[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] bad write performance with qdisk with larger files in pv-domU
Im using the following 32-bit setup: - xen 4.1.0 - upstream linux-kernel 2.6.39 as dom0 - linux 2.6.32 pv-domU that has several ext3 partitions mounted with qdisk (same behaviour with a 2.6.39 kernel, so i continued the investigation with the 2.6.32er kernel) Die read performance is good (ca. 60 MB/s) For smaller files (< 30-40 MB) the write-speed is ok. But if i copy a larger file (ca > 40 MB), the write speed decreases to ca. 0,5 MB/s, after the first ca. 40 MBs are written. One reason for the bad performance might be that qdisk doesnt use AIO. For testing purposes i activated AIO in hw/xen_disk.c (i set use_aio=1), but the domU freezed shortly after the domU-kernel started. Is this performance-impact expected when no AIO is used? I compared the raw-block implementation from xen-qemu 4.1.0 and current upstream, in case xen-qemu has some missing bugfixes and found the following patch that looks a bit interesting commit 4899d10d142e97eea8f64141a3507b2ee1a64f52 Author: Stefan Hajnoczi <stefanha@xxxxxxxxxxxxxxxxxx> Date: Mon Apr 19 13:34:11 2010 +0100 raw-posix: Use pread/pwrite instead of lseek+read/write This patch combines the lseek+read/write calls to use pread/pwrite instead. This will result in fewer system calls and is already used by AIO. From the first look the patch cannot be backported 1:1, so i havent tried it yet, because i doubt that it can make such a huge difference. Or would it be worth a try? Any other ideas how/what to investigate this issue further, in case the write- speed should be better also without AIO? I know that the qdisk implementation is expected to be slower, but i would expect at least lets say 5 MB/s. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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