Windows: heavy disk reads while downloading with SetFileValidData() enabled (libtorrent 2.1.1)

[pulbitz]

What happens

On Windows, while downloading a multi-file torrent, my disk gets almost as much read traffic as write traffic, even though I'm only downloading. The reads are on the very files that are being written at that moment.

This happens with libtorrent 2.1.1. With a libtorrent 1.2 based build, the same torrent on the same drive with the same settings produced only about a fifth of the disk reads.

Downloading at about 50 MB/s, Resource Monitor (Disk tab) shows:

Process	Read	Write
qbittorrent.exe	38.1 MB/s	0
System	0	35.8 MB/s

Per file, for the torrent being downloaded:

File	Read	Write	Read / Write
1	31.8 MB/s	24.9 MB/s	1.27
2	6.4 MB/s	5.5 MB/s	1.16
3	0	5.3 MB/s	-

These are real disk reads, not cache hits — the drive sat at 88% active time.
My upload rate was low at the time, so this isn't seeding traffic.

Environment

• Tested with a qBittorrent 5.3.0alpha1 GitHub Actions build using libtorrent 2.1.1
• OS cache enabled for both read and write, disk queue size 100 MiB, pread/pwrite disk IO type
• Windows 11, NTFS with 4 KiB clusters, HDD
• "Pre-allocate disk space for all files" enabled
• Running qBittorrent as administrator, so SetFileValidData() succeeds

Workaround

Turning off "Pre-allocate disk space for all files" so the files are created sparse, and no longer starting qBittorrent as administrator, means SetFileValidData() is never used. With that, the reads almost completely disappear at the same download speed.

With libtorrent 1.2 this was fine as it was.

[ValeZAA]

Running qBittorrent as administrator, so SetFileValidData() succeeds

So this is the key issue

[arvidn]

@pulbitz have you identified what's being read in process explorer?
it does sound counter-intuitive that SetValidData() would cause more reads.

[pulbitz]

That's what the per-file table in the report already is. I should have been clearer about where it came from: it's Resource Monitor (resmon.exe), Disk tab, "Disk Activity" section, which lists I/O per file per process. "File 1/2/3" are the files of the torrent that was downloading at that moment.

So what's being read is the torrent's own files, the same ones being written at the same time, and the reads are charged to qbittorrent.exe while the writes on those same files are charged to System. Nothing else on the volume was reading them.