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When multifd is used along with mapped-ram, we can take benefit of a filesystem that supports the O_DIRECT flag and perform direct I/O in the multifd threads. This brings a significant performance improvement because direct-io writes bypass the page cache which would otherwise be thrashed by the multifd data which is unlikely to be needed again in a short period of time. To be able to use a multifd channel opened with O_DIRECT, we must ensure that a certain aligment is used. Filesystems usually require a block-size alignment for direct I/O. The way to achieve this is by enabling the mapped-ram feature, which already aligns its I/O properly (see MAPPED_RAM_FILE_OFFSET_ALIGNMENT at ram.c). By setting O_DIRECT on the multifd channels, all writes to the same file descriptor need to be aligned as well, even the ones that come from outside multifd, such as the QEMUFile I/O from the main migration code. This makes it impossible to use the same file descriptor for the QEMUFile and for the multifd channels. The various flags and metadata written by the main migration code will always be unaligned by virtue of their small size. To workaround this issue, we'll require a second file descriptor to be used exclusively for direct I/O. The second file descriptor can be obtained by QEMU by re-opening the migration file (already possible), or by being provided by the user or management application (support to be added in future patches). Reviewed-by: Peter Xu <peterx@redhat.com> Signed-off-by: Fabiano Rosas <farosas@suse.de>
26 lines
965 B
C
26 lines
965 B
C
/*
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* Copyright (c) 2021-2023 Oracle and/or its affiliates.
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or later.
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* See the COPYING file in the top-level directory.
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*/
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#ifndef QEMU_MIGRATION_FILE_H
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#define QEMU_MIGRATION_FILE_H
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#include "qapi/qapi-types-migration.h"
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#include "io/task.h"
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#include "channel.h"
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#include "multifd.h"
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void file_start_incoming_migration(FileMigrationArgs *file_args, Error **errp);
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void file_start_outgoing_migration(MigrationState *s,
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FileMigrationArgs *file_args, Error **errp);
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int file_parse_offset(char *filespec, uint64_t *offsetp, Error **errp);
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void file_cleanup_outgoing_migration(void);
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bool file_send_channel_create(gpointer opaque, Error **errp);
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int file_write_ramblock_iov(QIOChannel *ioc, const struct iovec *iov,
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int niov, RAMBlock *block, Error **errp);
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int multifd_file_recv_data(MultiFDRecvParams *p, Error **errp);
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#endif
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