Problems with swap on ZFS ZVOL?

nsayer · #1 December 6th, 2010, 04:35

I've recently set up a VPS at rootbsd. I set it up with GPT+ZFS almost the same way as my old physical machine (the difference is that I'm not using a mirror for the tank, since the underlying storage has redundancy, and the virtual disk is much smaller than the physical disk I had before): the only things on the disk are the freebsd-boot partition and the tank. Swap is provided on a zvol.

The system only has 768 MB of RAM, but this page has some tuning that suggests that it ought to work.

Under load, the system winds up using a couple dozen MB of swap, but then all of a sudden it locked up with repeated messages:

Code:

swap_pager: indefinite wait buffer: bufobj:

All I get from teh g00g on this is a thread from September of 2007 on freebsd-current.

Is there still problems swapping to a zvol? What can I do to make it more reliable?

graudeejs · #2 December 6th, 2010, 07:39

You forgot to mention FreeBSD version....

I didn't have problems with swapping.... Once I made bug in a program, that filled ram and swapped 500MB in a about minute

until I reset PC manually
Check this, maybe you're missing something related to swap itself:
http://wiki.bsdroot.lv/freebsd:quick...te_swap_on_zfs

Or maybe it's memory amount issue or old FreeBSD release

nsayer · #3 December 6th, 2010, 18:01

Sorry I forgot to mention it - 8.1-RELEASE.

nsayer · #4 December 6th, 2010, 18:07

I checked out the page you referenced. Turning checksums off is a good idea. I'll do that. I also read elsewhere that creating the zvol with a 4k block size was a good idea.

The example also creates a traditional swap partition. I didn't do that, and now it would be a day of work to repartition the drive. But if you're not usig that swap partition, then your results should still be valid.

But this is a very RAM constrained (virtual) machine. I wonder if that's the difference.

danbi · #5 December 7th, 2010, 09:36

Some people say enabling compression on the swap volume helps as well.

nsayer · #6 December 7th, 2010, 21:02

Quote:

Originally Posted by danbi

Some people say enabling compression on the swap volume helps as well.

That's counter-intuitive. Do you have any references that explain why?

graudeejs · #7 December 7th, 2010, 21:07

On one hand you need to read less data... on slow disks and fast cpu, this could be beneficial (perfect for floppy disks

).
On other hand you need to decompress data, which means, more cpu cycles, which means bigger latency.

uchman · #8 June 14th, 2011, 10:50

Was this issue resolved? I'm having the same problem.

nsayer · #9 September 16th, 2012, 20:05

No, this is still going on. I just caught my VPS stuck with this same problem.

The provider gave us all a RAM upgrade for free not too long ago. It's up to 1.3 GB of RAM, but that still isn't a lot for a ZFS equipped server.

I have a single zvol for swap, but things like building a JVM from the ports will easily wind up getting it stuck this way.

gkontos · #10 September 16th, 2012, 23:26

You are using ZFS on root on a very low memory system. (less significant in this case)
You are using ZFS on a virtualized vdev (more significant in this case)

Backup and reinstall using UFS or UFS+J.

If you want to implement a ZFS on root then get a dedicated server.

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