forked from pool/util-linux
43 lines
1.7 KiB
Plaintext
43 lines
1.7 KiB
Plaintext
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From: Jeff Mahoney <jeffm@suse.com>
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Subject: swapon: Document btrfs limitation with swapfiles
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References: bnc#616617
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Btrfs, as of 2.6.35, is unable to allow swapfiles to be used on its
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filesystems. This is due to the swapfile implementation wanting to build
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an extent map of each block in the file and expecting it to be static
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for the life of the swapfile.
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Btrfs can't guarantee this and refuses to return the mapping. The swapfile
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implementation just makes a comment about there being holes in the file -
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but that's how btrfs denies the mapping.
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This patch adds a section to the swapon manpage to document it.
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Signed-off-by: Jeff Mahoney <jeffm@suse.com>
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---
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mount/swapon.8 | 12 ++++++++++++
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1 file changed, 12 insertions(+)
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--- a/mount/swapon.8
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+++ b/mount/swapon.8
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@@ -167,6 +167,18 @@ automatically detects and rewrites swap
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suspend data (e.g S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
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do it, then we get data corruption the next time an attempt at unsuspending is
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made.
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+.PP
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+.B swapon
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+may not work correctly when using a swap file with some versions of btrfs.
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+This is due to the swap file implementation in the kernel expecting to be able
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+to write to the file directly, without the assistance of the file system.
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+Since btrfs is a copy-on-write file system, the file location may not be
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+static and corruption can result. Btrfs actively disallows the use of files
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+on its file systems by refusing to map the file. This can be seen in the system
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+log as "swapon: swapfile has holes." One possible workaround is to map the
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+file to a loopback device. This will allow the file system to determine the
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+mapping properly but may come with a performance impact.
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+
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.SH SEE ALSO
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.BR swapon (2),
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.BR swapoff (2),
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