I don't understand what this has to do with devices that support DISCARD / UNMAP, such as SSDs. The question is about freeing part of a ZVOL in response to UNMAP/DISCARD requests from HFS+, right? The end result being that (maybe) the ZVOL uses less space. ZFS is not issuing any UNMAP/DISCARDs to the underlying device (e.g. SSD).
I guess I don't really understand why HFS+ is issuing all these UNMAPs on mount. Does it do that on other storage? E.g. HFS+ on a SSD (no ZFS involved). Is it sending a zillion TRIM commands every time my laptop boots, or I plug in an external SSD? --matt On Wed, Dec 17, 2014 at 10:21 PM, Jorgen Lundman <[email protected]> wrote: > > > > Matthew Ahrens wrote: > > I think you're saying you have a zvol with HFS+ on top, and that when you > > mount the HFS+ volume, it sends a lot of unmap requests to the zvol, > which > > is slow. > > > > Before we get into complicated solutions, I have some stupid questions: > > > > - Why does it need to issue a zillion unmaps every time you mount? > > Ask Apple! > > But yes, I suppose if you have a device that supports DISCARDs, and you > (the user) wants to receive unmaps, XNU kernel goes through all empty areas > on mount and issues unmap for each. Even for areas that have already been > discarded, it seems. > > I don't think we can detect those unmap requests for already unmapped > areas? Easily? > > > > > > - Could you just ignore the UNMAPs? (obvious answer is yes, but does it > > hurt anything else) > > Of course you can. Don't buy SSDs is one way! Disabling unmap support in OS > also works. But I guess they added unmap to devices, and operating systems, > for a reason, and should the user want them enabled, 20 mins to mount is > undesirable.. So we were checking to see if we could do something trivial > to lessen the effect. People running the experimental code report > happiness, but that doesn't mean it's correct :) > > > > > > - Do you have this unmap performance fix? > > 4873 zvol unmap calls can take a very long time for larger datasets > > Yes. Makes little difference. It is more the high number of commits that > take a while to go through. > > I suspect only OSX and FreeBSD have this concern, as the other platforms do > not yet fully support devices with DISCARD. But I don't know for sure. > > > -- > Jorgen Lundman | <[email protected]> > Unix Administrator | +81 (0)3 -5456-2687 ext 1017 (work) > Shibuya-ku, Tokyo | +81 (0)90-5578-8500 (cell) > Japan | +81 (0)3 -3375-1767 (home) > _______________________________________________ > developer mailing list > [email protected] > http://lists.open-zfs.org/mailman/listinfo/developer >
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