Updated on 19\/08\/2025<\/b><\/p>\n\n\n\n
Due, in part, to the different views and opinions regarding the usage of hot spare disks in our previous post, we\u2019ve decided to add an update for clarification. <\/span><\/i><\/p>\n\n\n\n As stated in almost every theory, using a hot spare disk<\/a> with ZFS, or in any other data storage environment is a good solution, as it automatically responds to malfunctions in a RAID and helps minimize the duration of a degraded array state.<\/p>\n\n\n\n That being said, the primary goal of creating a RAID is to ensure continuous operation and prevent data loss<\/a> in the event of a disk failure. Therefore, anything that increases the risk of data loss could be considered a bad idea. Let\u2019s take a closer look at some of the problematic aspects of using hot spare disks.<\/span><\/p>\n\n\n\n The primary issue with hot spare disks is that they enable the rebuilding (resilvering) of a system that is still in active use as a production server. While the resilvering process<\/a> is taking place, the system will also continue to process the usual production data reads and writes. <\/span><\/p>\n\n\n\n Resilvering is a process that consumes significant server resources. When executed while the server is still in use, it must compete with production workloads. Because it is treated as a low-priority task, the resilvering process can take an extended amount of time \u2013 sometimes even several days. This prolonged operation at maximum throughput can put considerable strain on the disks, especially HDDs, and may lead to serious wear or potential failures<\/a>.<\/span><\/p>\n\n\n\n Having decades worth of experience, we\u2019ve realized that the use of hot spare disks in <\/strong><\/span>complex enterprise systems<\/strong><\/b> increases the probability of additional disks failing<\/strong> as the resilvering process starts to put more and more stress on the existing disks and the system itself. <\/span><\/p>\n\n\n\n The next flaw of a hot spare disk is that it degrades over time. From the moment it is connected to the system, it keeps on working. And when, eventually, it\u2019s time for it to be used as a damaged disk\u2019s replacement; the hot spare disk itself could simply not be in a good enough state to actually replace the damaged disk.<\/span><\/p>\n\n\n\n Another issue with hot spare disks is that they are activated automatically when a disk failure is detected, even if the failed disk is still connected to the system. The faulty disk might attempt to reconnect and operate again while the hot spare is taking over its role, creating additional stress on the system. This can impact overall performance and, in some cases, increase the risk of data loss<\/a>.<\/span><\/p>\n\n\n\n If your goal is to build a system with no single point of failure<\/strong>, relying on a hot spare disk won\u2019t provide much confidence. The process of automatically replacing a failed disk can sometimes fail (partially or completely), which may lead to data loss.<\/p>\n\n\n\n From our decades of experience providing data storage solutions with Open-E<\/a>, we\u2019ve seen many cases where a hot spare disk actually caused a full server failure or even permanent data loss. The risk comes from automation: once triggered, it can set off a domino effect, especially in older infrastructures where hardware has already experienced years of wear.<\/p>\n\n\n\n These problematic aspects of hot spare disks are why our advice would be to not rely on hot spare disks in complex data storage architectures and to use other business continuity solutions instead. This is On- & Off-site Data Protection<\/a> <\/span>with user defined backup retention-interval plans reducing RPO & RTO to minutes.<\/p>\n\n\n\nThe Problematic Aspects of Using a Hot Spare Disk<\/span><\/h2>\n\n\n\n
Hot Spare Disks Add Stress to Vulnerable Systems<\/span><\/h2>\n\n\n\n
Problems in Overall Hot Spare Disk Design<\/span><\/h2>\n\n\n\n
Hot Spare Disks Create a Single Point of Failure<\/span><\/h2>\n\n\n\n
Recommended Procedure in Case of a Disk Failure<\/span><\/h2>\n\n\n\n