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pr 34583 in NetBSD. quoting author:

"Add a 3rd entry in the cache, which keeps the end position
from just before extending a file.
This has the desired effect of keeping the write speed constant."

please test, comment

Index: denode.h
================================================== =================
RCS file: /cvs/src/sys/msdosfs/denode.h,v
retrieving revision 1.18
diff -u -p -r1.18 denode.h
--- denode.h 1 Mar 2005 14:24:33 -0000 1.18
+++ denode.h 12 Oct 2006 13:41:09 -0000
@@ -116,10 +116,11 @@ struct fatcache {
* cache is probably pretty worthless if a file is opened by multiple
* processes.
*/
-#define FC_SIZE 2 /* number of entries in the cache */
+#define FC_SIZE 3 /* number of entries in the cache */
#define FC_LASTMAP 0 /* entry the last call to pcbmap() resolved
* to */
#define FC_LASTFC 1 /* entry for the last cluster in the file */
+#define FC_NEXTTOLASTFC 2 /* entry for a close to the last cluster in the file */

#define FCE_EMPTY 0xffffffff /* doesn't represent an actual cluster # */

@@ -129,6 +130,12 @@ struct fatcache {
#define fc_setcache(dep, slot, frcn, fsrcn) \
(dep)->de_fc[slot].fc_frcn = frcn; \
(dep)->de_fc[slot].fc_fsrcn = fsrcn;
+
+#define fc_last_to_nexttolast(dep) \
+ do { \
+ (dep)->de_fc[FC_NEXTTOLASTFC].fc_frcn = (dep)->de_fc[FC_LASTFC].fc_frcn; \
+ (dep)->de_fc[FC_NEXTTOLASTFC].fc_fsrcn = (dep)->de_fc[FC_LASTFC].fc_fsrcn; \
+ } while (0)

/*
* This is the in memory variant of a dos directory entry. It is usually
Index: msdosfs_fat.c
================================================== =================
RCS file: /cvs/src/sys/msdosfs/msdosfs_fat.c,v
retrieving revision 1.18
diff -u -p -r1.18 msdosfs_fat.c
--- msdosfs_fat.c 3 Oct 2006 19:49:06 -0000 1.18
+++ msdosfs_fat.c 12 Oct 2006 13:41:09 -0000
@@ -82,6 +82,34 @@ int fc_bmapcalls; /* # of times pcbmap
int fc_lmdistance[LMMAX]; /* counters for how far off the last
* cluster mapped entry was. */
int fc_largedistance; /* off by more than LMMAX */
+int fc_wherefrom, fc_whereto, fc_lastclust;
+int pm_fatblocksize;
+
+#ifdef MSDOSFS_DEBUG
+void print_fat_stats(void);
+
+void
+print_fat_stats(void)
+{
+ int i;
+
+ printf("fc_fileextends=%d fc_lfcempty=%d fc_bmapcalls=%d "
+ "fc_largedistance=%d [%d->%d=%d] fc_lastclust=%d pm_fatblocksize=%d\n",
+ fc_fileextends, fc_lfcempty, fc_bmapcalls, fc_largedistance,
+ fc_wherefrom, fc_whereto, fc_whereto-fc_wherefrom,
+ fc_lastclust, pm_fatblocksize);
+
+ fc_fileextends = fc_lfcempty = fc_bmapcalls = 0;
+ fc_wherefrom = fc_whereto = fc_lastclust = 0;
+
+ for (i = 0; i < LMMAX; i++) {
+ printf("%d:%d ", i, fc_lmdistance[i]);
+ fc_lmdistance[i] = 0;
+ }
+
+ printf("\n");
+}
+#endif

static void fatblock(struct msdosfsmount *, uint32_t, uint32_t *, uint32_t *,
uint32_t *);
@@ -114,6 +142,8 @@ fatblock(pmp, ofs, bnp, sizep, bop)
*sizep = size;
if (bop)
*bop = ofs % pmp->pm_fatblocksize;
+
+ pm_fatblocksize = pmp->pm_fatblocksize;
}

/*
@@ -205,9 +235,12 @@ pcbmap(dep, findcn, bnp, cnp, sp)
*/
i = 0;
fc_lookup(dep, findcn, &i, &cn);
- if ((bn = findcn - i) >= LMMAX)
+ if ((bn = findcn - i) >= LMMAX) {
fc_largedistance++;
- else
+ fc_wherefrom = i;
+ fc_whereto = findcn;
+ fc_lastclust = dep->de_fc[FC_LASTFC].fc_frcn;
+ } else
fc_lmdistance[bn]++;

/*
@@ -1020,6 +1053,8 @@ extendfile(dep, count, bpp, ncp, flags)
if (error != E2BIG)
return (error);
}
+
+ fc_last_to_nexttolast(dep);

while (count > 0) {
/*