[zfs-discuss] Performance and stability difference between writing to a zvol and a zfs file

Ryan How ryan at zbit.net.au
Tue May 7 01:26:29 EDT 2013

I tried to make it disappear again. It didn't disappear, but after 
writing it went super speed.

dd if=/dev/zero of=/dev/zvol/pool/testzvol bs=4k
^C1241918+0 records in
1241918+0 records out
5086896128 bytes (5.1 GB) copied, 2.2908 s, 2.2 GB/s

I guess something is being smart and it knows I just wrote zeros to it? 
I don't have compression or dedup enabled.

Tried writing from /dev/urandom . No activity in zpool iostat. So not 
sure where the write is going. Limited speed, but that would be urandom. 
Then tried writing again with /dev/zero and it was still super speed 
(Thought maybe it was skipping writes because the data was already there)

Seems I have an issue :/

I'll try testing on the ubuntu build you suggested. Might be some kernel 
version or config or something?

You don't get the same buffering / hanging effect I am getting?

Thanks, Ryan

On 7/05/2013 12:07 PM, Fajar A. Nugraha wrote:
> On Tue, May 7, 2013 at 10:51 AM, Ryan How <ryan at zbit.net.au 
> <mailto:ryan at zbit.net.au>> wrote:
>     Still need to be careful, the zvols aren't accessible for me until
>     I reboot. But they still show up in zfs list.
> That's another weird thing. I don't get that on my system. Can you try:
> - udevadm trigger /devices/virtual/block/zd0 (or whatever your 
> previous block device is. zvols start at zd0)
> - udevadm test /devices/virtual/block/zd0 -> look for something that 
> says "preserve already existing symlink" and "creating link"
> - rename the zvol to something else, and then back
> If the last step still doesn't work, you should try Ubuntu raring with 
> Darik's ppa. Not sure which difference cause it, but zvols works fine 
> on my system.
> -- 
> Fajar

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