4 ms·

> An HDD/SSD without ECC will not work, and wouldn't have worked for a long time now. Many of the reads performed will require some ECC use. Yeah, I expressed

by cbsmith 10y ago

> An HDD/SSD without ECC will not work, and wouldn't have worked for a long time now. Many of the reads performed will require some ECC use. Yeah, I expressed that badly. I always forget about the ECC bytes that are in the firmware. > The probability of failure during rebuild is not really directly linked to drive size, the usual interpretation of the drive BER is wrong (media BER is stated across a large population of drives rather than just one drive). Regardless of interpretations of BER, I can't agree about drive sizes. The phenomenon of failures during rebuild is well documented and the driving principle behind double-parity RAID. Adam Leventhal (who ought to know this stuff better than either of us) wrote a paper several years back on the need for triple-parity RAID, and it was entirely driven by increased drive densities: http://queue.acm.org/detail.cfm?id=1670144 http://queue.acm.org/detail.cfm?id=1670144 The reality is the higher drive densities mean you lose more bytes at a time when you have a drive failure, and that means more bytes you want to have "recovered".

baruch 10y ago
Adam Leventhal uses the wrong interpretation of the BER value so I wouldn't take his words at face value. The whole argument of Adam is by the disk BER and not by density. I do assume though that the RAID array does media scrub (BMS) periodically, if you don't you are at risk anyway and I'd call that negligence in maintaining your RAID. If you do use scrubbing the risk that another drive has a bad media spot is low as it must have developed in the time from the last scan and that is a bounded time (week, two, four) so the risk of two drives having a bad sector is now even lower (though never non-zero, backups are still a thing). If you couple that also with TLER and proper media handling instead of dropping the disk on media error the risk to the data becomes very low since there isn't a very high likelyhood that two disks will have a bad sector in the same stripe. I've been working with HDDs and SSDs and developing software for enterprise storage systems for a number of years now, I've worked in XIV and for all the thousands of systems and hundred of thousand disks of many models I've never seen two disks fail to read the same stripe and RAID recovery was always possible. Other problems are more likely sooner than an actual RAID failure (technician shutting down the system by pressing the UPS buttons or a software bug). I did learn of several failure modes that can increase the risk but they depend on specific workloads that are not generally applicable. One of those is that if you write to one track all too often you may affect nearby tracks and if the workload is high enough you don't give the disk the time to fix this (the disk tracks this failure mode and will workaround it in idle time). In such a case same stripe can be affected in multiple disks and the time to develop may (or may not) be shorter than the media scrub time. And even then a thin provisioned raid structure would reduce the risk of this failure mode and giving disks some rest time (even just a few seconds) would allow the drive to fix this and other problems it knows about. All in all, RAID is not dead (yet).
- cbsmith 10y ago
  So why would one ever bother with RAID-6 (or its moral equivalent with RAID-Z)? I've yet to hear someone justify it from a requirement being to actually survive two drives failing simultaneously.
  - baruch 10y ago
    The main problem is not two drives failing simultaneously but rather one failing shortly after the other. The more drives you have the more the likelihood of that increases, the rebuild time also increases with large drives and so you are more open to such a second disk failure. If you take the MTTF of the drive (or the mostly equivalent probability of failure) you find the risk increases since the MTTF of the drives does not increase at all with size, it is rather constant at least as it is specified by the vendors (HDDs are usually 1.2M hours). The more drives you have the more likely one of them to have a failure and once one fails and since the rebuild time increases with size your chance of failure during rebuild increases as well. Some systems mitigate that by having a more evenly distributed RAID such that the rebuild time doesn't increase that much as the drive size increases and is actually rather low. XIV systems are like that.
    - cbsmith 10y ago
      > The more drives you have the more likely one of them to have a failure and once one fails and since the rebuild time increases with size your chance of failure during rebuild increases as well. That was exactly what I was saying. Given the same sustained transfer rate, the bigger the drive, the longer the rebuild time, hence the greater the chance you'll have a failure during the rebuild. While you might think they would, throughput increases have not grown to match increases in bit density & storage capacity. SSD's have helped a bit in this area because their failure profiles are different and under the covers they are kind of like a giant array of mini devices, but AFAIK they still present challenges during RAID rebuilds.

by cbsmith 10y ago

baruch 10y ago

Adam Leventhal uses the wrong interpretation of the BER value so I wouldn't take his words at face value. The whole argument of Adam is by the disk BER and not by density. I do assume though that the RAID array does media scrub (BMS) periodically, if you don't you are at risk anyway and I'd call that negligence in maintaining your RAID. If you do use scrubbing the risk that another drive has a bad media spot is low as it must have developed in the time from the last scan and that is a bounded time (week, two, four) so the risk of two drives having a bad sector is now even lower (though never non-zero, backups are still a thing). If you couple that also with TLER and proper media handling instead of dropping the disk on media error the risk to the data becomes very low since there isn't a very high likelyhood that two disks will have a bad sector in the same stripe. I've been working with HDDs and SSDs and developing software for enterprise storage systems for a number of years now, I've worked in XIV and for all the thousands of systems and hundred of thousand disks of many models I've never seen two disks fail to read the same stripe and RAID recovery was always possible. Other problems are more likely sooner than an actual RAID failure (technician shutting down the system by pressing the UPS buttons or a software bug). I did learn of several failure modes that can increase the risk but they depend on specific workloads that are not generally applicable. One of those is that if you write to one track all too often you may affect nearby tracks and if the workload is high enough you don't give the disk the time to fix this (the disk tracks this failure mode and will workaround it in idle time). In such a case same stripe can be affected in multiple disks and the time to develop may (or may not) be shorter than the media scrub time. And even then a thin provisioned raid structure would reduce the risk of this failure mode and giving disks some rest time (even just a few seconds) would allow the drive to fix this and other problems it knows about. All in all, RAID is not dead (yet).

cbsmith 10y ago
So why would one ever bother with RAID-6 (or its moral equivalent with RAID-Z)? I've yet to hear someone justify it from a requirement being to actually survive two drives failing simultaneously.
- baruch 10y ago
  The main problem is not two drives failing simultaneously but rather one failing shortly after the other. The more drives you have the more the likelihood of that increases, the rebuild time also increases with large drives and so you are more open to such a second disk failure. If you take the MTTF of the drive (or the mostly equivalent probability of failure) you find the risk increases since the MTTF of the drives does not increase at all with size, it is rather constant at least as it is specified by the vendors (HDDs are usually 1.2M hours). The more drives you have the more likely one of them to have a failure and once one fails and since the rebuild time increases with size your chance of failure during rebuild increases as well. Some systems mitigate that by having a more evenly distributed RAID such that the rebuild time doesn't increase that much as the drive size increases and is actually rather low. XIV systems are like that.
  - cbsmith 10y ago
    > The more drives you have the more likely one of them to have a failure and once one fails and since the rebuild time increases with size your chance of failure during rebuild increases as well. That was exactly what I was saying. Given the same sustained transfer rate, the bigger the drive, the longer the rebuild time, hence the greater the chance you'll have a failure during the rebuild. While you might think they would, throughput increases have not grown to match increases in bit density & storage capacity. SSD's have helped a bit in this area because their failure profiles are different and under the covers they are kind of like a giant array of mini devices, but AFAIK they still present challenges during RAID rebuilds.