On Sep 18, 2009 15:40 -0400, Martin K. Petersen wrote: > >>>>> "Eric" == Eric Sandeen <sandeen@xxxxxxxxxx> writes: > >> Also, are you guys affected by the > >> previously-acked-sectors-are-now-gone problems with 512-byte > >> logical/4KB physical drives? > > Eric> previously-acked-sectors-are-now-gone? I guess I haven't been > Eric> keeping up. What do you mean by this? > > We already discussed this on irc. But in case anybody else are > wondering... > > On a disk with 4KB physical blocks emulating 512-byte logical blocks the > drive firmware must resort to read-modify-write cycles and that opens up > a new error scenario: > > 4KB physical block: | 0 | > 512b logical block: | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | > ^ ERROR > > In this case we have successfully written LBA 0 - 6. However, when the > drive attempts to write LBA 7 it gets an I/O error and the previous 7 > logical blocks (that have previously been acknowledged as written) are > lost. IOW, the drive write atomicity is at the physical block level and > not the logical ditto. In some sense, this is no worse than "real" sectors 0-6 going bad right after they were written, or in fact even having the writes silently fail. Yes, there is more chance that writing sector 7 (due to 4k sector r-m-w) will cause collateral damage, but the truth even today is that disks are not going to fail a single 512-byte sector at one time, but more likely 64kB (or whatever the remapping unit size is), so this isn't really introducing a new failure mode. Cheers, Andreas -- Andreas Dilger Sr. Staff Engineer, Lustre Group Sun Microsystems of Canada, Inc. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
