On 01/06/2011 03:53 PM, Duy Le (Dan) wrote:
Hi there, I used a raw disk image to host a VM. I later used blktrace to capture write requests issued from the guest to commit data to a physical disk and found that the request size of the host is 8 blocks (4KB) larger than that of the guest. Here is a part of the trace. --- Guest 251,32 0 100 2.399993525 10 D W 18747926 + 8 [kblockd/0] 251,32 0 101 2.399996297 10 D W 18752462 + 24 [kblockd/0] 251,32 0 102 2.399998367 10 D W 18756566 + 8 [kblockd/0] --- Host 8,16 0 155 2.415036111 32357 D W 19146095 + 16 [kvm] 8,16 0 156 2.415042667 32357 D W 19150639 + 32 [kvm] 8,16 0 158 2.415063867 32349 D W 19154743 + 16 [kvm] The content of that additional written sectors on the disk can either be "zero" or something else. Please let me know if you have a clue or tell me which file in the package should I be looking at closer to understand this behavior.
Most likely your guest partitions are not aligned to a 4k boundary; this causes the host to add padding to the nearest 4k, which is consistent with what you saw. You can use fdisk to confirm this.
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