I have philosophical question about the RAID software. In a redundant array, in the situation where a small number of blocks on a disk fail during or before a read request it appears that the software will down the disk and put the array into degraded mode. This is fine since the disk has returned an unrecoverable error. However, at that moment in time the RAID software knows from other array elements what the contents of those blocks should be, and if it were to try to rewrite the blocks, most disks would then have the opportunity to remap the bad blocks with spares and continue normal operation. Logically, the disk has no chance to remap the blocks if the software only issues read requests for them, it doesn't know what the data on the blocks should have been, but the RAID software does. This would be useful since if the software degrades an array because of a disk which could have been repaired on the fly the array's data may be seriously jeopardized in the event of a second disk failure. The same thing essentially happens when a failed disk is simply readded to an array: the data is rewritten to the device, any bad blocks are automatically remapped by the disk because the disk is receiving only write requests (which can be recovered) and voila! The array works fine again. Only problem with this is that it involves time during which another disk could fail, user intervention, and even if a spare disk is defined, a resync during which the array is vulnerable to total failure due to a second disk failing. I have personally been in multiple situations where I have lost important data on large level 5 arrays due to multiple disk failures where at least one of the failed disks was able to map out bad blocks (in subsequent post-mortem testing by issuing writes to the device) and resume normal operation, at least for a while. If the RAID software could have done that and issued a warning rather than degrading the array I would not have lost any data. I can think of a few arguments against such a technique which I should probably also present: The RAID software should be device independent, and rewriting blocks after read errors would constitute a hack to accommodate the way most hard disks work. This is true but it could also be an option flagged in the array configuration or in the kernel source. I think the vast majority of RAID users are using hard disks and I can't think of a different kind of block device that would suffer from attempting to rewrite damaged blocks. Yes, I suppose it's still a hack but heck, it's a hack that could save data. When a disk starts loosing blocks it's usually starting to fail anyway, even if it can be temporarily recovered. I think if anything like this existed it should log visible messages if it ever has to help a disk remap bad blocks which would be a good warning that a disk is starting to fail. Besides, a disk is usually doing this kind of remapping anyway during writes which we get no notification of. So if this is a bad idea would someone take a moment to tell me why? -Kanoa - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
