[Libguestfs] Communication issues between NBD driver and NBDKit server

[Richard W.M. Jones]

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 at 4836316160,32768B). Runtime 30 seconds
> May 15 16:18:25 vostro.rath.org kernel: block nbd0: Possible stuck request
> 000000007094eddc: control (write at 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 at 8057764864,32768B). Runtime 30 seconds
> May 15 16:18:41 vostro.rath.org kernel: block nbd0: Possible stuck request
> 000000002d1b3e8b: control (write at 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
> 
> --
> GPG Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F
> 
>              »Time flies like an arrow, fruit flies like a Banana.«
> 
> 

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-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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