[Qemu-devel] Re: [PATCH 1/5] block: add discard support
On 12/10/2010 02:38 PM, Christoph Hellwig wrote: if it's smaller than the block size we'll zero out the remainder of the block. I think it should fail at VM startup time, or even better do nothing at all. When you write in the middle of an absent block, and a partially-zero block is created, this is not visible: a read cannot see the difference between all zeros because it's sparse and all zeros because it's zero. If I ask you to (optionally) punch a 1kb hole but all you can do is punch a 2kb hole, I do care about the second kilobyte of data. Since the hole punching of bdrv_discard is completely optional, it should not be done in this case. Paolo
[Qemu-devel] Re: [PATCH 1/5] block: add discard support
On Mon, Dec 13, 2010 at 05:07:27PM +0100, Paolo Bonzini wrote: On 12/10/2010 02:38 PM, Christoph Hellwig wrote: if it's smaller than the block size we'll zero out the remainder of the block. I think it should fail at VM startup time, or even better do nothing at all. What should fail? When you write in the middle of an absent block, and a partially-zero block is created, this is not visible: a read cannot see the difference between all zeros because it's sparse and all zeros because it's zero. You can not see from a VM if a block is not allocated or zeroed. Then again we'll never create a fully zeroed block anyway unless we get really stupid discard patterns from the guest OS. If I ask you to (optionally) punch a 1kb hole but all you can do is punch a 2kb hole, I do care about the second kilobyte of data. Since the hole punching of bdrv_discard is completely optional, it should not be done in this case. Of course we do not discard the second KB in that case. If you issue a 1k UNRSVSP ioctl on a 2k block size XFS filesystem it will zero exactly the 1k you specified, which is required for the semantics of the ioctl. Yes, it's not optimal, but qemu can't easily know what block size the underlying filesystem has.
[Qemu-devel] Re: [PATCH 1/5] block: add discard support
On 12/13/2010 05:15 PM, Christoph Hellwig wrote: On Mon, Dec 13, 2010 at 05:07:27PM +0100, Paolo Bonzini wrote: On 12/10/2010 02:38 PM, Christoph Hellwig wrote: if it's smaller than the block size we'll zero out the remainder of the block. I think it should fail at VM startup time, or even better do nothing at all. What should fail? Nothing -- you wrote if it's smaller than the block size we'll zero out the remainder of the block which I interpreted the wrong way, i.e. as XFS will round up the size to the remainder of the block and zero that part out as well. Thanks for the clarification. Paolo
