On Wednesday July 13, david@xxxxxxxxxxxx wrote: > I guess each raw device does some readahead, then the md0 does some > readahead and then the lvm does some readahead. Theoretically the md0 > and lvm should overlap - but I guess that much of the raw device level > readahead is discarded. No. Devices don't to readahead (well, modern drives may well read-ahead into an on-drive buffer, but that is completely transparent and separate from any readahead that linux does). Each device just declares how much readahead it thinks is appropriate for that devices. The linux mm layer does read-ahead by requesting devices for blocks that haven't actually been asked for by upper layers. The amount of readahead depends on the behaviour of the app doing the reads, and the setting declared by the devices. raid5 declares a read-ahead size of twice the stripe size. i.e. chunks * (disks-1) * 2. Possibly it should make it bigger if the underlying devices would all be happy with that, however I haven't given the issue a lot of thought, and it is tunable from userspace. 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
