On Tue, May 01, 2012 at 05:52:45PM +0200, Spelic wrote: > I'm looking at it right now > Well, I was thinking at a parent snapshot and child snapshot (or > anyway an older and a more recent snapshot of the same device) so > I'm not sure that's the feature I needed... probably I'm missing > something and need to study more I'm not really following you here. You can have arbitrary depth of snapshots (snaps of snaps) if that helps. > I'm not sure I have understood the full nomenclature of dm-thin yet > :-) ... "dm-thin blocks" would be the same thing as so called "pool > blocksize" as talked in the thread " Re: [PATCH 2/2] dm thin: > support for non power of 2 pool blocksize" right? so that's > customizable now and not necessarily in power of 2... > > But those are anyway quite big, default is what, 64 megabytes? > (which is in fact a good thing for preventing excessive > fragmentation...) Yes, this is the pool block size, it's the atomic unit used for provisioning and copy-on-write. I think the LVM2 tools default this to be 512 _k_. You'd only set it to 64M if you had little interest in snapshot performance. > Now an obvious question: > If userspace sends multiple smaller discards eventually covering the > whole block, the block will still be unmapped correctly, right? No, I don't track anything smaller than a block. (Note, blocks are typically much smaller than you've been envisioning.) > If yes: so you do preserve the information of what part of the block > is has already been discarded, and what part is not... so it would > be possible to return zeroes if the unmapped sub-part of the block > is being read... right? No, but the underlying device may do ... > >then the mappings will be removed. Any future io to that block will > >trigger the block to be reprovisioned. > > (note: here we are talking of a full block now unmapped, different > situation from above) > Ok, supposing I do *not* write, so it does not get reprovisioned, > what does reading from there return; does it return zeroes, or it > returns nonzero data coming from the parent snapshot at the same > offset? zeroes. > >... > >As for identifying empty blocks from userland: there is an inherant > >race here. What would you do with the info? > > You are right , I would definitely need to take a snapshot prior to > reading that... so consider my question related to reading a > snapshot of a device which has been partially discarded... Y, I'll provide tools to let you do this. If you wish to help with writing a replicator please email me. It's a project I'm keen to get going. - Joe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
