5 ms·

I'm going to go out on a limb and bet that this is because too many people use RAID as a backup, instead of something in place to help prevent downtime.

lolc 8y ago
They are specific about the RAID levels. RAID 5 is troubelsome because your data is smeared across disks. That has all kinds of negative consequences when it comes to performance (write amplification) and recovery (unlikely).
- cthalupa 8y ago
  >RAID 5 is troubelsome because your data is smeared across disks. So is RAID0. >negative consequences when it comes to performance (write amplification) If you're using SSDs, not really a concern with spinning rust. >recovery (unlikely) Really? I've successfully rebuilt literally thousands of RAID5 arrays successfully over the years, with maybe a two dozen failing over that time period, and in most of those cases the rebuilds were not kicked off shortly after the initial disk failure. (I used to work in the hosting industry. Many a customer opted for RAID5 in the servers they leased from us) I've opted for RAIDZ3 at home, and don't know that I would recommend anything under RAID6, but I think it's pretty silly to claim that recovery is unlikely.
  - simonh 8y ago
    For smearing read striping. I think 'recovery (unlikely)' means a problem during recovery is unlikely. I think RAID 3/4/5 can be better that nothing, they're not completely worthless, but come with significant down sides compared to better schemes that make them undesirable these days. The I in RAID was put there back when disks were many orders of magnitude more expensive than today, it's just a false economy not going to RAID10 or some other better scheme.
  - lsc 8y ago
    >>negative consequences when it comes to performance (write amplification) >If you're using SSDs, not really a concern with spinning rust. This is not true. Raid5 has the same problem with "you must write a unit of x at once" that SSD have. If I've got a 64K chunk size on your raid5, assuming that the disk has been written to, if you are writing a whole 64K chunk, you are good. but if I'm writing a 32K chunk to a stripe that already has data (and you have the same 'I wrote the data and deleted it but maybe my underlying block doesn't know that it's deleted' problem you have with ssd) you need to read all the data in that chunk you want to keep, recalculate parity (the fast part) and then write that whole 64K chunk back out. It really is a lot like the problems you have with SSD, raid or no, only you have more choice over the chunk size when building the array. All that said, I have also managed a lot of raid5 on spinning disk in my day... it absolutely screams for read-only workloads, and write back caching can usually mitigate the write amplification issues (of course, adding in more data consistency issues when you lose power) and while yeah, there is danger of losing a few sectors during rebuild, people also leave the hardware write back caches enabled on their disks. Given the choice, I'd trust data on a raid5 over data on disks with write-back cache enabled in your typical 'unexpected power loss every three years' situation. Either way, technically, yeah, it's not 100% safe, but most of the time it's good enough if you need the performance per dollar.
  - tinus_hn 8y ago
    The reliability of RAID level 5 rebuilding on disks depends on regular checking. If you never read most of the data on the disks, which many people do, you stand quite the chance of discovering part of it is unreadable the first time you try to read it in years, which will be during a rebuild. And then, even if all of the data may well still be available, the rebuild will fail.

lolc 8y ago

They are specific about the RAID levels. RAID 5 is troubelsome because your data is smeared across disks. That has all kinds of negative consequences when it comes to performance (write amplification) and recovery (unlikely).

cthalupa 8y ago
>RAID 5 is troubelsome because your data is smeared across disks. So is RAID0. >negative consequences when it comes to performance (write amplification) If you're using SSDs, not really a concern with spinning rust. >recovery (unlikely) Really? I've successfully rebuilt literally thousands of RAID5 arrays successfully over the years, with maybe a two dozen failing over that time period, and in most of those cases the rebuilds were not kicked off shortly after the initial disk failure. (I used to work in the hosting industry. Many a customer opted for RAID5 in the servers they leased from us) I've opted for RAIDZ3 at home, and don't know that I would recommend anything under RAID6, but I think it's pretty silly to claim that recovery is unlikely.
- simonh 8y ago
  For smearing read striping. I think 'recovery (unlikely)' means a problem during recovery is unlikely. I think RAID 3/4/5 can be better that nothing, they're not completely worthless, but come with significant down sides compared to better schemes that make them undesirable these days. The I in RAID was put there back when disks were many orders of magnitude more expensive than today, it's just a false economy not going to RAID10 or some other better scheme.
- lsc 8y ago
  >>negative consequences when it comes to performance (write amplification) >If you're using SSDs, not really a concern with spinning rust. This is not true. Raid5 has the same problem with "you must write a unit of x at once" that SSD have. If I've got a 64K chunk size on your raid5, assuming that the disk has been written to, if you are writing a whole 64K chunk, you are good. but if I'm writing a 32K chunk to a stripe that already has data (and you have the same 'I wrote the data and deleted it but maybe my underlying block doesn't know that it's deleted' problem you have with ssd) you need to read all the data in that chunk you want to keep, recalculate parity (the fast part) and then write that whole 64K chunk back out. It really is a lot like the problems you have with SSD, raid or no, only you have more choice over the chunk size when building the array. All that said, I have also managed a lot of raid5 on spinning disk in my day... it absolutely screams for read-only workloads, and write back caching can usually mitigate the write amplification issues (of course, adding in more data consistency issues when you lose power) and while yeah, there is danger of losing a few sectors during rebuild, people also leave the hardware write back caches enabled on their disks. Given the choice, I'd trust data on a raid5 over data on disks with write-back cache enabled in your typical 'unexpected power loss every three years' situation. Either way, technically, yeah, it's not 100% safe, but most of the time it's good enough if you need the performance per dollar.
- tinus_hn 8y ago
  The reliability of RAID level 5 rebuilding on disks depends on regular checking. If you never read most of the data on the disks, which many people do, you stand quite the chance of discovering part of it is unreadable the first time you try to read it in years, which will be during a rebuild. And then, even if all of the data may well still be available, the rebuild will fail.