On 02/04/2011 03:54 PM, Mike Snitzer wrote: > On Fri, Feb 04 2011 at 9:12am -0500, > Bryn M. Reeves <bmr@redhat.com> wrote: > >> Hi Folks, >> >> Recently had a query from someone using an array supporting thin provisioning >> with zero page reclaim[1]. >> >> They wanted a method to write zeros to all the unallocated space in a VG to >> trigger a reclaim of unused regions in the VG's PVs. >> >> My suggestion was to lvcreate a -l100%FREE LV named "filler" and overwrite it >> from /dev/zero. >> >> It seems like this might something a lot of users want as this functionality >> gets to be more common. >> >> I wondered if it could be worth adding a script to automate this or even an >> option to vgchange (e.g. --zero-free-space)? >> >> Didn't have time to try putting anything together yet but it seems such a >> feature could be useful. > > Do these HDS arrays also support discard? Would seem to me that issuing > discards for the free space would be better (more standard) than writing > zeros to accomplish the same. > > This variant would be: vgchange --discard-free-space There are two variants of reclamation currently supported by external storage products. The WRITE_SAME/TRIM based explicit discard support is superior when the entire storage stack supports it and I agree it would be good to support this mode of reclaim too (superior in that you lose the zero-write overhead and associated reclaim lag and can get better space utilisation since the host "knows" better which blocks are unused). Several vendors (at least HDS/EMC that I know of) also support an implicit reclaim mode known as zero page/space reclaim in which the storage de-allocates blocks that are overwritten with zeros. I suppose the ideal solution would be to detect whether the storage supports one or the other automatically but I don't know that there is a way to achieve this right now (TRIM/WRITE_SAME possibly but I think the zero page stuff is entirely transparent). Regards, Bryn. _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
