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Re: [Qemu-block] [Qemu-devel] [RFC] qemu-img: Drop BLK_ZERO from convert

From: Stefan Hajnoczi
Subject: Re: [Qemu-block] [Qemu-devel] [RFC] qemu-img: Drop BLK_ZERO from convert
Date: Tue, 27 Feb 2018 16:17:43 +0000
User-agent: Mutt/1.9.2 (2017-12-15)

On Mon, Feb 26, 2018 at 06:03:13PM +0100, Max Reitz wrote:
> There are filesystems (among which is tmpfs) that have a hard time
> reporting allocation status.  That is definitely a bug in them.
> However, there is no good reason why qemu-img convert should query the
> allocation status in the first place.  It does zero detection by itself
> anyway, so we can detect unallocated areas ourselves.
> Furthermore, if a filesystem driver has any sense, reading unallocated
> data should take just as much time as lseek(SEEK_DATA) + memset().  So
> the only overhead we introduce by dropping the manual lseek() call is a
> memset() in the driver and a buffer_is_zero() in qemu-img, both of which
> should be relatively quick.

This makes sense.  Which file systems did you test this patch on?

XFS, ext4, and tmpfs would be a good minimal test set to prove the
patch.  Perhaps with two input files:
1. A file that is mostly filled with data.
2. A file that is only sparsely populated with data.

The time taken should be comparable with the time before this patch.

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