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Re: [PATCH for-5.1] file-posix: Mitigate file fragmentation with extent

From: Eric Blake
Subject: Re: [PATCH for-5.1] file-posix: Mitigate file fragmentation with extent size hints
Date: Tue, 7 Jul 2020 09:47:06 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 7/7/20 9:23 AM, Kevin Wolf wrote:
Espeically when O_DIRECT is used with image files so that the page cache


indirection can't cause a merge of allocating requests, the file will
fragment on the file system layer, with a potentially very small
fragment size (this depends on the requests the guest sent).

On Linux, fragmentation can be reduced by setting an extent size hint
when creating the file (at least on XFS, it can't be set any more after
the first extent has been allocated), basically giving raw files a
"cluster size" for allocation.

This adds an create option to set the extent size hint, and changes the
default from not setting a hint to setting it to 1 MB. The main reason
why qcow2 defaults to smaller cluster sizes is that COW becomes more
expensive, which is not an issue with raw files, so we can choose a
larger file. The tradeoff here is only potentially wasted disk space.

For qcow2 (or other image formats) over file-posix, the advantage should
even be greater because they grow sequentially without leaving holes, so
there won't be wasted space. Setting even larger extent size hints for
such images may make sense. This can be done with the new option, but
let's keep the default conservative for now.

The effect is very visible with a test that intentionally creates a
badly fragmented file with qemu-img bench (the time difference while
creating the file is already remarkable) and then looks at the number of
extents and the take a simple "qemu-img map" takes.


Signed-off-by: Kevin Wolf <kwolf@redhat.com>
  qapi/block-core.json      | 11 ++++++----
  include/block/block_int.h |  1 +
  block/file-posix.c        | 42 +++++++++++++++++++++++++++++++++++++++
  3 files changed, 50 insertions(+), 4 deletions(-)

+    /*
+     * Try to set the extent size hint. Failure is not fatal, and a warning is
+     * only printed if the option was explicitly specified.
+     */
+    {
+        struct fsxattr attr;
+        result = ioctl(fd, FS_IOC_FSGETXATTR, &attr);
+        if (result == 0) {
+            attr.fsx_xflags |= FS_XFLAG_EXTSIZE;
+            attr.fsx_extsize = file_opts->extent_size_hint;
+            result = ioctl(fd, FS_IOC_FSSETXATTR, &attr);
+        }
+        if (result < 0 && file_opts->has_extent_size_hint) {
+            warn_report("Failed to set extent size hint: %s",
+                        strerror(errno));
+        }
+    }

That's a neat ioctl to learn.

Reviewed-by: Eric Blake <eblake@redhat.com>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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