4 ms·

> RAID risk is about your rebuild failure rate, in this case URE (Unrecoverable Read Error). This seems to be the keystone to your reasoning, and I'm not sure

by mmt 8y ago

> RAID risk is about your rebuild failure rate, in this case URE (Unrecoverable Read Error). This seems to be the keystone to your reasoning, and I'm not sure it's true, in practice. IME and in the literature I've read, modern RAIDs operate assuming a whole-disk failure (both on write and on read). Sub-whole-device errors seem to be an issue of concern limited to ECC in RAM. Do you have pointers to literature about this in the context of RAID? > A failover is still a failover and can reduce performance and it reduces your remaining failover margin. I think that means that the node-level redundancy isn't actually equivalent to RAID1-style redundancy (where there's no explicit failover), which is important to mention. Of course, my question wasn't really "why bother" so much as asking if that architecture isn't far more wasteful than, say, RAID1 (or even just RAID6/RAIDZ3) intra-node and less node-level redundancy. > You want to keep your failover rate low I'm pretty sure best practices are the opposite of this. One can only know if the redundant copy actually works by using it, so a system that can failover more often is better. Ideally, the system is, like RAID1, requires no explicit failover and merely load balances both copies. > RAID6 will in the next few years hit the spot where the risk of loosing 2 drives during a rebuild, due to the large drive sizes, becomes large You'll need to quantify these, though, since, though drive capacities have continued to grow faster than transfer speeds (for mechanical drives), it's still in the OOM of 30 hours for minimum rebuild time. What's the risk of a second and third drive failure in 300 hours (1/10th the maximum rebuild rate)?

