labist Posted March 2, 2012 Posted March 2, 2012 Dear all My question is that if i configure Raid 5 with 3 hard drives and if 2 of them fails will it be possible to recover data Thanks.
Steve21 Posted March 2, 2012 Posted March 2, 2012 Dear all My question is that if i configure Raid 5 with 3 hard drives and if 2 of them fails will it be possible to recover data Thanks. With 2 disks down no it's not. With 1 down yes it is. Steve
sonofsanta Posted March 2, 2012 Posted March 2, 2012 No. Even with a hot spare as well, a second drive could fail whilst the array was rebuilding. The only true protection against two-disk failure is RAID 6.
PotNoodleTech Posted March 2, 2012 Posted March 2, 2012 Yes but the chances of two drives failing within 24 hours are slim. Possible but slim. Raid 6 has nasty write overheads. I used to run it but don't any more gone back to raid 5 + hospare for most things.
elsiegee40 Posted March 2, 2012 Posted March 2, 2012 It's scary stuff hot-swapping a RAID 5 drive, but I have done it and survived! It wasn't difficult; it's just the awful hoping that what I'm doing won't affect anything else! As others have said, it would be really bad luck to lose 2 in short succession. Nothing is impossible, so it could happen, but hopefully it won't (Said she clutching the nearest piece of wood with crossed fingers)
danbuntu Posted March 2, 2012 Posted March 2, 2012 No. Even with a hot spare as well, a second drive could fail whilst the array was rebuilding. The only true protection against two-disk failure is RAID 6. or raid 5 with more disks
plexer Posted March 2, 2012 Posted March 2, 2012 or raid 5 with more disks Nope RAID5 with more disks still only protects you against one failure and the more drives you have the bigger your array the longer your rebuild time and the higher chance of more failures. Ben
jamesfed Posted March 2, 2012 Posted March 2, 2012 Lets remember that just RAID on its own (even RAID6) isn't 100% sure that you won't loose data at some stage due to drive failure/ect. No system will ever be 100% but a good backup system is always a must - ideally a mix of both disk (for high speed backup/restore) and tape (for offsite).
ToyHeartsFan Posted March 2, 2012 Posted March 2, 2012 Nope RAID5 with more disks still only protects you against one failure and the more drives you have the bigger your array the longer your rebuild time and the higher chance of more failures. Ben Raid5 But more drives mean the array keeps working if one fails, without redundancy and with a performance hit though. It might depend on the controller but from experience if a single drive fails in a 3 drive raid5 array the server will crash and you can't use the array until the faulty drive has been replaced and the array rebuilt. If on the other hand a single drive fails in a 5 disk raid5 array the server keeps running and the array should rebuild in the background when the faulty drive is replaced - dont ask me about a 4 disk array because I dont know. So if you can go for a 5 disk array +1hotspare +1sat on a shelf Raid5 only gives you redundancy it still needs a backup solution because if 2 drives fail you can wave goodbye to your data.
sonofsanta Posted March 5, 2012 Posted March 5, 2012 It might depend on the controller but from experience if a single drive fails in a 3 drive raid5 array the server will crash and you can't use the array until the faulty drive has been replaced and the array rebuilt. I don't think that should happen - the whole point of RAID 5 is you can lose a disk and still keep running. More disks in RAID5 is actually a greater threat, as others have said, as it increases the odds of a disk failing while you're still recovering the missing disk. A hot spare is not really any greater protection, other than the fact that it reduces the time from "disk goes wrong" to "array rebuilding" to effectively zero, rather than the variable "when you notice" time span.
Hawkeyez Posted March 5, 2012 Posted March 5, 2012 Yes but the chances of two drives failing within 24 hours are slim. Possible but slim. Raid 6 has nasty write overheads. I used to run it but don't any more gone back to raid 5 + hospare for most things. Yep, slim, but can happen. Member of my team here had his Raid5 fail at home, only for a 2nd disk to fail while the 1st disk was still rebuilding!. Like others have said, still need to have a backup solution, just in case that 1 in a million happens, and 2 disks fail.
localzuk Posted March 5, 2012 Posted March 5, 2012 There was a post about RAID5 and 6 recently on the Spiceworks community - basically saying that RAID5 is poor practice now due to the probability of a URE occuring, especially on a SATA array. (The article linked was here - Data Storage: The Myth of Redundancy - Datamation). Basically, the chance of an unrecoverable error occurring on a RAID array during rebuild is 50% on a 6TB array which uses standard SATA disks. SAS disks generally have a higher rating for UREs, so it isn't strictly relevant for them at the moment (but in a few years time when we hit the array sizes of them too it will be). The article goes to say that RAID 0 is sometimes more reliable than RAID 5 - which in itself is a ridiculous claim, however the original point does stand somewhat - that RAID5 is not really an ideal choice any more. RAID 6 adds a slight extra level of redundancy, but doesn't increase the reliability - the chance of the array going belly up during rebuild still exists. So, the end result is advice to go with a RAID 10 array instead. If you are looking at a 3 disk array, switching to 4 disks isn't a huge extra cost either - and the size of the array would be the same at the end of it.
Naughty_Nigel Posted March 5, 2012 Posted March 5, 2012 Yep, slim, but can happen. Member of my team here had his Raid5 fail at home, only for a 2nd disk to fail while the 1st disk was still rebuilding!. Like others have said, still need to have a backup solution, just in case that 1 in a million happens, and 2 disks fail. It might be one in a million, but the MTBF of any one drive in an array will be just the same as the other drives, so if one drive fails after several years service the chances are that another one will fail soon! Plus, if there are factors which have contributed to the failure of one drive, (temperature, shock, power failure, etc), those same factors will also apply to other drives in the array. NN
MYK-IT Posted March 5, 2012 Posted March 5, 2012 (edited) RAID5 can only cope with (a maximum of) 1 HDD failing. As sonofsanta mentioned, having a HDD configured as a spare/hotswap just means the RAID contoller will automatically use that drive (and rebuild the RAID) instead of you manually removing the failed one and inserting a working HDD. It makes no difference (as far as redundancy is concerned) as to how may HDDs are in the RAID5 Array, it can still only cope with one HDD failing; plus there is a 50% performance hit whilst a HDD has failed. Where people express concern with RAID5 is if another HDD fails before the original HDD has been replaced AND the RAID has not been rebuilt; but the same could be said for RAID1. Only RAID6 can cope with any 2 HDD failures (but write performance is 50% slower than RAID5), and RAID 10, 50 etc can cope with 1 per sub-array. You could go on and specify multiple RAID Controllers etc, but it is a balance of performance, redundancy and cost and more importantly backup strategy. Edited March 5, 2012 by MYK-IT
Arthur Posted March 5, 2012 Posted March 5, 2012 There's an interesting paper by Adam Leventhal about how unsuitable RAID-5 is and RAID-6 will be... Triple-Parity RAID and Beyond
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