On Saturday October 21, mangoo@xxxxxxxx wrote: > Neil Brown wrote: > > On Tuesday October 17, mirek@xxxxxxxxxxxxxxxx wrote: > >> I would like to propose an enhancement of raid 1 driver in linux kernel. > >> The enhancement would be speedup of data reading on mirrored partitions. > >> The idea is easy. > >> If we have mirrored partition over 2 disks, and these disk are in sync, there is > >> possibility of simultaneous reading of the data from both disks on the same way > >> as in raid 0. So it would be chunk1 read from master, chunk2 read from slave at > >> the same time. > >> As result it would give significant speedup of read operation (comparable with > >> speed of raid 0 disks). > > > > This is not as easy as it sounds. > > Skipping over blocks within a track is no faster than reading blocks > > in the track, so you would need to make sure that your chunk size is > > larger than one track - probably it would need to be several tracks. > > What you said is certainly true when we read one file at a given moment. > > What if we read two different files at a given time? Certainly, it would > be faster if DRIVE_1 reads FILE_1, and DRIVE_2 reads FILE_2. And it is possible that md/raid1 currently does that (I think it depends a bit on the size and location of the files). But what if you are reading 3 separate areas of the filesystem... Sharing that over 2 drives when you don't know in-advance what will be done is probably non-trivial :-) > > > > Raid1 already does some read-balancing, though it is possible (even > > likely) that it doesn't balance very effectively. Working out how > > best to do the balancing in general in a non-trivial task, but would > > be worth spending time on. > > Probably what I said before isn't very correct, as RAID-1 has no idea of > the filesystem that is on top of it; rather, it will see attempts to > access differend areas of the array? > Exactly. It just sees block address. It tends to prefer to send a request to a drive that has recently been asked to read a block nearby the new request. 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
