[ccache] [RFC PATCH] Use fast (copy-on-write) copies on btrfs

[Tobias Geerinckx-Rice]

> this is a feature request. I think it would make sense to add CoW (Copy-on-
> Write) support for ccache (instead of using dangerous hard-links)

I think so too.

> for filesystems which support it (zfs, btrfs, more to come?).

I've hacked up a simple patch to do this unconditionally on btrfs on
Linux, silently falling back to a regular copy if the ioctl fails.
The implementation is btrfs-specific; ZFS doesn't seem to support file-
level CoW at the moment and there is no cross-fs kernel interface (yet).

Notes/questions:

  - It's been tested only by running a few 'make check's on both my
    native btrfs system and an ext4 loopback image. Caveat so much
    emptor.

  - I've done no performance testing at all. I hope to do so eventually, 
    but free time is scarce. Egon? Anyone? :-)

  - I really don't think this ought to be run-time configurable. It's
    a transparent optimisation that either always works, or always falls 
    back to a full copy.
    On non-Linux platforms, this should compile away. On Linux systems,
    I'm reasonably assuming the cost of a failed attempt to be less than
    that of a configuration check, without the ugliness of the latter.

  - I'm not too fond of the resulting copy_file(), what with the FIXME
    and the goto (even if goto seems to be used liberally elsewhere).
    IMHO, copy_file tries to be too transparent, leading to convoluted
    code for little benefit.
    I'd like to split it up into separate copy_{to,from}_cache that only
    do what is needed, but I'm too afraid of having missed something.
 
  - So please: tell me why I'm stupid and wrong and save me some work!

Regards,

T G-R

>From a1e00d82ab1eae1bc39122482b35954bf2304af0 Mon Sep 17 00:00:00 2001
From: Tobias Geerinckx-Rice <tobias.geerinckx.rice at gmail.com>
Date: Tue, 16 Sep 2014 20:23:14 +0200
Subject: [RFC PATCH] Use fast (copy-on-write) copies on btrfs

Modern file systems like btrfs and ZFS support metadata-only file
copies. These are similar to hard links, but without the risks:
if either the source or destination files are later modified, the
contents of the other file(s) do not change.

(Currently uses hard-coded #ifdef/#defines which should go away
as soon as all this is properly supported in the kernel.)
---
 util.c | 44 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 37 insertions(+), 7 deletions(-)

diff --git a/util.c b/util.c
index 156c0be..b5e5ae8 100644
--- a/util.c
+++ b/util.c
@@ -19,6 +19,7 @@
 
 #include "ccache.h"
 
+#include <sys/ioctl.h>
 #include <zlib.h>
 
 #ifdef HAVE_PWD_H
@@ -224,6 +225,26 @@ mkstemp(char *template)
 #endif
 
 /*
+ * Copy the contents of src_fd to dest_fd in one efficient operation.
+ * Currently only supports btrfs on Linux, returns -1 on failure.
+ */
+int
+clone_file(int src_fd, int dest_fd)
+{
+#ifdef __linux__
+#undef BTRFS_IOCTL_MAGIC
+#define BTRFS_IOCTL_MAGIC 0x94
+#undef BTRFS_IOC_CLONE
+#define BTRFS_IOC_CLONE _IOW (BTRFS_IOCTL_MAGIC, 9, int)
+	return ioctl(dest_fd, BTRFS_IOC_CLONE, src_fd);
+#else	/* __linux__ */
+	(void) src_fd;
+	(void) dest_fd;
+	return -1;
+#endif	/* __linux__ */
+}
+
+/*
  * Copy src to dest, decompressing src if needed. compress_level > 0 decides
  * whether dest will be compressed, and with which compression level.
  */
@@ -260,13 +281,6 @@ copy_file(const char *src, const char *dest, int compress_level)
 		goto error;
 	}
 
-	gz_in = gzdopen(fd_in, "rb");
-	if (!gz_in) {
-		cc_log("gzdopen(src) error: %s", strerror(errno));
-		close(fd_in);
-		goto error;
-	}
-
 	if (compress_level > 0) {
 		/*
 		 * A gzip file occupies at least 20 bytes, so it will always
@@ -289,6 +303,21 @@ copy_file(const char *src, const char *dest, int compress_level)
 			goto error;
 		}
 		gzsetparams(gz_out, compress_level, Z_DEFAULT_STRATEGY);
+	} else {
+		/* FIXME: this introduces unnecessary open/close overhead */
+		if (!file_is_compressed(src)) {
+			/* try to create a fast CoW copy first */
+			if (clone_file(fd_in, fd_out) == 0) {
+				goto out;
+			}
+		}
+	}
+
+	gz_in = gzdopen(fd_in, "rb");
+	if (!gz_in) {
+		cc_log("gzdopen(src) error: %s", strerror(errno));
+		close(fd_in);
+		goto error;
 	}
 
 	while ((n = gzread(gz_in, buf, sizeof(buf))) > 0) {
@@ -335,6 +364,7 @@ copy_file(const char *src, const char *dest, int compress_level)
 		return -1;
 	}
 
+out:
 	gzclose(gz_in);
 	gz_in = NULL;
 	if (gz_out) {
-- 
2.1.0