From: Dushyant Bansal <cs5070214@cse.iitd.ac.in>
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] Re: KVM call agenda for Jan 25
Date: Sun, 27 Feb 2011 03:20:40 +0530 [thread overview]
Message-ID: <4D6975B0.4060309@cse.iitd.ac.in> (raw)
In-Reply-To: <AANLkTik6de8nkuS6xz6btOkqjTMTKmOm_B+Jy9DCa2Tk@mail.gmail.com>
>> In cp, it just copies all the disk blocks actually occupied by the file.
>> And, with qemu-img convert, it checks all the sectors and copy those, which
>> contains atleast one non-NUL byte.
>> The better performance of cp over qemu-img convert is the result of overhead
>> of this checking.
>>
> How did you find out what cp(1) and qemu-img do?
> How does cp(1) know which disk blocks are actually occupied?
>
I have looked into their source code.
Yes, that was not correct. cp also checks each block and copy those
which contains non-null byte.
qemu-img does the same thing with sectors.
>> I tried a few variations:
>> 1. just copy all the sectors without checking
>> So, actual size becomes equal to virtual size.
>>
> Did that make qemu-img faster for the image file you tested?
>
No, in fact it becomes slower. I guess it is due to the increase in disk
write.
>> 2. In is_allocated_sectors,out of n sectors, if any sector has a non-NUL
>> byte then break and copy all n sectors.
>> As expected, resultant raw image was quite large in size.
>>
> This is kind of like what cp(1) does, except it limits n to 32 KB
> maximum at a time. Maybe if you add this tweak they will show similar
> performance. The drawback is that the output image is larger than
> with the current approach.
>
> Stefan
>
>
Disk block size is usually 512 bytes and in qemu-img, sector size is
also 512B. And, this change would copy n sectors even if only one of
them actually contains data (while cp checks and copies one
block(=sector) at a time). Therefore, it will end up writing more data
than cp.
virtual size: 10G (10737418240 bytes)
disk size: 569M
convert-> original
time 0m52.522s
convert-> modified (resultant disk size: 5.3G)
time 2m12.744s
cp
time 0m51.724s (same disk size)
---------------------------------------------------------------------------
virtual size: 10G (10737418240 bytes)
disk size: 3.6G
convert-> original
time 1m52.249s
convert-> modified (resultant disk size: 7.1G)
time 3m2.891s
cp
time 1m55.320s (same disk size)
---------------------------------------------------------------------------
In these results, we can see that resultant disk size has increased.
Thanks,
Dushyant
next prev parent reply other threads:[~2011-02-26 21:52 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-01-24 13:25 KVM call agenda for Jan 25 Chris Wright
2011-01-24 13:25 ` [Qemu-devel] " Chris Wright
2011-01-24 22:06 ` Anthony Liguori
2011-01-24 22:06 ` [Qemu-devel] " Anthony Liguori
2011-01-25 13:57 ` Luiz Capitulino
2011-01-25 14:02 ` Luiz Capitulino
2011-01-25 14:13 ` Stefan Hajnoczi
2011-01-25 14:13 ` Stefan Hajnoczi
2011-01-29 10:50 ` Dushyant Bansal
2011-01-29 13:16 ` Stefan Hajnoczi
2011-02-25 17:42 ` Dushyant Bansal
2011-02-26 14:05 ` Stefan Hajnoczi
2011-02-26 21:50 ` Dushyant Bansal [this message]
2011-02-27 10:49 ` Stefan Hajnoczi
2011-02-28 7:36 ` Markus Armbruster
2011-02-28 20:41 ` Dushyant Bansal
2011-03-01 9:40 ` Stefan Hajnoczi
2011-03-14 15:13 ` Dushyant Bansal
2011-03-15 10:27 ` Kevin Wolf
2011-03-16 14:17 ` Dushyant Bansal
2011-03-16 17:47 ` Stefan Hajnoczi
2011-03-17 10:07 ` Kevin Wolf
2011-03-26 21:56 ` Dushyant Bansal
2011-03-28 10:26 ` Kevin Wolf
2011-01-25 14:11 ` Aurelien Jarno
2011-01-25 14:11 ` Aurelien Jarno
2011-01-25 14:27 ` Anthony Liguori
2011-01-25 14:27 ` Anthony Liguori
2011-01-25 14:42 ` Kevin Wolf
2011-01-25 14:42 ` Kevin Wolf
2011-01-25 15:29 ` Aurelien Jarno
2011-01-25 14:26 ` Avi Kivity
2011-01-25 14:26 ` [Qemu-devel] " Avi Kivity
2011-01-25 14:35 ` Stefan Hajnoczi
2011-01-25 14:35 ` [Qemu-devel] " Stefan Hajnoczi
2011-01-26 9:58 ` Avi Kivity
2011-01-26 9:58 ` [Qemu-devel] " Avi Kivity
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