On Thu, Aug 18, 2016 at 11:27:55AM +0800, Brad Campbell wrote: > On 18/08/16 11:04, Chris Dunlop wrote: >> G'day all, >> >> What options are there to safely rewrite a disk that's part of a live MD >> raid1? >> >> Specifically, I have smartctl reporting a Current_Pending_Sector of 360 on a >> member of a raid1 set. >> >> A 'check' of the raid comes up clean. I'd like to see if I can clear the >> pending sector count by rewriting the sectors. Whilst rewriting just those >> sectors would be ideal, I don't know which they are, so it looks like a >> whole disk write is the way to go. > > A smartctl -t long on the drive will error out at the first problematic > sector and put that LBA in the SMART log, so there's a start. I should have mentioned: a 'smartctl -t long' on the drive came up clean. > Another way to determine it is run dd from the drive, and it will abort on > the first error telling you how many records it managed to copy. With the > default bs of 512, that gives you a sector number. A 'dd' read of the whole disk also came up clean. >From what I can gather, a "pending sector" is one that's a bit suspect, but may actually be ok. It seems mine are ok (at least for reading), but the pending count won't clear until a write succeeds (or fails, and the sector is remapped). >> Or is this 'dd' stuff just nuts, a case of "well that's a novel way of >> trashing your data..." and/or "you're welcome to try, but you get to keep >> all the pieces and don't come crying to us for help!"? > > Pretty much. If a RAID check is not touching them, then they are likely in > the vacant area around the superblock. Nothing touches that, and playing > with it can lead to tears if you misfire and hit the superblock or the data. Sure - I understand the risks. > If the superblock is ok, and the errors are outside of the data area I've > taken a drive out of the array, used dd_rescue to clone the area of the > drive in question and then written that back to the disk and re-added to the > array. That just re-writes the good data and with zeros where the bad > sectors were. > > That is a horrible, horrible procedure that I did on an array I use for > testing and has no valuable data on. I would not recommend it if you care > about your array or data. I'm interested to see if there's a way of essentially doing the above on a live system, assuming there's appropriate care taken to not trash any existing data (including superblocks). I.e. is it *theoretically* possible to write the same data back to the whole disk safely. E.g. using 'dd' from/to the same disk is almost there, but, as described, there's a window of opportunity where you could get stale data on the disk and a raid repair could then copy that stale data to the good disk. > Brad Thanks, Chris -- 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
