Host's write request is 4KB larger than that of the guest?

Host's write request is 4KB larger than that of the guest?

[Duy Le (Dan)]

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.

I really appreciate that.

-- Dan

Re: Host's write request is 4KB larger than that of the guest?

[Avi Kivity]

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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