Re: [Qemu-devel] [PATCH] block: Use bdrv functions to replace file operation in qcow.c

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Nov 18, 2011 at 11:10 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 11/18/2011 11:59 AM, Stefan Hajnoczi wrote:
>>
>> +    tmp = g_malloc0(sizeof(uint64_t)*l1_size);
>> >  +    ret = bdrv_pwrite(qcow_bs, header_size, tmp,
>> > sizeof(uint64_t)*l1_size);
>> >  +    g_free(tmp);
>> >  +    if (ret != sizeof(uint64_t)*l1_size) {
>> >  +        goto exit;
>> >       }
>>
>> That means 400 MB of RAM for the zero L1 table for a 100 TB image.
>> Since qcow is a legacy format this probably doesn't matter in practice
>> but in theory this approach can require a noticable amount of RAM.
>
> 4 MB / TB is not a big deal (you probably would like the L1 table to be in
> memory all the time), but why write the L1 table at all?  Since the file was
> CREATed, it is already zero and you can just leave a hole in the file.

I thought the same thing then remember sometimes people want to use
image formats on block devices.  I think at least making image
creation not depend on has_zero_init is a good idea.

Stefan