On Thu, 25 Feb 2010 10:19:30 +0100 Asdo <asdo@xxxxxxxxxxxxx> wrote: > Neil Brown wrote: > > On Wed, 24 Feb 2010 11:12:09 -0500 > > "Martin K. Petersen" <martin.petersen@xxxxxxxxxx> wrote: > > > > > >> So realistically both disk blocks are wrong and there's a window until > >> the new, correct block is written. That window will only cause problems > >> if there is a crash and we'll need to recover. My main concern here is > >> how big the discrepancy between the disks can get, and whether we'll end > >> up corrupting the filesystem during recovery because we could > >> potentially be matching metadata from one disk with journal entries from > >> another. > >> > > > > After a crash, md will only read from one of the devices (the first) until a > > resync has completed. So there should be no room for more confusion than you > > would expect on a single device. > Not enough, I'd say. > The reads are from a single device, the first, but it's the writes which > you don't know if they go to firstly to the first device or in the > reverse order. So I'd still be concerned by what Martin says. I'm getting bored of repeating myself, so I won't respond to this. > > In addition in this ML there are people reporting that the mismatches > occur even when the system is always on, no crashes. So I think there is > another mechanism for mismatches (not sure if in addition or it's the > only mechanism). Ditto > > Besides, if the mechanism for mismatches is correct I'd go for the copy > (or page lock if possible). All raids have copy, except raid0 maybe, and > they are not slow. Here the copy would only occur on writes, and raid-1 > is not targeted to be SO fast on writes... Also raid-1's are usually on > few disk, like no more than 3, so the copy is not likely to bottleneck > the speed of the writes. I'm sure it would be a measurable slowdown, though < 20%. Probably < 10%. I doubt everyone would be happy with that, though you might. > > What about raid-10? Are there copies for the raid-1 part of raid-10? > No. Neither raid1 nor raid10 copy the data, only raid456. NeilBrown -- 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
