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Block layer discard requestsBlock layer discard requestsPosted Aug 14, 2008 17:27 UTC (Thu) by GreyWizard (guest, #1026)In reply to: Block layer discard requests by willy Parent article: Block layer discard requests This also means that no existing device on the market can take advantage of it. What about virtual machines? At present once a block is allocated on a dynamic disk the host has no way to know that it can later be discarded, even if almost every file in the guest is deleted. Isn't this exactly the same problem? Changing the software that emulates an ATA device could probably happen more quickly than hardware changes.
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Block layer discard requests Posted Aug 14, 2008 17:42 UTC (Thu) by willy (subscriber, #9762) [Link] You're quite correct that a virtual machine could make some use of it. If we have an interface to the filesystem that allows us to punch out blocks (see eg http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi?co...) and use sparse files, we could "overprovision". The software that emulates an ATA device can probably be changed about as quickly as real ATA devices -- remember drives are full of firmware too. The holdup is getting T13 to decide on a command number and publish ATA8. Then it's a 'simple matter' of getting users to upgrade ... and I think a large number of them are going to be wary about further updates from at least one major virtualisation provider from now on. Oh, of course drive vendors aren't typically quick to publish microcode updates. You've already bought the drive, so they have little to gain from giving you firmware updates, and as every firmware update is potentially dangerous, they have a certain liability there too.
Block layer discard requests Posted Aug 14, 2008 19:09 UTC (Thu) by GreyWizard (guest, #1026) [Link] That makes sense. Thanks for the extra information.
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