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The readahead filter is a self-configuring filter that makes sequential reads faster when the plugin is slow (and all of the plugins we use here are always slow). I observed the behaviour of the readahead filter with our qcow2 overlay when converting a guest from a vCenter source. Even when doing random reads, qemu issues 64K reads which happen to also be the minimum request size of the readahead filter, so there is no extra overhead. When doing the sequential copy the readahead filter performed better than qemu curl’s readahead because it scaled the prefetched data appropriately on long contiguous stretches and then shrunk it back to 64K around fragmented parts of the base image.
Libguestfs is tools and a library for accessing and modifying guest disk images. For more information see the home page: http://libguestfs.org/ For discussion, development, patches, etc. please use the mailing list: http://www.redhat.com/mailman/listinfo/libguestfs To find out how to build libguestfs from source, read: docs/guestfs-building.pod http://libguestfs.org/guestfs-building.1.html man docs/guestfs-building.1 Copyright (C) 2009-2019 Red Hat Inc. The library is distributed under the LGPLv2+. The programs are distributed under the GPLv2+. Please see the files COPYING and COPYING.LIB for full license information. The examples are under a very liberal license.