Asynchronous writing to zvols (ZFS)

[Pawel Jakub Dawidek]

On Sat, Jul 26, 2008 at 08:05:46PM +0200, Peter Schuller wrote:
> Hello,
> 
> I was finally playing around with iSCSI, having never used it before. For 
> convenience, and also because it may be a future use case for real use, I 
> used zvols for my targets.
> 
> I could not get write speed above roughly 1 MB/second even in simple cases 
> like dd:ing with an 8 MB block size, with the zvol:s on a 6-disk raidz2. The 
> individual disk utilization of constituent drives remains small (< 3%). 
> Switching to a memory disk target yielded expected performance 
> characteristics.
> 
> I notice that there were confirmed issues with writes to zvol:s:
> 
>    http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6496356
>    http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6496344
> 
> The problem is that I'm not really sure how to translate "snv_59" into 
> something that I can compare with the version of ZFS in FreeBSD. Do the 
> above "bugs" still apply to the ZFS version in FreeBSD, or am I hitting 
> something else?

The problem is that we don't between async and sync I/O request on GEOM
level, that's why I decided to commit a ZIL log after each write, which
wasn't very smart it seems. This is handled differently in version I've
in perforce. Could you try the below patch and see how it performs now?

	http://people.freebsd.org/~pjd/patches/zvol.c.patch

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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