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Re: [Qemu-devel] [RFC] qemu-img: Drop BLK_ZERO from convert
From: |
Stefan Hajnoczi |
Subject: |
Re: [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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