[PATCH 7/7] xfs_scrub: tune fstrim minlen parameter based on free space histograms

Darrick J. Wong
2023-12-31T22:52:21Z

Currently, phase 8 runs very slowly on filesystems with a lot of small
free space extents. To reduce the amount of time spent on fstrim
activities during phase 8, we want to balance estimated runtime against
completeness of the trim. In short, the goal is to reduce runtime by
avoiding small trim requests.

At the start of phase 8, a CDF is computed in decreasing order of extent
length from the histogram buckets created during the fsmap scan in phase
7. A point corresponding to the fstrim percentage target is chosen from
the CDF and mapped back to a histogram bucket, and free space extents
smaller than that amount are ommitted from fstrim.

On my aging /home filesystem, the free space histogram reported by
xfs_spaceman looks like this:

from to extents blocks pct blkcdf extcdf
1 1 121953 121953 0.04 100.00 100.00
2 3 124741 299694 0.09 99.96 81.16
4 7 113492 593763 0.18 99.87 61.89
8 15 109215 1179524 0.36 99.69 44.36
16 31 76972 1695455 0.52 99.33 27.48
32 63 48655 2219667 0.68 98.82 15.59
64 127 31398 2876898 0.88 98.14 8.08
128 255 8014 1447920 0.44 97.27 3.23
256 511 4142 1501758 0.46 96.82 1.99
512 1023 2433 1768732 0.54 96.37 1.35
1024 2047 1795 2648460 0.81 95.83 0.97
2048 4095 1429 4206103 1.28 95.02 0.69
4096 8191 1045 6162111 1.88 93.74 0.47
8192 16383 791 9242745 2.81 91.87 0.31
16384 32767 473 10883977 3.31 89.06 0.19
32768 65535 272 12385566 3.77 85.74 0.12
65536 131071 192 18098739 5.51 81.98 0.07
131072 262143 108 20675199 6.29 76.47 0.04
262144 524287 80 29061285 8.84 70.18 0.03
524288 1048575 39 29002829 8.83 61.33 0.02
1048576 2097151 25 36824985 11.21 52.51 0.01
2097152 4194303 32 101727192 30.95 41.30 0.01
4194304 8388607 7 34007410 10.35 10.35 0.00

From this table, we see that free space extents that are 16 blocks or
longer constitute 99.3% of the free space in the filesystem but only
27.5% of the extents. If we set the fstrim minlen parameter to 16
blocks, that means that we can trim over 99% of the space in one third
of the time it would take to trim everything.

Add a new -o fstrim_pct= option to xfs_scrub just in case there are
users out there who want a different percentage. For example, accepting
a 95% trim would net us a speed increase of nearly two orders of
magnitude, ignoring system call overhead. Setting it to 100% will trim
everything, just like fstrim(8).

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
libfrog/histogram.c | 2 +
libfrog/histogram.h | 1 +
man/man8/xfs_scrub.8 | 16 +++++++++++
scrub/phase8.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++---
scrub/vfs.c | 4 ++-
scrub/vfs.h | 2 +
scrub/xfs_scrub.c | 38 +++++++++++++++++++++++++
scrub/xfs_scrub.h | 12 ++++++++
8 files changed, 141 insertions(+), 9 deletions(-)

diff --git a/libfrog/histogram.c b/libfrog/histogram.c
index 54e2bac0f73..61ecda16ffe 100644
--- a/libfrog/histogram.c
+++ b/libfrog/histogram.c
@@ -109,7 +109,7 @@ hist_free(
* of small extents, e.g. 98% of the free space extents are larger than 31
* blocks.
*/
-static int
+int
hist_cdf(
const struct histogram *hs,
struct histogram *cdf)
diff --git a/libfrog/histogram.h b/libfrog/histogram.h
index ec788344d4c..ecac66d240d 100644
--- a/libfrog/histogram.h
+++ b/libfrog/histogram.h
@@ -39,6 +39,7 @@ void hist_add(struct histogram *hs, long long len);
void hist_init(struct histogram *hs);
void hist_prepare(struct histogram *hs, long long maxlen);
void hist_free(struct histogram *hs);
+int hist_cdf(const struct histogram *hs, struct histogram *cdf);
void hist_print(const struct histogram *hs);
void hist_summarize(const struct histogram *hs);

