[pve-devel] [PATCH common] tools: file_set_contents: use syswrite instead of print

[Filip Schauer]

The use of `print` can be inefficient for writing larger files due to
its default buffering in 8 KiB blocks.

This is especially problematic on `pmxcfs` where files are written in
4 KiB blocks due to the defaults of `libfuse2`. This leads to
significant write amplification on files larger than 4 KiB.

Patch (fix #5728: pmxcfs: allow bigger writes than 4k for fuse) [1]
addresses this by enabling `big_writes`, allowing up to 128 KiB blocks.
But due to the use of `print` in `file_set_contents`, writes are still
only buffered in 8 KiB blocks.

To further address this, this commit switches to using `syswrite`
instead of `print` to mitigate the block size limit imposed by `print`.
Combined with patch [1], file writes to `/etc/pve/` are now buffered in
128 KiB blocks.

The table below illustrates the drastic reduction in write
amplification when writing files of different sizes to `/etc/pve/` using
`file_set_contents`:

           print                big_writes+print     big_writes+syswrite
data size  written     amplif.  written     amplif.  written    amplif.
    1 KiB      48 KiB     48.0      45 KiB     45.0     41 KiB     41.0
    2 KiB      48 KiB     24.0      45 KiB     22.5     62 KiB     31.0
    4 KiB      82 KiB     20.5      80 KiB     20.0     73 KiB     18.3
    8 KiB     121 KiB     15.1      90 KiB     11.3     89 KiB     11.1
   16 KiB     217 KiB     13.6     146 KiB      9.1    113 KiB      7.1
   32 KiB     506 KiB     15.8     314 KiB      9.8    158 KiB      4.9
   64 KiB    1472 KiB     23.0     826 KiB     12.9    259 KiB      4.0
  128 KiB    5585 KiB     43.6    3765 KiB     29.4    452 KiB      3.5
  256 KiB   20424 KiB     79.8   10743 KiB     42.0   2351 KiB      9.2
  512 KiB   86715 KiB    169.4   43650 KiB     85.3   3204 KiB      6.3
 1024 KiB  369568 KiB    360.9  187496 KiB    183.1  15845 KiB     15.5

[1] https://lists.proxmox.com/pipermail/pve-devel/2024-September/065396.html

Signed-off-by: Filip Schauer <f.schauer at proxmox.com>
---
 src/PVE/Tools.pm | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/PVE/Tools.pm b/src/PVE/Tools.pm
index bd305bd..0edf166 100644
--- a/src/PVE/Tools.pm
+++ b/src/PVE/Tools.pm
@@ -285,10 +285,17 @@ sub file_set_contents {
 	}
 	die "unable to open file '$tmpname' - $!\n" if !$fh;
 
-	binmode($fh, ":encoding(UTF-8)") if $force_utf8;
+	$data = encode("utf8", $data) if $force_utf8;
 
-	die "unable to write '$tmpname' - $!\n" unless print $fh $data;
-	die "closing file '$tmpname' failed - $!\n" unless close $fh;
+	my $offset = 0;
+	my $len = length($data);
+
+	while ($offset < $len) {
+	    $offset += syswrite($fh, $data, $len - $offset, $offset)
+		or die "unable to write '$tmpname' - $!\n";
+	}
+
+	close $fh or die "closing file '$tmpname' failed - $!\n";
     };
     my $err = $@;
 
-- 
2.39.2