Hi, > LVM copies the buffer!? > I don't think so... > LVM is near-zero overhead, so I would be surprised if it copied the buffer. well, I'm not so sure it is near-zero overhead (I'll exaplain below), and even if, making copies could be still "near-zero" overhead, it depends on where the bottlenecks are. I'm not an LVM insider, so this are just random thoughts. About the near-zero overhead, maybe this could open a different thread, but just to give some numbers... I've a bunch of RAID-6 volumes, made of USB disks, i.e. using PATA<->USB bridges. This volumes are aggregated using LVM and, on top of that, there is a LUKS container. The raw read perfomance on the RAID-6 is, in the best case, around about 48MB/s, which is pretty good for USB, I guess it will be difficult to get more. The raw read perfomance of the LVM volume is i~38MB/s. The raw read performance of the LUKS is ~28MB/s (actually maybe a bit less). Each further layer loses about 10MB/s. I guess this is much more visible in USB than in SATA/SAS situations, since going from 205 to 195 might get unnoticed. This is not a CPU problem, since the PC is dual core, one core runs and it never exceeds 30%. The USB is slow enough to allow all the operations to be performed in real-time. Nevertheless, LVM is doing something there, in this setup is has an overhead of about 20%, far from zero. So, the 10MB/s loss could be, again I've no idea on how LVM works, caused by copying. Could also be something else, of course, it would be interesting to have more information from some expert (also to optimize my USB setup, if possible). > Also I don't think it was needed in their case, except maybe if Maybe, but if the filesystem can play with the buffer while submitted, then I would rather copy the data. Again, some expert opinion would be appreciated. > LVM also does not merge requests AFAIR. (visible with mdstat -x 1) BTW, what's that? I mean "mdstat -x 1"... > But me also I have never seen mismatches and the only raid-1's I > have are above LVM. (except /boot but that's almost never modified) Well, that's good, you confirmed my experience. I've also RAID-10 on LVM and never got mismatches, while the plain RAID-10 got sometimes. bye, -- piergiorgio -- 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
