Re: [Qemu-devel] callout to *file in bdrv_co_get_block_status

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/03/2017 15:21, Peter Lieven wrote:
>>>
>>> I stumbled across the issue with lseek on a tmpfs because in the
>>> build process for our templates
>>> I temporarily have vmdks on a tmpfs and it takes ages before qemu-img
>>> convert starts to run (it iterates
>>> over every 64kb cluster with that callout to find_allocation and for
>>> some reason lseek is very slow on tmpfs).
>> Ok, thanks.  Perhaps it's worth benchmarking tmpfs specifically.  Apart
>> from the system call overhead (which does not really matter if you're
>> going to do a read), lseek on other filesystems should not be any slower
>> than read.
> 
> Okay, but the even the read is not really necessary if the metadata is
> correct?

Yeah, what I mean is:

- if you're going to do a read of non-zero blocks, the lseek you do
before reading those blocks should not matter.

- if you're going to skip the read of non-zero blocks, the lseek you do
is always going to be faster than reading them and then checking with
buffer_is_nonzero.

> Would it be an idea to introduce an inverse flag live BDRV_BLOCK_NOT_ZERO
> for cases where we know that there is really DATA and thus can avoid the
> second callout?

How would you know that a block is nonzero?

Paolo