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gefs: fix file truncation performance (thanks cinap)

We were inserting very oversized deletion messages when
removing a file, instead of inserting the right size
message. This also batches the insertions, reducing the
number of upserts.

--- a/sys/src/cmd/gefs/fs.c

+++ b/sys/src/cmd/gefs/fs.c

@@ -443,7 +443,7 @@

 static int

 readb(Tree *t, Fid *f, char *d, vlong o, vlong n, vlong sz)

 {

-	char buf[17], kvbuf[17+32];

+	char buf[Offksz], kvbuf[Offksz+32];

 	vlong fb, fo;

 	Bptr bp;

 	Blk *b;

@@ -2442,6 +2442,7 @@

 runsweep(int id, void*)

 {

 	char buf[Kvmax];

+	Msg mb[Kvmax/Offksz];

 	Bptr bp, nb, *oldhd;

 	vlong off;

 	Tree *t;

@@ -2448,7 +2449,6 @@

 	Arena *a;

 	Amsg *am;

 	Blk *b;

-	Msg m, mb[2];

 	int i, nm;

 	if((oldhd = calloc(fs->narena, sizeof(Bptr))) == nil)

@@ -2591,26 +2591,30 @@

 			if(am->dent != nil)

 				qlock(&am->dent->trunclk);

 			fs->snap.dirty = 1;

+			nm = 0;

 			for(off = am->off; off < am->end; off += Blksz){

-				qlock(&fs->mutlk);

-				if(waserror()){

+				mb[nm].op = Oclearb;

+				mb[nm].k = buf + Offksz * nm;

+				mb[nm].nk = Offksz;

+				mb[nm].k[0] = Kdat;

+				PACK64(mb[nm].k+1, am->qpath);

+				PACK64(mb[nm].k+9, off);

+				mb[nm].v = nil;

+				mb[nm].nv = 0;

+				if(++nm >= nelem(mb) || off + Blksz >= am->end){

+					qlock(&fs->mutlk);

+					if(waserror()){

+						qunlock(&fs->mutlk);

+						nexterror();

+					}

+					epochstart(id);

+					upsert(am->mnt, mb, nm);

+					epochend(id);

+					epochclean();

 					qunlock(&fs->mutlk);

-					nexterror();

+					poperror();

+					nm = 0;

 				}

-				epochstart(id);

-				m.k = buf;

-				m.nk = sizeof(buf);

-				m.op = Oclearb;

-				m.k[0] = Kdat;

-				PACK64(m.k+1, am->qpath);

-				PACK64(m.k+9, off);

-				m.v = nil;

-				m.nv = 0;

-				upsert(am->mnt, &m, 1);

-				epochend(id);

-				epochclean();

-				qunlock(&fs->mutlk);

-				poperror();

 			}

 			if(am->dent != nil){

 				am->dent->trunc = 0;

-- 

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