On 11/10/2013 4:54 PM, Adam Goryachev wrote: > On 11/11/13 09:36, Stan Hoeppner wrote: >> On 11/10/2013 2:34 PM, NeilBrown wrote: >>> The firmware can only relocate a sector if it reads it when it is >>> marginal >>> but not yet completely lost. If a sector is not read for a long time >>> and >>> during that time the media degraded beyond recovery the firmware >>> cannot do >>> anything. But RAID1 can - it can get it from the other device. >> But is a scrub required for this? Isn't this exactly what occurs during >> normal operation with md/RAID1? I.e. a read fails with disk error, so >> we grab the sector from the mirror? So what advantage is there to >> scrubbing md/RAID1? > Wouldn't a check of the raid cause each member to be read in full, > therefore helping the disk to notice that the sector is marginal, and/or > the RAID layer to notice that the sector is no longer readable and > therefore read from the other member, and re-write the sector. Consider > a sector that is very rarely accessed... > > Or are you suggesting that a smart command issued to the underlying > devices can solve both of those scenarios? No, what I suggest is that drive instrumentation will often alert one to drive problems before you see a read error at the kernel. Assuming this is true then scrubbing isn't necessary. What Neil describes is a case where a sector is written once and read very infrequently, or possibly years after the write, i.e. long term archiving. In this case a scrub may discover a media defect which may go unnoticed by the drive firmware or normal md array operation. -- Stan -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
