[PATCH] Dump flush write performance

[Ben Guthro <bguthro@virtualiron.com>]

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Two changes to avoid disk driver using up kernel memory and triggering 
oom-killer
while creating guest dump to a disk file:

1) Decreased write chunks from 16MB to 2MB;
2) Flush (fsync) after each 2MB write.

To make sure we don't suffer significant write performance penalty, each 
of the
following cases have been tested several times to dump a 1GB guest to a 
file on
a SAN disk:

With current code (16MB writes, sync once after all writes); ~38s
16MB writes, sync after each write: ~46s
2MB writes, sync after each write: ~49s

Sync-often costs us about 20%, smaller-write-size costs another 8%. The 
total is
less than 30% and about 10 seconds more for a 1GB guest.

Signed-off-by: Ben Guthro <bguthro@virtualron.com>
Signed-off-by: Daniel Li <dli@virtualiron.com>

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diff -r 9d263e1fcd02 tools/libxc/xc_core.c
--- a/tools/libxc/xc_core.c	Wed Oct 24 16:25:11 2007 -0400
+++ b/tools/libxc/xc_core.c	Wed Oct 24 16:26:04 2007 -0400
@@ -56,7 +56,7 @@
 #include <unistd.h>
 
 /* number of pages to write at a time */
-#define DUMP_INCREMENT (4 * 1024)
+#define DUMP_INCREMENT (512)
 
 /* Don't yet support cross-address-size core dump */
 #define guest_width (sizeof (unsigned long))
@@ -875,7 +875,7 @@ static int local_file_dump(void *args, c
     if (length >= DUMP_INCREMENT*PAGE_SIZE) {
         // Now dumping pages -- make sure we discard clean pages from
         // the cache after each write
-        discard_file_cache(da->fd, 0 /* no flush */);
+        discard_file_cache(da->fd, 1 /* flush */);
     }
 
     return 0;

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