Darrick, it bugged me a bit that the way this patch worked was to go through
one set of calculations to sort out a log size, then throw in a new heuristic
or two to change that result. And TBH the 7/8 bit seems very ad hoc when
we already had the prior 95% heuristic.
While I know that most of this goes away by the last patch, I'm considering
deferring patches 4 & 5 for the next release because of the impact on
fstests, so would like to land somewhere clean by patch 3.
What do you think of this patch as a replacement for this patch 3/5? It's a
bit clearer to me, and results in (almost) the same net results as your patch,
with minor differences around 512MB filesystems due to the removal of the
7/8 limit.
Depending on what you think, I can tweak your commit log as needed, leave
your authorship, and add a:
[sandeen: consolidate heuristics into something a bit more clear]
or something like that... or, claim authorship (and all ensuing bugs) ;)
Thanks,
-Eric
diff --git a/include/xfs_multidisk.h b/include/xfs_multidisk.h
index a16a9fe2..ef4443b0 100644
--- a/include/xfs_multidisk.h
+++ b/include/xfs_multidisk.h
@@ -17,8 +17,6 @@
#define XFS_MIN_INODE_PERBLOCK 2 /* min inodes per block */
#define XFS_DFL_IMAXIMUM_PCT 25 /* max % of space for inodes */
#define XFS_MIN_REC_DIRSIZE 12 /* 4096 byte dirblocks (V2) */
-#define XFS_DFL_LOG_FACTOR 5 /* default log size, factor */
- /* with max trans reservation */
#define XFS_MAX_INODE_SIG_BITS 32 /* most significant bits in an
* inode number that we'll
* accept w/o warnings
diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
index ad776492..cf1d64a2 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.
@@ -3297,7 +3305,7 @@ calculate_log_size(
struct xfs_mount *mp)
{
struct xfs_sb *sbp = &mp->m_sb;
- int min_logblocks;
+ int min_logblocks; /* absolute minimum */
struct xfs_mount mount;
/* we need a temporary mount to calculate the minimum log size. */
@@ -3339,29 +3347,19 @@ _("external log device size %lld blocks too small, must be at least %lld blocks\
/* internal log - if no size specified, calculate automatically */
if (!cfg->logblocks) {
- if (cfg->dblocks < GIGABYTES(1, cfg->blocklog)) {
- /* tiny filesystems get minimum sized logs. */
- cfg->logblocks = min_logblocks;
- } else if (cfg->dblocks < GIGABYTES(16, cfg->blocklog)) {
+ /* Use a 2048:1 fs:log ratio for most filesystems */
+ cfg->logblocks = (cfg->dblocks << cfg->blocklog) / 2048;
+ cfg->logblocks = cfg->logblocks >> cfg->blocklog;
- /*
- * For small filesystems, we want to use the
- * XFS_MIN_LOG_BYTES for filesystems smaller than 16G if
- * at all possible, ramping up to 128MB at 256GB.
- */
- cfg->logblocks = min(XFS_MIN_LOG_BYTES >> cfg->blocklog,
- min_logblocks * XFS_DFL_LOG_FACTOR);
- } else {
- /*
- * With a 2GB max log size, default to maximum size
- * at 4TB. This keeps the same ratio from the older
- * max log size of 128M at 256GB fs size. IOWs,
- * the ratio of fs size to log size is 2048:1.
- */
- cfg->logblocks = (cfg->dblocks << cfg->blocklog) / 2048;
- cfg->logblocks = cfg->logblocks >> cfg->blocklog;
- }
+ /* But don't go below a reasonable size */
+ cfg->logblocks = max(cfg->logblocks,
+ XFS_MIN_REALISTIC_LOG_BLOCKS(cfg->blocklog));
+
+ /* And for a tiny filesystem, use the absolute minimum size */
+ if (cfg->dblocks < MEGABYTES(512, cfg->blocklog))
+ cfg->logblocks = min_logblocks;
+ /* Enforce min/max limits */
clamp_internal_log_size(cfg, mp, min_logblocks);
validate_log_size(cfg->logblocks, cfg->blocklog, min_logblocks);
