>From: George Herman [mailto:[email protected]]
...
>./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.
>What write caching enabled does, seems to be an area that is
misunderstood.... or at least misunderstood by me. I >understood that if it
was disabled (the default), every IO operation to the disk was followed by a
synchronize cache >operation. With it on, I understood that this no longer
happened on async operations but applications that specified >sync
transactions the synchronize cache operation would still happen as usual.
Turning it on, evidently has much more >impact that this. I'm being told
that it impacts synchronous operations, in that it treats all sync
operations as async. >If this is indeed the case, is there anyway to turn
off synchronize cache for async operations only?
As Richard wrote you are confusing terms here.
zvol_wce disables/enables sync/async behaviour when writing to a zvol (btw:
you can use zfs set sync=disabled now).
SCSI synchronize commands will still be send at the end of each txg commit.
If you want to disable the SCSI_SYNCHRONIZE then either do: echo
"zfs_nocacheflush/W1" | mdb -kw
(it will revert back to the default after reboot - if you want to make it
persistant, then put in /etc/system: set zfs:zfs_nocacheflush = 1, notice it
has a global effect - no scsi_cache_synchronize commands will be send by zfs
to any device, you can set it per device as well via sd.conf if you want).
Btw: you can check if scsi cache synchronize commands are send via dtrace:
dtrace -n fbt::*SYNCHRONIZE_CACHE*:entry'{printf("%Y\n", walltimestamp);}'
>It's the combination of both that is causing the confusing behavior. If
what I've described above is indeed true, I >understand why I'm seeing the
strange behavior. As for measurements, I'm seeing the following:
>
>WCE and IO Throttling on (default): ~ <10 MB/sec
>WCE enabled and IO Throttling on : ~130 MB/sec
>WCE enable and IO Throttling off: ~ 230MB/sec
>
>I did testing of the drive as a raw device, and I get a maximum transfer
rate of 145 MB/sec... the specs say that is it >147 MB/sec.
I'm actually surprised that you only get 230MB/s here - but it depends on
how much free memory you have, what kind of CPU, etc... The 10MB/s is also
fine, as Richard pointed out.
--
Robert Milkowski
http://milek.blogspot.com
-------------------------------------------
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