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[Qemu-devel] [Bug 1353545] Re: QED does not deliver flush on synchronize
From: |
Nybble |
Subject: |
[Qemu-devel] [Bug 1353545] Re: QED does not deliver flush on synchronized write |
Date: |
Wed, 06 Aug 2014 15:32:29 -0000 |
** Changed in: qemu
Status: New => Invalid
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https://bugs.launchpad.net/bugs/1353545
Title:
QED does not deliver flush on synchronized write
Status in QEMU:
Invalid
Bug description:
At least one flush method should be provided by qed driver
(common/co/aio).
I was doing "sync write" benchmarking on varies qemu supported formats
(on ssd). I was surprised that QED shows significant high performance
over several other formats (qcow2/vmdk/vdi..). In some test cases QED
even runs a little bit faster than raw format!
After careful reading of the code I found QED skipped the necessary
flush by providing nothing on flush api. unlike read/write which
require at least one type of implementation, flush API is not
mandatory because your format could guarantee that synchronization is
done in write path or others. However that's not the case in QED
because data is buffered in the "bs->file" and QED needs to propagate
the flush command to it to make data safe.
However I'm not a skilled qemu developer. Above is what I got from my
thinking and study. Please correct me if in any way I misunderstood
the design. I really appreciate!
The patch is short:
https://github.com/wuxb45/qemu/commit/fe46e640f53a006907091e910fb4868a878ea0a8
qemu release version: 2.1.0. But this should have been there for a long time.
The patch is based on this recent commit:
69f87f713069f1f70f86cb65883f7d43e3aa21de
Thanks.
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