[dm-devel] striped targets with LVM2 logics

[EXT / DEVOTEAM VAROQUI Christophe <ext devoteam varoqui sncf fr>]

Hello,

I would like to raise the issue of striped target logics with LVM2 :
With the current stable release, one can set the number of stripes at LV
creation time.
Then pvmove is allowed to move a stripe to a PV that already host a stripe
of the same LV,
thus killing the performance for this LV and for every other LV hosted on
this PV.

I guess you are already aware of that pb, 
and would like to know what policy you intend to implement to resolve this
issue.

I wish one can just bypass the nb of stripes setting and let LVM2 decide
what mapping would give the best perf.
Maybe adding a "-mi / --min-stripes" and a "-Mi / --max-stripes" could give
useful input to the algorithm.
A default max-stripes value of 5 or 6 seems to be standard (for storageworks
firmware at least)

Then pvmove logics should check if a segment is available on a PV that do
not host a stripe for this LV.
If there is none, then reduce the number of stripes for this LV.

regards,
cvaroqui