Forwarding to dm-devel as it appears the issue to be resolved for VDO to receive discards correctly needs a code change in dm-thin ... See start of thread here for full details: https://www.redhat.com/archives/vdo-devel/2018-July/msg00000.html ---------- Forwarded message ---------- From: James Hogarth <james.hogarth@xxxxxxxxx> Date: 11 July 2018 at 11:48 Subject: Re: [Vdo-devel] Trying to test thin provisioned LVM on VDO To: vdo-devel@xxxxxxxxxx Cc: Michael Sclafani <sclafani@xxxxxxxxxx>, snitzer@xxxxxxxxxx On 11 July 2018 at 11:26, James Hogarth <james.hogarth@xxxxxxxxx> wrote: > On 11 July 2018 at 10:40, Michael Sclafani <sclafani@xxxxxxxxxx> wrote: >> Based on the error message and a quick scan of the code, it appears dm-thin >> disables discards because VDO's max_discard_sectors = 4KB is smaller than >> dm-thin's 64KB+ block size. I have no idea why it does that, but if it >> neither discards nor zeros out blocks it has written to VDO, that space will >> not be reclaimed. >> > > Thanks for confirming the line of thought I was following ... > > Annoyingly this makes the RHEL documentation pretty useless to follow > for carrying out thin provisioned volumes... > > Unfortunately I don't have a support account to hand to raise this as > a RHEL7.5 issue to resolve ... > > Looking at the lvcreate man page it's not possible to set a block size > for a thin pool below 64K > > -c|--chunksize Size[k|UNIT] > The size of chunks in a snapshot, cache pool or thin > pool. For snapshots, the value > must be a power of 2 between 4KiB and 512KiB and the > default value is 4. For a cache > pool the value must be between 32KiB and 1GiB and the > default value is 64. For a thin > pool the value must be between 64KiB and 1GiB and the > default value starts with 64 and > scales up to fit the pool metadata size within 128MiB, > if the pool metadata size is not > specified. The value must be a multiple of 64KiB. See > lvmthin(7) and lvmcache(7) for > more information. > > What's going to be the best approach to resolve this so that thin > provisioning works as expected? It's obviously not advisable to use in > this configuration due to the inevitable disk exhaustion issue that > will arise. Mike you wrote the relevant patch that appears to be causing the conflict and prevents dm-thin passing the discard to VDO here: https://www.redhat.com/archives/dm-devel/2012-August/msg00381.html I know it was a while back but do you recall what the reason for the max_discard_sector and sectors_per_block comparison was for? >From the VDO code it appears untenable to increase maxDiscardSector without major performance impact - to the extent of I/O stalls. So it looks like the only way to make this work is a change to dm-thin to ensure the discards are still passed to the VDO layer below it. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
