Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
---
mkfs/xfs_mkfs.c | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
index ad776492..84dbb799 100644
--- a/mkfs/xfs_mkfs.c
+++ b/mkfs/xfs_mkfs.c
@@ -18,6 +18,14 @@
#define GIGABYTES(count, blog) ((uint64_t)(count) << (30 - (blog)))
#define MEGABYTES(count, blog) ((uint64_t)(count) << (20 - (blog)))
+/*
+ * Realistically, the log should never be smaller than 64MB. Studies by the
+ * kernel maintainer in early 2022 have shown a dramatic reduction in long tail
+ * latency of the xlog grant head waitqueue when running a heavy metadata
+ * update workload when the log size is at least 64MB.
+ */
+#define XFS_MIN_REALISTIC_LOG_BLOCKS(blog) (MEGABYTES(64, (blog)))
+
/*
* Use this macro before we have superblock and mount structure to
* convert from basic blocks to filesystem blocks.
@@ -3259,6 +3267,28 @@ validate_log_size(uint64_t logblocks, int blocklog, int min_logblocks)
}
}
+/*
+ * Ensure that the log is large enough to provide reasonable performance on a
+ * modern system.
+ */
+static void
+calc_realistic_log_size(
+ struct mkfs_params *cfg)
+{
+ unsigned int realistic_log_blocks;
+
+ realistic_log_blocks = XFS_MIN_REALISTIC_LOG_BLOCKS(cfg->blocklog);
+
+ /*
+ * If the "realistic" size is more than 7/8 of the AG, this is a tiny
+ * filesystem and we don't care.
+ */
+ if (realistic_log_blocks > (cfg->agsize * 7 / 8))
+ return;
+
+ cfg->logblocks = max(cfg->logblocks, realistic_log_blocks);
+}
+
static void
clamp_internal_log_size(
struct mkfs_params *cfg,
@@ -3362,6 +3392,8 @@ _("external log device size %lld blocks too small, must be at least %lld blocks\
cfg->logblocks = cfg->logblocks >> cfg->blocklog;
}
+ calc_realistic_log_size(cfg);
+
clamp_internal_log_size(cfg, mp, min_logblocks);
validate_log_size(cfg->logblocks, cfg->blocklog, min_logblocks);
