Re: [Qemu-devel] [RFC 0/8] block: Live backup prototype

[Stefan Hajnoczi]

On Sun, Mar 10, 2013 at 11:50 AM, Dietmar Maurer <address@hidden> wrote:
>> About 4KB zero tracking:
>>
>> The vma.py module does not check for zeros, the mask is always 0xffff.
>>  There is a pathalogical case of a disk with every 2nd 4 KB block zeroed, 
>> here
>> vma.py would create a fully-allocated file.But the more common case is that 
>> the
>> whole 64 KB cluster is zero - think about the fact that qcow2/qed only do 
>> discard
>> (zero clusters) at cluster level
>> (64 KB).
>>
>> Also, if you try hard to zero 4 KB blocks then two things can happen:
>> 1. You fragment the file, saving space now but turning sequential access into
>> random access once those zero regions get filled in.
>> 2. The underlying file system ignores the holes to reduce fragmentation and 
>> all
>> the effort was wasted - I guess this can be checked with ioctl(FIEMAP/FIBMAP)
>> to see how ext4/xfs/btrfs handle it.
>
> We track zero blocks at 4KB level to get small backup files. Restore can 
> handle those
> blocks differently, either pre-allocate or create holes. That can even be a 
> restore option.

You are right, the behavior can be set at restore time.

I'm curious how much of a win the 4 KB zero detection is on random
Linux or Windows guest images.  Have you collected numbers?

Stefan