[libvirt] [PATCH] qemu: reduce file padding requirements
Daniel P. Berrange
berrange at redhat.com
Thu Jun 10 10:16:29 UTC 2010
On Wed, Jun 09, 2010 at 08:33:49PM -0600, Eric Blake wrote:
> Followup to https://bugzilla.redhat.com/show_bug.cgi?id=599091,
> commit 20206a4b, to reduce disk waste in padding.
>
> * src/qemu/qemu_monitor.h (QEMU_MONITOR_MIGRATE_TO_FILE_BS): Drop
> back to 512.
> (QEMU_MONITOR_MIGRATE_TO_FILE_TRANSFER_SIZE): New macro.
> * src/qemu/qemu_driver.c (qemudDomainSaveFlag): Update comment.
> * src/qemu/qemu_monitor_text.c (qemuMonitorTextMigrateToFile): Use
> two invocations of dd to output non-aligned large blocks.
> * src/qemu/qemu_monitor_json.c (qemuMonitorJSONMigrateToFile):
> Likewise.
> ---
>
> This should result in much less wasted space (but I don't have a good
> environment set up for testing migration at the moment; Laine, would
> you mind testing this patch, since you wrote the last one?).
>
> Also, should we bump QEMU_MONITOR_MIGRATE_TO_FILE_BS to 4k, for the
> benefit of newer disks that have 4k blocks?
>
> src/qemu/qemu_driver.c | 7 -------
> src/qemu/qemu_monitor.h | 12 ++++++++----
> src/qemu/qemu_monitor_json.c | 13 +++++++++----
> src/qemu/qemu_monitor_text.c | 13 +++++++++----
> 4 files changed, 26 insertions(+), 19 deletions(-)
>
> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> index bc8dcfa..4edf5ec 100644
> --- a/src/qemu/qemu_driver.c
> +++ b/src/qemu/qemu_driver.c
> @@ -5000,13 +5000,6 @@ static int qemudDomainSaveFlag(virDomainPtr dom, const char *path,
> * we need to ensure there's a 512 byte boundary. Unfortunately
> * we don't have an explicit offset in the header, so we fake
> * it by padding the XML string with NULLs.
> - *
> - * XXX: This means there will be (QEMU_MONITOR_MIGRATE_TO_FILE_BS
> - * - strlen(xml)) bytes of wastage in each file.
> - * Unfortunately, a large BS is needed for reasonable
> - * performance. It would be nice to find a replacement for dd
> - * that could specify the start offset in bytes rather than
> - * blocks, to eliminate this waste.
> */
> if (offset % QEMU_MONITOR_MIGRATE_TO_FILE_BS) {
> unsigned long long pad =
> diff --git a/src/qemu/qemu_monitor.h b/src/qemu/qemu_monitor.h
> index e03b4df..8391ebd 100644
> --- a/src/qemu/qemu_monitor.h
> +++ b/src/qemu/qemu_monitor.h
> @@ -1,7 +1,7 @@
> /*
> * qemu_monitor.h: interaction with QEMU monitor console
> *
> - * Copyright (C) 2006-2009 Red Hat, Inc.
> + * Copyright (C) 2006-2010 Red Hat, Inc.
> * Copyright (C) 2006 Daniel P. Berrange
> *
> * This library is free software; you can redistribute it and/or
> @@ -261,10 +261,14 @@ int qemuMonitorMigrateToCommand(qemuMonitorPtr mon,
> unsigned int background,
> const char * const *argv);
>
> -/* In general, a larger BS means better domain save performance,
> - * at the expense of a larger resulting file - see qemu_driver.c
> +/* In general, BS is the smallest fundamental block size we can use to
> + * access a block device; everything must be aligned to a multiple of
> + * this. However, operating on blocks this small is painfully slow,
> + * so we also have a transfer size that is larger but only aligned to
> + * the smaller block size.
> */
> -# define QEMU_MONITOR_MIGRATE_TO_FILE_BS (1024llu * 1024)
> +# define QEMU_MONITOR_MIGRATE_TO_FILE_BS 512llu
> +# define QEMU_MONITOR_MIGRATE_TO_FILE_TRANSFER_SIZE (1024llu * 1024)
>
> int qemuMonitorMigrateToFile(qemuMonitorPtr mon,
> unsigned int background,
> diff --git a/src/qemu/qemu_monitor_json.c b/src/qemu/qemu_monitor_json.c
> index e89ad43..5e6cedd 100644
> --- a/src/qemu/qemu_monitor_json.c
> +++ b/src/qemu/qemu_monitor_json.c
> @@ -1695,10 +1695,15 @@ int qemuMonitorJSONMigrateToFile(qemuMonitorPtr mon,
> goto cleanup;
> }
>
> - if (virAsprintf(&dest, "exec:%s | dd of=%s bs=%llu seek=%llu",
> - argstr, safe_target,
> - QEMU_MONITOR_MIGRATE_TO_FILE_BS,
> - offset / QEMU_MONITOR_MIGRATE_TO_FILE_BS) < 0) {
> + /* Two dd processes, sharing the same stdout, are necessary to
> + * allow starting at an alignment of 512, but without wasting
> + * padding to get to the larger alignment useful for speed. */
> + if (virAsprintf(&dest, "exec:%s | { dd bs=%llu seek=%llu if=/dev/null && "
> + "dd bs=%llu; } >%s",
Does this work with block devices as the target ? We previously
switched from cat>> to dd, because it didn't work correctly with block
devs.
Regards,
Daniel
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