diff --git a/man/man8/xfs_scrub.8 b/man/man8/xfs_scrub.8
index 404baba696e..b9f253e1b07 100644
--- a/man/man8/xfs_scrub.8
+++ b/man/man8/xfs_scrub.8
@@ -100,6 +100,22 @@ The
supported are:
.RS 1.0i
.TP
+.BI fstrim_pct= percentage
+To constrain the amount of time spent on fstrim activities during phase 8,
+this program tries to balance estimated runtime against completeness of the
+trim.
+In short, the program avoids small trim requests to save time.
+
+During phase 7, a log-scale histogram of free space extents is constructed.
+At the start of phase 8, a CDF is computed in decreasing order of extent
+length from the histogram buckets.
+A point corresponding to the fstrim percentage target is chosen from the CDF
+and mapped back to a histogram bucket.
+Free space extents at least as long as the bucket size are trimmed.
+Smaller extents are ignored.
+
+By default, the percentage threshold is 99%.
+.TP
.BI iwarn
Treat informational messages as warnings.
This will result in a nonzero return code, and a higher logging level.
diff --git a/scrub/phase8.c b/scrub/phase8.c
index f1a854a3e56..c6845555579 100644
--- a/scrub/phase8.c
+++ b/scrub/phase8.c
@@ -11,6 +11,7 @@
#include "list.h"
#include "libfrog/paths.h"
#include "libfrog/workqueue.h"
+#include "libfrog/histogram.h"
#include "xfs_scrub.h"
#include "common.h"
#include "progress.h"
@@ -57,10 +58,12 @@ static int
fstrim_fsblocks(
struct scrub_ctx *ctx,
uint64_t start_fsb,
- uint64_t fsbcount)
+ uint64_t fsbcount,
+ uint64_t minlen_fsb)
{
uint64_t start = cvt_off_fsb_to_b(&ctx->mnt, start_fsb);
uint64_t len = cvt_off_fsb_to_b(&ctx->mnt, fsbcount);
+ uint64_t minlen = cvt_off_fsb_to_b(&ctx->mnt, minlen_fsb);
int error;

while (len > 0) {
@@ -68,7 +71,7 @@ fstrim_fsblocks(

run = min(len, FSTRIM_MAX_BYTES);

- error = fstrim(ctx, start, run);
+ error = fstrim(ctx, start, run, minlen);
if (error == EOPNOTSUPP) {
/* Pretend we finished all the work. */
progress_add(len);
@@ -78,9 +81,10 @@ fstrim_fsblocks(
char descr[DESCR_BUFSZ];

snprintf(descr, sizeof(descr) - 1,
- _("fstrim start 0x%llx run 0x%llx"),
+ _("fstrim start 0x%llx run 0x%llx minlen 0x%llx"),
(unsigned long long)start,
- (unsigned long long)run);
+ (unsigned long long)run,
+ (unsigned long long)minlen);
str_liberror(ctx, error, descr);
return error;
}
@@ -93,6 +97,64 @@ fstrim_fsblocks(
return 0;
}

+/* Compute a suitable minlen parameter for fstrim. */
+static uint64_t
+fstrim_compute_minlen(
+ const struct scrub_ctx *ctx,
+ const struct histogram *freesp_hist)
+{
+ struct histogram cdf;
+ uint64_t ret = 0;
+ double blk_threshold = 0;
+ unsigned int i;
+ unsigned int ag_max_usable;
+ int error;
+
+ /*
+ * The kernel will reject a minlen that's larger than m_ag_max_usable.
+ * We can't calculate or query that value directly, so we guesstimate
+ * that it's 95% of the AG size.
+ */
+ ag_max_usable = ctx->mnt.fsgeom.agblocks * 95 / 100;
+
+ if (freesp_hist->totexts == 0)
+ goto out;
+
+ if (debug > 1)
+ hist_print(freesp_hist);
+
+ /* Insufficient samples to make a meaningful histogram */
+ if (freesp_hist->totexts < freesp_hist->nr_buckets * 10)
+ goto out;
+
+ hist_init(&cdf);
+ error = hist_cdf(freesp_hist, &cdf);
+ if (error)
+ goto out_free;
+
+ blk_threshold = freesp_hist->totblocks * ctx->fstrim_block_pct;
+ for (i = 1; i < freesp_hist->nr_buckets; i++) {
+ if (cdf.buckets[i].blocks < blk_threshold) {
+ ret = freesp_hist->buckets[i - 1].low;
+ break;
+ }
+ }
+
+out_free:
+ hist_free(&cdf);
+out:
+ if (debug > 1)
+ printf(_("fstrim minlen %lld threshold %lld ag_max_usable %u\n"),
+ (unsigned long long)ret,
+ (unsigned long long)blk_threshold,
+ ag_max_usable);
+ if (ret > ag_max_usable)
+ ret = ag_max_usable;
+ if (ret == 1)
+ ret = 0;
+ return ret;
+}
+
/* Trim each AG on the data device. */
static int
fstrim_datadev(
@@ -100,8 +162,11 @@ fstrim_datadev(
{
struct xfs_fsop_geom *geo = &ctx->mnt.fsgeom;
uint64_t fsbno;
+ uint64_t minlen_fsb;
int error;

+ minlen_fsb = fstrim_compute_minlen(ctx, &ctx->datadev_hist);
+
for (fsbno = 0; fsbno < geo->datablocks; fsbno += geo->agblocks) {
uint64_t fsbcount;

