On 10/26/2016 02:46 PM, Mikulas Patocka wrote:
I don't like the idea of complicating the code by turning discards into writes.
That's not what my patch series does. The only writes added by my patch series are those for the non-aligned head and tail of the range passed to blkdev_issue_zeroout().
The flag "discard_zeroes_data" is actually misdesigned, because the storage stack can change dynamically while bios are in progress. You can send a discard bio to a device mapper device that has "discard_zeroes_data" - and while the bio is in progress, the device mapper stack can be reconfigured to redirect the bio to another device that doesn't have "discard_zeroes_data" - and the bio won't zero data and the caller that issued it has no way to find it out. I think the proper thing would be to move "discard_zeroes_data" flag into the bio itself - there would be REQ_OP_DISCARD and REQ_OP_DISCARD_ZERO - and if the device doesn't support REQ_OP_DISCARD_ZERO, it rejects the bio and the caller is supposed to do zeroing manually.
Sorry but I don't like this proposal. I think that a much better solution would be to pause I/O before making any changes to the discard_zeroes_data flag.
Bart. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