zaarn 8y ago
>Do you have pointers to literature about this in the context of RAID? URE Rate is documented in most handbooks or manuals on harddisks, it's usually about 10^-12, though this is a worst case error rate and -13 or -14 is more realistic. At that rate you means your harddrive will return a read error every terabyte to every hundred terabytes. A URE can cause the raid controller to think the HDD is defective and crash the array if no parity is left to compensate. >Of course, my question wasn't really "why bother" so much as asking if that architecture isn't far more wasteful than, say, RAID1 (or even just RAID6/RAIDZ3) intra-node and less node-level redundancy. Depends on your use case. >I'm pretty sure best practices are the opposite of this. Best practise is to test your failover regularly, correct, but this doesn't mean you shouldn't minimize your actual failover rate. >What's the risk of a second and third drive failure in 300 hours (1/10th the maximum rebuild rate)? It's not that low, especially since a lot of arrays contain similar enough harddrives that concurrent failure is possible (although you can make it less likely by avoiding similar batches). And again, that URE rate has a risk of marking of your array as dead if you get unlucky.
- mmt 8y ago
  > URE Rate is documented in most handbooks or manuals on harddisks, it's usually about 10^-12, though this is a worst case error rate and -13 or -14 is more realistic. The kind of literature I'm looking for is based on real world data, since spec sheets are generally marketing documents and therefore, at best, uninformative. This is particularly noticeable when a single number is used for a statistic, as in this situation. Let's take a current datasheet [1] which actually says 1 sector (unclear if that's 512 or 4k, but let's assume 512) per 10E15, max, a full 3 OOMs higher than what you mentioned. That's 512PB, which, at the highest listed transfer rate, would take almost 545k hours, corresponding to slightly above a 1.6% AFR for worst-case URE-caused failures. Since I don't believe the spec sheet's 0.35% (average) AFR, but since real-world AFRs are in the low single-percent, I'm staying with the conclusion that read errors need not be considered separately from any other underlying cause of drive failure. > Depends on your use case. I'm not convinced that "waste" can ever depend on use case, when examined under the narrow lense of redundancy. Regardless, it was a use case you, specifically brought up, so the question remains open. > Best practise is to test your failover regularly, correct You're misunderstanding my assertion, which wasn't about regularity but frequency. I made no statement as to regularity (and it may even be better, with discrete failovers, to do so irregularly, rather than regularly). My assertion is that best practice dictates frequent failover. > It's not that low > risk of marking of your array as dead if you get unlucky. You still haven't actually quantified the risk. "Unlucky" and "possible" are too hand-wavy to engineer around. Quantifying the risk, even if it's a very coarse approximation, is a prerequisite to avoiding FUD-based decisions/actions. Usually, the most visible consequence of the latter is the waste alluded to earlier, manifesting as higher cost and/or lower performance. The less visible consequence is misallocated resources/attention from something relatively likely (e.g. human error causing catastrophic data loss) to something much less likely (e.g. single sector read error causing catastrophic data loss). [1] https://www.seagate.com/www-content/datasheets/pdfs/exos-x-12-DS1946-2-1712US-en_US.pdf https://www.seagate.com/www-content/datasheets/pdfs/exos-x-1...
  - zaarn 8y ago
    >Let's take a current datasheet [1] Of a high profile and high quality enterprise disk, unlikely to appear in 99% of RAID setups. Congratulations. Most RAID setups in the wild consist of low quality enterprise or even desktop platters, high quality setups are rare and expensive. Esp. considering that lots of consumers have a NAS or MyCloud/eqv. at home and SMB doesn't buy those harddrives either, I don't think this is a valid comparison. [https://www.seagate.com/files/www-content/product-content/nas-fam/nas-hdd/en-us/docs/nas-hdd-data-sheet-ds1789-1-1305us.pdf https://www.seagate.com/files/www-content/product-content/na...] [https://www.seagate.com/staticfiles/docs/pdf/datasheet/disc/desktop-hdd-data-sheet-ds1770-1-1212us.pdf https://www.seagate.com/staticfiles/docs/pdf/datasheet/disc/...] These drives here have an OOM higher error rate on the data sheet and they're still not very common, plus the error rate is given in bits here, shaving of 3 or 5 OOM again, calculating it out gives you an bad bit every 12 TB or so. Depending on how your RAID works, this can ruin the entire stripe or the sector it gets the hit on. 12TB isn't a lot and there is a good probability that a 3x4TB RAID5 array will hit such an error and a 3x2TB RAID5 has a 50/50 chance of having it during a rebuild. >I'm not convinced that "waste" can ever depend on use case It definitely can. Consider the use case of "office documents" vs "high performance video storage". With office documents a high throughput is not needed so we can reduce storage costs by using a RAID5 or 6 depending on array size. Video storage for example for CCTV requires more throughput an a RAID10 will give you better performance at some lost effective storage. You can of course put your office documents on a RAID10 but that is the wasted space. It is not necessary to maximize the RAID performance and the low storage needed for office documents means you can use RAID5 on small arrays fairly safely, RAID6 if you need more. Waste is a matter of efficiency in all domains, in this case means picking the RAID level that maximizes storage efficiency, performance goals met and safety as best as possible. Picking one that trades off one for the other is waste or even dangerous. >My assertion is that best practice dictates frequent failover. I don't think I ever met a sysadmin that insistent frequent failover due to failure is considered best practise. Regular testing yes, regular failure, no. Failure is expensive, testing not. >You still haven't actually quantified the risk. "Unlucky" and "possible" are too hand-wavy to engineer around. They're not really, plus you can easily quantify risks as you have done using datasheets. Though the moment your array isn't a singular batch of exactly the same harddrive these estimations become a lot harder. You don't need to exactly quantify the risk, it is sufficient to know the margins of the risk or even if you are going into the margins of the risk, ie a risk model. I don't need to know the exact and perfect probability of a read error on the array, I need to know if the array is likely to experience one (with likely being >1% or any other >X% during a rebuild or during normal operation). This question is more easily answered and doesn't require more than estimations. This trades cost efficiency for safety margins meaning that an array has a comfortably low actualized risk of failing. That is what I consider actual engineering there.

by mmt 8y ago

zaarn 8y ago

>Do you have pointers to literature about this in the context of RAID? URE Rate is documented in most handbooks or manuals on harddisks, it's usually about 10^-12, though this is a worst case error rate and -13 or -14 is more realistic. At that rate you means your harddrive will return a read error every terabyte to every hundred terabytes. A URE can cause the raid controller to think the HDD is defective and crash the array if no parity is left to compensate. >Of course, my question wasn't really "why bother" so much as asking if that architecture isn't far more wasteful than, say, RAID1 (or even just RAID6/RAIDZ3) intra-node and less node-level redundancy. Depends on your use case. >I'm pretty sure best practices are the opposite of this. Best practise is to test your failover regularly, correct, but this doesn't mean you shouldn't minimize your actual failover rate. >What's the risk of a second and third drive failure in 300 hours (1/10th the maximum rebuild rate)? It's not that low, especially since a lot of arrays contain similar enough harddrives that concurrent failure is possible (although you can make it less likely by avoiding similar batches). And again, that URE rate has a risk of marking of your array as dead if you get unlucky.