@@ -112,7 +177,7 @@ fstrim_datadev(
*/
progress_add(geo->blocksize);
fsbcount = min(geo->datablocks - fsbno + 1, geo->agblocks);
- error = fstrim_fsblocks(ctx, fsbno + 1, fsbcount);
+ error = fstrim_fsblocks(ctx, fsbno + 1, fsbcount, minlen_fsb);
if (error)
return error;
}
diff --git a/scrub/vfs.c b/scrub/vfs.c
index cc958ba9438..22c19485a2d 100644
--- a/scrub/vfs.c
+++ b/scrub/vfs.c
@@ -300,11 +300,13 @@ int
fstrim(
struct scrub_ctx *ctx,
uint64_t start,
- uint64_t len)
+ uint64_t len,
+ uint64_t minlen)
{
struct fstrim_range range = {
.start = start,
.len = len,
+ .minlen = minlen,
};

if (ioctl(ctx->mnt.fd, FITRIM, &range) == 0)
diff --git a/scrub/vfs.h b/scrub/vfs.h
index 1af8d80d1de..f0cfd53c27b 100644
--- a/scrub/vfs.h
+++ b/scrub/vfs.h
@@ -24,6 +24,6 @@ typedef int (*scan_fs_tree_dirent_fn)(struct scrub_ctx *, const char *,
int scan_fs_tree(struct scrub_ctx *ctx, scan_fs_tree_dir_fn dir_fn,
scan_fs_tree_dirent_fn dirent_fn, void *arg);

-int fstrim(struct scrub_ctx *ctx, uint64_t start, uint64_t len);
+int fstrim(struct scrub_ctx *ctx, uint64_t start, uint64_t len, uint64_t minlen);

#endif /* XFS_SCRUB_VFS_H_ */
diff --git a/scrub/xfs_scrub.c b/scrub/xfs_scrub.c
index c66469a0703..37b95aa1e67 100644
--- a/scrub/xfs_scrub.c
+++ b/scrub/xfs_scrub.c
@@ -622,11 +622,13 @@ report_outcome(
*/
enum o_opt_nums {
IWARN = 0,
+ FSTRIM_PCT,
O_MAX_OPTS,
};

static char *o_opts[] = {
[IWARN] = "iwarn",
+ [FSTRIM_PCT] = "fstrim_pct",
[O_MAX_OPTS] = NULL,
};

@@ -635,8 +637,11 @@ parse_o_opts(
struct scrub_ctx *ctx,
char *p)
{
+ double dval;
+
while (*p != '\0') {
char *val;
+ char *endp;

switch (getsubopt(&p, o_opts, &val)) {
case IWARN:
@@ -647,6 +652,35 @@ parse_o_opts(
}
info_is_warning = true;
break;
+ case FSTRIM_PCT:
+ if (!val) {
+ fprintf(stderr,
+ _("-o fstrim_pct requires a parameter\n"));
+ usage();
+ }
+
+ errno = 0;
+ dval = strtod(val, &endp);
+
+ if (*endp) {
+ fprintf(stderr,
+ _("-o fstrim_pct must be a floating point number\n"));
+ usage();
+ }
+ if (errno) {
+ fprintf(stderr,
+ _("-o fstrim_pct: %s\n"),
+ strerror(errno));
+ usage();
+ }
+ if (dval <= 0 || dval > 100) {
+ fprintf(stderr,
+ _("-o fstrim_pct must be larger than 0 and less than 100\n"));
+ usage();
+ }
+
+ ctx->fstrim_block_pct = dval / 100.0;
+ break;
default:
usage();
break;
@@ -659,7 +693,9 @@ main(
int argc,
char **argv)
{
- struct scrub_ctx ctx = {0};
+ struct scrub_ctx ctx = {
+ .fstrim_block_pct = FSTRIM_BLOCK_PCT_DEFAULT,
+ };
struct phase_rusage all_pi;
char *mtab = NULL;
FILE *progress_fp = NULL;
diff --git a/scrub/xfs_scrub.h b/scrub/xfs_scrub.h
index 1a28f0cc847..7d48f4bad9c 100644
--- a/scrub/xfs_scrub.h
+++ b/scrub/xfs_scrub.h
@@ -90,8 +90,20 @@ struct scrub_ctx {

/* Free space histograms, in fsb */
struct histogram datadev_hist;
+
+ /*
+ * Pick the largest value for fstrim minlen such that we trim at least
+ * this much space per volume.
+ */
+ double fstrim_block_pct;
};

+/*
+ * Trim only enough free space extents (in order of decreasing length) to
+ * ensure that this percentage of the free space is trimmed.
+ */
+#define FSTRIM_BLOCK_PCT_DEFAULT (99.0 / 100.0)
+
/* Phase helper functions */
void xfs_shutdown_fs(struct scrub_ctx *ctx);
int scrub_cleanup(struct scrub_ctx *ctx);