On Sun, May 15, 2022 at 04:45:11PM +0100, Nikolaus Rath wrote:
Hi,
I am observing some strange errors when using the Kernel's NBD driver with NBDkit.
On the kernel side, I see:
May 15 16:16:11 vostro.rath.org kernel: nbd0: detected capacity change from 0 to 104857600 May 15 16:16:11 vostro.rath.org kernel: nbd1: detected capacity change from 0 to 104857600 May 15 16:18:23 vostro.rath.org kernel: block nbd0: Possible stuck request 00000000ae5feee7: control (write@4836316160,32768B). Runtime 30 seconds May 15 16:18:25 vostro.rath.org kernel: block nbd0: Possible stuck request 000000007094eddc: control (write@5372947456,10240B). Runtime 30 seconds May 15 16:18:27 vostro.rath.org kernel: block nbd0: Suspicious reply 89 (status 0 flags 0) May 15 16:18:31 vostro.rath.org kernel: block nbd0: Possible stuck request 0000000075f8b9bc: control (write@8057764864,32768B). Runtime 30 seconds May 15 16:18:41 vostro.rath.org kernel: block nbd0: Possible stuck request 000000002d1b3e8b: control (write@14499979264,32768B). Runtime 30 seconds [...]
Does it really take over 30 seconds for nbdkit to respond? You might want to insert some debugging into the S3 plugin to see what stage of the request cycle is taking so long, although I'm going to guess it's the remote Amazon server itself. It seems like you can adjust this timeout using the nbd-client -t flag (it calls ioctl(NBD_SET_TIMEOUT) in the kernel). If I understand the logic correctly, the nbd timeout is currently set to 0, which causes the default socket timeout to be used. Using the -t flag overrides this. So I guess try setting it larger and see if the problem goes away. Rich.
And userspace ('zfs snapshot" in this instance) is stuck afterwards.
On the NBDkit side, there seemingly are write errors when replying back to the kernel:
$ nbdkit --unix /tmp/tmpi5o59_y_/nbd_socket_sb --foreground --filter=exitlast --filter=stats --threads 16 S3 size=50G bucket=nikratio-backup key=sb statsfile =/tmp/tmpi5o59_y_/stats_sb.txt object-size=32K & $ nbd-client -unix /tmp/tmpi5o59_y_/nbd_socket_sb /dev/nbd2 Warning: the oldstyle protocol is no longer supported. This method now uses the newstyle protocol with a default export Negotiation: ..size = 51200MB Connected /dev/nbd0 [....] nbdkit: python.10: error: write reply: NBD_CMD_WRITE: Broken pipe
What's the best way to narrow down who's the culprit here (kernel vs NBD server)?
Best, -Nikolaus
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