On Tuesday May 26, goswin-v-b@xxxxxx wrote: > Neil Brown <neilb@xxxxxxx> writes: > > > On Monday May 25, goswin-v-b@xxxxxx wrote: > >> That really seems to scream for LVM to support more raid levels. It > >> already has linear, raid0 and raid1 support (although I have no idea > >> how device mapper raid1 compares to md raid1). > > > > Note that LVM (a suite of user-space tools) could conceivably use > > md/raid1, md/raid5 etc. The functionality doesn't have to go in dm. > > > > Neil > > How would you do this? Worst case you can have a LV made up of totaly > non linear PEs, meaning lots of 4MB (default PE size) big chunks in > random order on random disks. > > Do you create a raid1/5 for each stripe? You surely run out of md > devices. We have 2^21 md devices easily (I think that is the number) and it wouldn't be hard to have more if that were an issue. > > Create dm mappings for all stripe 0s, stripe 1s, stripe 2s, ... and > then a raid1/5 over those stripe devices? That might be an option. > > What if the LV has segments with different raid configurations (number > of disks in a stripe or even different levels)? Create a raid for each > segment and then a dm mapping for a linear raid? > Yes. > > You can get a flood of intermediate devices there. A /proc/mdstat with > 200 entries would be horrible. iostat output would be totaly > useless. ... > Yep, these would be interesting problems to solve. /proc/mdstat is a bit of a wart on the design - getting the entry in /proc/mdstat optional might be a good idea. As for iostat - where does it get info from ? /proc/partitions? /proc/diskinfo? Maybe /sys/block? Either way, we could probably find a way to say "this block device is 'hidden'" . If you want to be able to slice and dice lot of mini-raid arrays into an LVM system, then whatever way you implement it you will need to be keeping track of all those bits. I think it makes most sense to use the "block device" as the common abstraction, then if we start finding issues: solve them. That way the solutions become available for others to use in ways we hadn't expected. > MfG > Goswin 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
