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Disk overload 100% - !ut stat() theory


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While investigating the Disk Overload phenomenon that's plagued some people lately, I found a few interesting things.

This seems to mainly effect NAS users, that is, ones whose primary download drive is a Samba share on another computer on your LAN.

As reported elsewhere, turning on preallocation and/or diskio.sparse_files seems to help.

However, even after doing so, I had very high smbd CPU usage, so I ran "sudo strace -p <pid_of_smbd>" and found an extreme number of lstat() calls regarding non-existing !ut files of torrents that are already completed.

I have over a hundred seeding torrents, some of them with several thousand files. Removing some of the biggest ones reduced smbd CPU from 50% to 5%.

It seems µTorrent doesn't remember the state of the files under its control, and re-checks them a lot. While this info of local drives is cached by the OS, making local stat() very fast, it is not so cached on network drives.

This theory explains why Disk Overload can be at 100% while no visible disk reads or writes are taking place.

I hope this info is accurate and can help the developers find a solution to this problem soon.

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Meneth's theory appears to be consistent with the problem I am seeing.

I have 3500 torrents, of which about 270 reside on a WD ShareSpace NAS server. This box has relatively slow response times.

I had not seen the disk overload problem recently, but it started again after the latest version was installed yesterday.

After re-booting Windows 7 and re-starting uTorrent:

- Disk overloaded 100% appeared shortly after the program completed starting the torrents and a couple of minutes after a download started. That download was writing to a local disk, not the NAS server.

- Resource monitor shows no disk activity by uTorrent.

- Resource monitor shows network activity by uTorrent

- uTorrent Speed/Disk Statistics show no activity

- The program window is "frozen" (not responding) but the Disk Statistics are refreshing

- Download is 2.3 kB/s

- Upload is 2.7 kB/s

- Disk Statistics are not changing, except the number of Corked jobs is increasing every second and is currently at 40759.

I hope this helps and that the problem is resolved soon as this behaviour makes the latest version unusable for me.

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