On Mon, Feb 26, 2007 at 04:33:37PM +1100, Neil Brown wrote: > Do we want a path in the other direction to handle write errors? The > file system could say "Don't worry to much if this block cannot be > written, just return an error and I will write it somewhere else"? > This might allow md not to fail a whole drive if there is a single > write error. Can someone with knowledge of current disk drive behavior confirm that for all drives that support bad block sparing, if an attempt to write to a particular spot on disk results in an error due to bad media at that spot, the disk drive will automatically rewrite the sector to a sector in its spare pool, and automatically redirect that sector to the new location. I believe this should be always true, so presumably with all modern disk drives a write error should mean something very serious has happend. (Or that someone was in the middle of reconfiguring a FC network and they're running a kernel that doesn't understand why short-duration FC timeouts should be retried. :-) > Or is that completely un-necessary as all modern devices do bad-block > relocation for us? > Is there any need for a bad-block-relocating layer in md or dm? That's the question. It wouldn't be that hard for filesystems to be able to remap a data block, but (a) it would be much more difficult for fundamental metadata (for example, the inode table), and (b) it's unnecessary complexity if the lower levels in the storage stack should always be doing this for us in the case of media errors anyway. > What about corrected-error counts? Drives provide them with SMART. > The SCSI layer could provide some as well. Md can do a similar thing > to some extent. Where these are actually useful predictors of pending > failure is unclear, but there could be some value. > e.g. after a certain number of recovered errors raid5 could trigger a > background consistency check, or a filesystem could trigger a > background fsck should it support that. Somewhat off-topic, but my one big regret with how the dm vs. evms competition settled out was that evms had the ability to perform block device snapshots using a non-LVM volume as the base --- and that EVMS allowed a single drive to be partially managed by the LVM layer, and partially managed by evms. What this allowed is the ability to do device snapshots and therefore background fsck's without needing to convert the entire laptop disk to using a LVM solution (since to this day I still don't trust initrd's to always do the right thing when I am constantly replacing the kernel for kernel development). I know, I'm weird, distro users have initrd that seem to mostly work, and it's only wierd developers that try to use bleeding edge kernels with a RHEL4 userspace that suffer, but it's one of the reasons why I've avoided initrd's like the plague --- I've wasted entire days trying to debug problems with the userspace-provided initrd being too old to support newer 2.6 development kernels. In any case, the reason why I bring this up is that it would be really nice if there was a way with a single laptop drive to be able to do snapshots and background fsck's without having to use initrd's with device mapper. - Ted - 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
