Do not use sync_file_range

[nbd.git] / nbd-server.c

diff --git a/nbd-server.c b/nbd-server.c
index 6c3953d..cd67e44 100644 (file)
--- a/nbd-server.c
+++ b/nbd-server.c
@@ -1089,7 +1089,38 @@ ssize_t rawexpwrite(off_t a, char *buf, size_t len, CLIENT *client, int fua) {
        if(client->server->flags & F_SYNC) {
                fsync(fhandle);
        } else if (fua) {
-#ifdef USE_SYNC_FILE_RANGE
+
+         /* This is where we would do the following
+          *   #ifdef USE_SYNC_FILE_RANGE
+          * However, we don't, for the reasons set out below
+          * by Christoph Hellwig <hch@infradead.org>
+          *
+          * [BEGINS] 
+          * fdatasync is equivalent to fsync except that it does not flush
+          * non-essential metadata (basically just timestamps in practice), but it
+          * does flush metadata requried to find the data again, e.g. allocation
+          * information and extent maps.  sync_file_range does nothing but flush
+          * out pagecache content - it means you basically won't get your data
+          * back in case of a crash if you either:
+          * 
+          *  a) have a volatile write cache in your disk (e.g. any normal SATA disk)
+          *  b) are using a sparse file on a filesystem
+          *  c) are using a fallocate-preallocated file on a filesystem
+          *  d) use any file on a COW filesystem like btrfs
+          * 
+          * e.g. it only does anything useful for you if you do not have a volatile
+          * write cache, and either use a raw block device node, or just overwrite
+          * an already fully allocated (and not preallocated) file on a non-COW
+          * filesystem.
+          * [ENDS]
+          *
+          * What we should do is open a second FD with O_DSYNC set, then write to
+          * that when appropriate. However, with a Linux client, every REQ_FUA
+          * immediately follows a REQ_FLUSH, so fdatasync does not cause performance
+          * problems.
+          *
+          */
+#if 0
                sync_file_range(fhandle, foffset, len,
                                SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE |
                                SYNC_FILE_RANGE_WAIT_AFTER);