mmt 8y ago
> URE Rate is documented in most handbooks or manuals on harddisks, it's usually about 10^-12, though this is a worst case error rate and -13 or -14 is more realistic. The kind of literature I'm looking for is based on real world data, since spec sheets are generally marketing documents and therefore, at best, uninformative. This is particularly noticeable when a single number is used for a statistic, as in this situation. Let's take a current datasheet [1] which actually says 1 sector (unclear if that's 512 or 4k, but let's assume 512) per 10E15, max, a full 3 OOMs higher than what you mentioned. That's 512PB, which, at the highest listed transfer rate, would take almost 545k hours, corresponding to slightly above a 1.6% AFR for worst-case URE-caused failures. Since I don't believe the spec sheet's 0.35% (average) AFR, but since real-world AFRs are in the low single-percent, I'm staying with the conclusion that read errors need not be considered separately from any other underlying cause of drive failure. > Depends on your use case. I'm not convinced that "waste" can ever depend on use case, when examined under the narrow lense of redundancy. Regardless, it was a use case you, specifically brought up, so the question remains open. > Best practise is to test your failover regularly, correct You're misunderstanding my assertion, which wasn't about regularity but frequency. I made no statement as to regularity (and it may even be better, with discrete failovers, to do so irregularly, rather than regularly). My assertion is that best practice dictates frequent failover. > It's not that low > risk of marking of your array as dead if you get unlucky. You still haven't actually quantified the risk. "Unlucky" and "possible" are too hand-wavy to engineer around. Quantifying the risk, even if it's a very coarse approximation, is a prerequisite to avoiding FUD-based decisions/actions. Usually, the most visible consequence of the latter is the waste alluded to earlier, manifesting as higher cost and/or lower performance. The less visible consequence is misallocated resources/attention from something relatively likely (e.g. human error causing catastrophic data loss) to something much less likely (e.g. single sector read error causing catastrophic data loss). [1] https://www.seagate.com/www-content/datasheets/pdfs/exos-x-12-DS1946-2-1712US-en_US.pdf https://www.seagate.com/www-content/datasheets/pdfs/exos-x-1...
- zaarn 8y ago
  >Let's take a current datasheet [1] Of a high profile and high quality enterprise disk, unlikely to appear in 99% of RAID setups. Congratulations. Most RAID setups in the wild consist of low quality enterprise or even desktop platters, high quality setups are rare and expensive. Esp. considering that lots of consumers have a NAS or MyCloud/eqv. at home and SMB doesn't buy those harddrives either, I don't think this is a valid comparison. [https://www.seagate.com/files/www-content/product-content/nas-fam/nas-hdd/en-us/docs/nas-hdd-data-sheet-ds1789-1-1305us.pdf https://www.seagate.com/files/www-content/product-content/na...] [https://www.seagate.com/staticfiles/docs/pdf/datasheet/disc/desktop-hdd-data-sheet-ds1770-1-1212us.pdf https://www.seagate.com/staticfiles/docs/pdf/datasheet/disc/...] These drives here have an OOM higher error rate on the data sheet and they're still not very common, plus the error rate is given in bits here, shaving of 3 or 5 OOM again, calculating it out gives you an bad bit every 12 TB or so. Depending on how your RAID works, this can ruin the entire stripe or the sector it gets the hit on. 12TB isn't a lot and there is a good probability that a 3x4TB RAID5 array will hit such an error and a 3x2TB RAID5 has a 50/50 chance of having it during a rebuild. >I'm not convinced that "waste" can ever depend on use case It definitely can. Consider the use case of "office documents" vs "high performance video storage". With office documents a high throughput is not needed so we can reduce storage costs by using a RAID5 or 6 depending on array size. Video storage for example for CCTV requires more throughput an a RAID10 will give you better performance at some lost effective storage. You can of course put your office documents on a RAID10 but that is the wasted space. It is not necessary to maximize the RAID performance and the low storage needed for office documents means you can use RAID5 on small arrays fairly safely, RAID6 if you need more. Waste is a matter of efficiency in all domains, in this case means picking the RAID level that maximizes storage efficiency, performance goals met and safety as best as possible. Picking one that trades off one for the other is waste or even dangerous. >My assertion is that best practice dictates frequent failover. I don't think I ever met a sysadmin that insistent frequent failover due to failure is considered best practise. Regular testing yes, regular failure, no. Failure is expensive, testing not. >You still haven't actually quantified the risk. "Unlucky" and "possible" are too hand-wavy to engineer around. They're not really, plus you can easily quantify risks as you have done using datasheets. Though the moment your array isn't a singular batch of exactly the same harddrive these estimations become a lot harder. You don't need to exactly quantify the risk, it is sufficient to know the margins of the risk or even if you are going into the margins of the risk, ie a risk model. I don't need to know the exact and perfect probability of a read error on the array, I need to know if the array is likely to experience one (with likely being >1% or any other >X% during a rebuild or during normal operation). This question is more easily answered and doesn't require more than estimations. This trades cost efficiency for safety margins meaning that an array has a comfortably low actualized risk of failing. That is what I consider actual engineering there.