diff --git a/blsl/vcfparallel.py b/blsl/vcfparallel.py
index f410565..cacaa57 100644
--- a/blsl/vcfparallel.py
+++ b/blsl/vcfparallel.py
@@ -10,10 +10,10 @@
 import shutil
 import subprocess
 import argparse
-import math
 import tempfile
 from concurrent.futures import ProcessPoolExecutor, as_completed
 from pathlib import Path
+import math
 import uuid
 import re
 
@@ -37,9 +37,8 @@ def one_chunk(vcf, chunk, temp_prefix, filterarg="", pipeline="", index=True):
         cmd = f"{cmd} | {pipeline}"
     if index:
         cmd = f"{cmd} | bcftools view -Ob0 --write-index -o {ofile}"
-    with open(ofile + ".log", "wb") as log:
-        with open(ofile, "wb") as ofh:
-            subprocess.run(cmd, shell=True, stdout=ofh, stderr=log, check=True)
+    cmd = f"({cmd}) >{ofile} 2>{ofile}.log"
+    subprocess.run(cmd, shell=True, check=True)
     return ofile
 
 
@@ -70,8 +69,8 @@ def merge_one(files, prefix, threads=1, merge_type="fast"):
     with open(fofn, "w") as fh:
         for file in sorted(files):
             print(file, file=fh)
-    merge = "--allow-overlaps --rm-dup all" if merge_type == "slow" else "--naive"
-    cmd = f"(bcftools concat --file-list {fofn} {merge} --threads {threads} -Ob0 --write-index -o {output}) &>{output.log}"
+    merge = "--allow-overlaps --rm-dup all" if merge_type == "slow" else ""
+    cmd = f"(bcftools concat --file-list {fofn} {merge} --threads {threads} -Ob0 --write-index -o {output}) >{output}.log 2>&1"
     try:
         subprocess.run(cmd, shell=True, check=True)
     except subprocess.CalledProcessError:
@@ -88,11 +87,12 @@ def merge_results(args, filestomerge):
                 with open(file, "rb") as fh:
                     shutil.copyfileobj(fh, ofh)
         return args.output
-    if args.threads > 4:
-        print(f"Grouped merge -> {args.threads} groups")
-        groups = [list() for _ in range(args.threads)]
-        for i, file in enumerate(filestomerge):
-            groups[i%args.threads].append(file)
+    filestomerge =list(filestomerge)
+    if len(filestomerge) > 1000:
+        n_groups = int(math.ceil(math.sqrt(len(filestomerge))))
+        print(f"Grouped merge -> {n_groups} groups")
+        groupsize = int(math.ceil(len(filestomerge) / n_groups))
+        groups = [filestomerge[i:min(i+groupsize, len(filestomerge))] for i in range(0, len(filestomerge), groupsize)]
         final_merge = []
         with ProcessPoolExecutor(args.threads) as exc:
             jobs = []
@@ -109,8 +109,9 @@ def merge_results(args, filestomerge):
         for file in sorted(final_merge):
             print(file, file=fh)
     index = "--write-index" if re.match(r"[zb]", args.outformat) else ""
-    merge = "--allow-overlaps --rm-dup all" if args.merge_type == "slow" else "--naive"
-    cmd = f"bcftools concat --file-list {fofn} {merge} --threads {args.threads} -O{args.outformat} -o {args.output} {index}"
+    merge = "--allow-overlaps --rm-dup all" if args.merge_type == "slow" else ""
+    print(f"Finally merging {len(final_merge)} files to {args.output}")
+    cmd = f"bcftools concat --file-list {fofn} {merge} --threads {args.threads} -O{args.outformat} -o {args.output} {index} >{args.output}.log 2>&1"
     subprocess.run(cmd, shell=True, check=True)
     return args.output
 
@@ -126,7 +127,7 @@ def main(argv=None):
             help="Output file format (passed directly to bcftools -O, see man bcftools but z=vcf.gz and b=bcf)")
     ap.add_argument("-T", "--temp-prefix", default=None,
             help="Temporary output directory")
-    ap.add_argument("-S", "--slow-merge", default="fast", type=str, action="store_const", const="slow", dest="merge_type",
+    ap.add_argument("-S", "--slow-merge", default="fast", action="store_const", const="slow", dest="merge_type",
             help="Use slow bcftools merging (with --allow-overlaps and --remove-duplicates)")
     ap.add_argument("-f", "--filter", default="", type=str,
             help="bcftools view arguments for variant filtering")