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


From: Dietmar Maurer
Subject: Re: [Qemu-devel] [RFC 0/8] block: Live backup prototype
Date: Sun, 10 Mar 2013 10:50:13 +0000

> 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.





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