwed. For example if A+B are mirrored by C+D, RAID 10 won't survive an A C fail or a B D fail, but will survive A D or B C failing. RAID 0 and RAID 10 are used only when write performance is important. Read performance is generally better with any type of RAID. Since in most cases you are doing a lot more reads than writes, you are unlikely to notice much difference between RAID varieties. If you need RAID 10, you probably also need some high-performance hardware RAID controllers. Of course, I don't know your situation, but for me, data integrity is important but so is capacity, so I use with RAID 5. I have the luxury of being take the server offline or shut down the GUI on my workstation, both of which use RAID 5, so that an array rebuild can proceed rapidly. Just don't ignore smartctl errors. When you get a failing disk, replace it immediately. -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of unsubscribe. Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/54813cce.30...@torfree.net
