![]() ![]() In the case of the ten disk RAID-6 group, this means that there are now nine drives but we can only lose one more drive before losing data protection (recall that RAID-6 allows you to lose two drives before the next lost drive results in unrecoverable data lost). The drive on which the error has occurred is then failed causing a rebuild. Given the information in the previous table, when about 10TB of data is read then there is almost a 100% chance of encountering a hard disk error. This is true even if the file system using the RAID-6 group has no data in it.įor example, if we are using ten 4TB Consumer SATA drives in a RAID-6 group, there is a total of 40TB of data. This means they have to read 100% of the remaining drives even if there is no data on portions of the drive.įor instance, if we have a RAID-6 group with 10 total drives and you lose a drive, then 100% of the seven remaining drives have to be read to rebuild the failed drive and regain the RAID-6 protection. ![]() Classic RAID groups will have to read all the disks that remain in the RAID group to rebuild the failed drive. Assuming the drive was part of a RAID group the controller will start a RAID rebuild using a spare drive. For Enterprise SAS drives about 1.1 PB of data can be read before approaching a 100% probability of hitting an unreadable sector (hard error).Īt the point where you encounter a hard error the controller assumes the drive has failed. On the other hand, using Enterprise SAS drives, a bit more data can read before encountering a read error. This is equivalent to reading roughly 111 TB of data (0.11 PB). SATA/SAS Nearline Enterprise drives improve the hard error rate by a factor of 10 but they are still 10 times more likely to encounter a hard read error (inability to read a sector) relative to an Enterprise SAS drive. ![]() If you read 10TB of data from Consumer SATA drives, the probability of encountering a read error approaches 100% (virtually guaranteed to get an unreadable sector resulting in a failed drive). What the table tells us is that Consumer SATA drives are 100 times more likely than Enterprise SAS drives to encounter a read error. The hard error rate for these drives is 10E16. For example, Seagate has a 10.5K drive with a SAS interface (no SATA interface). The third class of drives, listed in the third row of the table as “Enterprise SAS/FC,” typically only has a SAS interface. The first one has a 12Gbps interface and the second one has a 6Gbps SATA interface but both have the same hard error rate, 10E15. Both drives are the same but have different interfaces. For example, Seagate has two enterprise drives, where the first one has a SAS 12 Gbps interface and the second one has a SATA 6 Gbps interface. The second class of drives, labeled as “SATA/SAS Nearline Enterprise” in the above table, can have a SATA or SAS interface (same drive for either interface). Notice that the hard error rate, referred to as “Nonrecoverable Read Errors per Bits Read, Max” in the linked document, is 10E14 as shown in the table above. Here is an example of a SATA consumer drive spec sheet. The first row in the table, which are drives listed as “SATA Consumer,” are drives that typically only have a SATA interface (no versions with a SAS interface). Table 1: Hard error rate for various storage media Most hardware will go through several retries to read the sector but after a certain number of retries and/or a certain period of time, it will fail and the drive reports the sector as unreadable and the drive has failed.īelow is a table from Henry’s previously mentioned article that lists the hard error rates for various drive types and how much data, in petabytes, would have to read before encountering an unreadable sector. When a drive encounters a read error it simply means that any data that was on the sector cannot be read. It is defined as the number of bits that are read before the probability of hitting a read error reaches 100% (i.e. This subject has been written about several times including one of Henry Newman’s recent articles. ![]()
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