Re: [Qemu-block] [PATCH v4 0/3] qemu-img check: format allocation info
31.07.2017 18:14, Eric Blake wrote: On 07/29/2017 11:41 AM, Vladimir Sementsov-Ogievskiy wrote: Hi all. See 01 patch for the doc. Question to discuss. If I understand correctly get_block_status flags allocated, zero, and data actually provide 5 possible combinations, which I combine into three. There are actually 8 possible bit combinations, but you are right that some of them are in practice impossible (since the allocated bit can only be set in cases where the underlying driver set the data or zero bit). allocated data zero 1 11\__ data This one is interesting - it means we know the contents read as zero, but that it occupies space on the disk instead of being a hole; reporting it as zero may make it easier to punch a hole. 1 10/ Yes, definitely data, and no clue if it can be turned into a hole. 1 01\__ zero 0 01/ Yes, definitely zero. (The former happens when a format layer directly reports that the current layer reads as zero; the latter is possible when a format layer doesn't have an allocation, but where we know unallocated clusters read as zero, perhaps because there is no backing file to further fall back to). 0 00___ discarded Could also mean hasn't been touched yet (discarded sort of implies that it has been touched at some point in the past) last time I don't like it too. What about renaming it to just 'unallocated'? The other bit patterns: 0 10- not possible: if a driver sets data, then the block layer sets allocated 0 11- ditto 1 00- not possible: nothing sets the allocated bit in isolation This division looks not bad, but it is not the only one possible. Separating data is really useful - it shows leaked clusters.. So the question is, don't we want to adjust the division? I'm ok with the current one. -- Best regards, Vladimir
Re: [Qemu-block] [PATCH v4 0/3] qemu-img check: format allocation info
On 07/29/2017 11:41 AM, Vladimir Sementsov-Ogievskiy wrote: > Hi all. > > See 01 patch for the doc. > > Question to discuss. > If I understand correctly get_block_status flags allocated, zero, and data > actually provide 5 possible combinations, which I combine into three. There are actually 8 possible bit combinations, but you are right that some of them are in practice impossible (since the allocated bit can only be set in cases where the underlying driver set the data or zero bit). > > allocated data zero > 1 11\__ data This one is interesting - it means we know the contents read as zero, but that it occupies space on the disk instead of being a hole; reporting it as zero may make it easier to punch a hole. > 1 10/ Yes, definitely data, and no clue if it can be turned into a hole. > 1 01\__ zero > 0 01/ Yes, definitely zero. (The former happens when a format layer directly reports that the current layer reads as zero; the latter is possible when a format layer doesn't have an allocation, but where we know unallocated clusters read as zero, perhaps because there is no backing file to further fall back to). > 0 00___ discarded Could also mean hasn't been touched yet (discarded sort of implies that it has been touched at some point in the past) The other bit patterns: 0 10- not possible: if a driver sets data, then the block layer sets allocated 0 11- ditto 1 00- not possible: nothing sets the allocated bit in isolation > > This division looks not bad, but it is not the only one possible. > Separating data is really useful - it shows leaked clusters.. > So the question is, don't we want to adjust the division? > I'm ok with the current one. > -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org signature.asc Description: OpenPGP digital signature
