Hi George,
On Dec 19, 2012, at 6:52 PM, George Herman <[email protected]> wrote:
> I'm observing some strange behavior that I hope someone might be able to
> explain...
>
> I run the following script:
>
> zpool create zpool c0t5000C50041ABABABd0
> zfs create -V 200G -b 4k zpool/zvol
> zfs set logbias=throughput zpool/zvol
This has no effect unless you have a slog.
> zfs set primarycache=none zpool/zvol
> zfs set secondarycache=none zpool/zvol
primarycache and secondarycache affect reads, not writes.
> ./zvol_wce /dev/zvol/rdsk/zpool/zvol 1
This enables write caching, so we then expect to not use the ZIL unless
the application issues a SYNCHRONIZE_CACHE-like command.
> vdbench -f Zvol_var
>
> I'm using the zvol_wce program that is described on Milak's blog to enable
> write caching. (I understand that it disables synchronize cache operation on
> each write)
> I turn off IO throttling (via 'echo zfs_no_write_throttle/w1 |mdb -kw').
The write throttle is a little tricky to manage. For a small RAM machine, it
will
behave very differently than a large RAM machine. Similarly, for slow disks,
it will behave differently. Have you tried to measure it's use prior to
disabling?
I left some scripts to do that at Nexenta, but for the benefits of the
community, try
dtrace -n 'fbt:zfs:txg_delay:entry {printf("%Y\n", walltimestamp)}'
If that doesn't tick over, then you aren't seeing any write throttle requests.
NB, at the last illumos hackathon, Basil was working on turning this into a
kstat
counter :-)
> I run write sequential operations using xfersize=4k and specify o_dsync and
> o_rsync in vdbench.
>
> I'm getting 200+MB/sec reported as the throughput (via vdbench) when the
> drive can only do 147 MB/sec.
Sure, you're caching.
> Another observation... when I control c the program, it takes 10+ secs to
> terminate, and there continues to be IO to the drive during the terminaton.
Sure, you're caching.
> (Seen via iostat) When either io throttling is disabled or I don't turn on
> WCE, vdbench reports <10 MB/sec and the problem will terminate normally.
That is 2,500 IOPS @ 4k, not bad for a disk that can do maybe 80.
-- richard
--
[email protected]
+1-760-896-4422
-------------------------------------------
illumos-discuss
Archives: https://www.listbox.com/member/archive/182180/=now
RSS Feed: https://www.listbox.com/member/archive/rss/182180/21175430-2e6923be
Modify Your Subscription:
https://www.listbox.com/member/?member_id=21175430&id_secret=21175430-6a77cda4
Powered by Listbox: http://www.listbox.com
