Reviewed: https://review.openstack.org/640781 Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=e7b64eaad82db38dd46f586b650da4ddde42533b Submitter: Zuul Branch: master
commit e7b64eaad82db38dd46f586b650da4ddde42533b Author: Kashyap Chamarthy <[email protected]> Date: Thu Feb 28 12:33:12 2019 +0100 qemu: Make disk image conversion dramatically faster tl;dr: Use 'writeback' instead of 'writethrough' as the cache mode of the target image for `qemu-img convert`. Two reasons: (a) if the image conversion completes succesfully, then 'writeback' calls fsync() to safely write data to the physical disk; and (b) 'writeback' makes the image conversion a _lot_ faster. Back-of-the-envelope "benchmark" (on an SSD) -------------------------------------------- (Ran both the tests thrice each; version: qemu-img-2.11.0) With 'writethrough': $> time (qemu-img convert -t writethrough -f qcow2 -O raw \ Fedora-Cloud-Base-29.qcow2 Fedora-Cloud-Base-29.raw) real 1m43.470s user 0m8.310s sys 0m3.661s With 'writeback': $> time (qemu-img convert -t writeback -f qcow2 -O raw \ Fedora-Cloud-Base-29.qcow2 5-Fedora-Cloud-Base-29.raw) real 0m7.390s user 0m5.179s sys 0m1.780s I.e. ~103 seconds of elapsed wall-clock time for 'writethrough' vs. ~7 seconds for 'writeback' -- IOW, 'writeback' is nearly _15_ times faster! Details ------- Nova commit e6ce9557f84cdcdf4ffdd12ce73a008c96c7b94a ("qemu-img do not use cache=none if no O_DIRECT support") was introduced to make instances boot on filesystems that don't support 'O_DIRECT' (which bypasses the host page cache and flushes data directly to the disk), such as 'tmpfs'. In doing so it introduced the 'writethrough' cache for the target image for `qemu-img convert`. This patch proposes to change that to 'writeback'. Let's addresses the 'safety' concern: "What about data integrity in the event of a host crash (especially on shared file systems such as NFS)?" Answer: If the host crashes mid-way during image conversion, then neither "data integrity" nor the cache mode in use matters. But if the image conversion completes _succesfully_, then 'writeback' will safely write the data to the physical disk, just as 'writethough' does. So we are as safe as we can, but with the extra benefit of image conversion being _much_ faster. * * * The `qemu-img convert` command defaults to 'cache=writeback' for the source image. And 'cache=unsafe' for the target, because if `qemu-img` "crashes during the conversion, the user will throw away the broken output file anyway and start over"[1]. And `qemu-img convert` supports[2] fsync() for the target image since QEMU 1.1 (2012). [1] https://git.qemu.org/?p=qemu.git;a=commitdiff;h=1bd8e175 -- "qemu-img convert: Use cache=unsafe for output image" [2] https://git.qemu.org/?p=qemu.git;a=commitdiff;h=80ccf93b -- "qemu-img: let 'qemu-img convert' flush data" Closes-Bug: #1818847 Change-Id: I574be2b629aaff23556e25f8db0d740105be6f07 Signed-off-by: Kashyap Chamarthy <[email protected]> Looks-good-to-me'd-by: Kevin Wolf <[email protected]> ** Changed in: nova Status: In Progress => Fix Released -- You received this bug notification because you are a member of Yahoo! Engineering Team, which is subscribed to OpenStack Compute (nova). https://bugs.launchpad.net/bugs/1818847 Title: Fix QEMU cache mode used for image conversion and Nova instances Status in OpenStack Compute (nova): Fix Released Bug description: Nova uses QEMU's disk image cache modes in two main areas: (1) When decicding what cache mode to use for the target disk image when converting (using `qemu-img convert`) images from one format to another (qcow2 <-> raw). See unprivileged_convert_image() in nova/privsep/qemu.py. (2) When configuring cache modes for running guests (Nova instances). Nova tells libvirt what cache mode to use, and libvirt will in turn configure block devices via QEMU (using its '-drive' command-line option). See disk_cachemode() in nova/virt/libvirt/driver.py. (And also for "volume drivers" like SMBFS and Virtuozzo Storage also use 'writethrough' -- refer smbfs.py and vzstorage.py.) In both cases Nova uses QEMU's a combination of cache modes 'none' and 'writethrough'. But that is incorrect, because of our misunderstanding of how cache modes work. E.g. Nova's libvirt driver currently assumes (refer disk_cachemode()) that 'writethrough' and 'none' cache modes have the same behaviour with respect to host crash safety, which is not at all true. Fix these wrong assumptions. (Also consult the QEMU Block Layer developers to double-check the behaviour of cache modes and where they are applicable.) To manage notifications about this bug go to: https://bugs.launchpad.net/nova/+bug/1818847/+subscriptions -- Mailing list: https://launchpad.net/~yahoo-eng-team Post to : [email protected] Unsubscribe : https://launchpad.net/~yahoo-eng-team More help : https://help.launchpad.net/ListHelp
