ext4: tune mballoc's default group prealloc size for bigalloc file systems

The default group preallocation size had been previously set to 512
blocks/clusters, regardless of the block/cluster size.  This is
probably too big for large cluster sizes.  So adjust the default so
that it is 2 megabytes or 32 clusters, whichever is larger.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 16 insertions, 2 deletions

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index f8e37cf2c2dd..63dd56703342 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -126,7 +126,8 @@
  * list. In case of inode preallocation we follow a list of heuristics
  * based on file size. This can be found in ext4_mb_normalize_request. If
  * we are doing a group prealloc we try to normalize the request to
- * sbi->s_mb_group_prealloc. Default value of s_mb_group_prealloc is
+ * sbi->s_mb_group_prealloc.  The default value of s_mb_group_prealloc is
+ * dependent on the cluster size; for non-bigalloc file systems, it is
  * 512 blocks. This can be tuned via
  * /sys/fs/ext4/<partition>/mb_group_prealloc. The value is represented in
  * terms of number of blocks. If we have mounted the file system with -O
@@ -2473,7 +2474,20 @@ int ext4_mb_init(struct super_block *sb, int needs_recovery)
         sbi->s_mb_stats = MB_DEFAULT_STATS;
         sbi->s_mb_stream_request = MB_DEFAULT_STREAM_THRESHOLD;
         sbi->s_mb_order2_reqs = MB_DEFAULT_ORDER2_REQS;
-        sbi->s_mb_group_prealloc = MB_DEFAULT_GROUP_PREALLOC;
+        /*
+         * The default group preallocation is 512, which for 4k block
+         * sizes translates to 2 megabytes.  However for bigalloc file
+         * systems, this is probably too big (i.e, if the cluster size
+         * is 1 megabyte, then group preallocation size becomes half a
+         * gigabyte!).  As a default, we will keep a two megabyte
+         * group pralloc size for cluster sizes up to 64k, and after
+         * that, we will force a minimum group preallocation size of
+         * 32 clusters.  This translates to 8 megs when the cluster
+         * size is 256k, and 32 megs when the cluster size is 1 meg,
+         * which seems reasonable as a default.
+         */
+        sbi->s_mb_group_prealloc = max(MB_DEFAULT_GROUP_PREALLOC >>
+                                       sbi->s_cluster_bits, 32);
         /*
          * If there is a s_stripe > 1, then we set the s_mb_group_prealloc
          * to the lowest multiple of s_stripe which is bigger than