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gefs: change log compression heuristic

compress when the log doubles in size, rather than
using a fixed size heuristic; this means that we
don't start compressing frequently as the log gets
big and the file system gets fragmented.

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

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

@@ -483,6 +483,7 @@

 	a->loghd = (Bptr){blks[0], -1, -1};

 	a->logtl = b;	/* written back by sync() later */

 	a->nlog = nlog;

+	a->lastlogsz = nlog;

 	/* May add blocks to new log */

 	for(; i < nblks; i++)

--- a/sys/src/cmd/gefs/dat.h

+++ b/sys/src/cmd/gefs/dat.h

@@ -596,6 +596,7 @@

 	vlong	used;

 	vlong	reserve;

 	/* allocation log */

+	vlong	lastlogsz;	/* size after last compression */

 	vlong	nlog;		/* number of blocks in log */

 	Bptr	loghd;		/* allocation log */

 	Blk	*logtl;		/* end of the log, open for writing */

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

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

@@ -2474,10 +2474,10 @@

 				oldhd[i] = Zb;

 				qlock(a);

 				/*

-				 * arbitrary heuristic -- 10% of our reserved

-				 * space seems like a fine time to compress.

+				 * arbitrary heuristic -- try compressing

+				 * when the log doubles in size.

 				 */

-				if(a->nlog >= a->reserve/(10*Blksz)){

+				if(a->nlog >= 2*a->lastlogsz){

 					oldhd[i] = a->loghd;

 					epochstart(id);

 					if(waserror()){

-- 

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