zfs discuss - Jul 2009 - questions regarding RFE 6334757 and CR 6322205 disk write cache. thanks (case 11356581)

Hello experts,

I would like consult you some questions regarding RFE 6334757
and CR 6322205 (disk write cache).

=========================================RFE 6334757
disk write cache should be enabled and should have a tool to
switch it on and off

CR 6322205
Enable disk write cache if ZFS owns the disk
=========================================
The cu found on SPARC Enterprise T5140, "Disk Write cache" was
disable when it was shifted from Sun factory, but after install ZFS,
"Disk Write cache" turned to be enable.

My questions are

1) When SPARC Enterprise T5140 was shifted from factory,
    is the value of Disk Write cache set to disabled, right ?

    From RFE 6334757, we can see it is disable when shifted
    from factory, but I am not sure..

2) On Solaris10, after installing ZFS and Zone, will
    the value of Disk Write cache be set to enable ?
    After what action is done, the value of Disk Write
    cache will be changed ?

    From CR 6322205, we can see as long as ZFS owns the disk,
    "Disk Write cache" will be set to enable, but what is
    the operation for "ZFS owns the disk" ?

    CR 6322205
    Enable disk write cache if ZFS owns the disk

3) If change the value of Disk Write cache from enabled
    to disable, is there any impact/problem to the system ?
?
4) For FRU parts, Write cache has been set to disabled, right ?

Thank you very much.
Best Regards
chunhuan

1) Turning on write caching is potentially dangerous because the disk will
indicate that data has been written (to cache) before it has actually been
written to non-volatile storage (disk). Since the factory has no way of knowing
how you''ll use your T5140, I''m guessing that they set the disk
write caches off by default.

2) Since ZFS "knows" about disk caches and ensures that it issues
synchronous writes where required, it is safe to turn on write caching when the
*ENTIRE* disk is used for ZFS. Accordingly, ZFS will attempt to turn on a
disk''s write cache whenever you add the *ENTIRE* disk to a zpool. If
you add only a disk slice to a zpool, ZFS will not try to turn on write caching
since it doesn''t know whether other portions of the disk will be used
for applications which are not write-cache safe.

zpool create pool01 c0t0d0        <- ZFS will try to turn on disk write cache
since using entire disk

zpool create pool02 c0t0d0s1    <- ZFS will not try to turn on disk write
cache (only using 1 slice)

To avoid future disk replacement problems (e.g. if the replacement disk is
slightly smaller), we generally create a single disk slice that takes up almost
the entire disk and then build our pools on these slices. ZFS doesn''t
turn on the write cache in this case, but since we know that the disk is only
being used for ZFS we can (and do!) safety turn on the write cache manually.

3) You can change the write (and read) cache settings using the
"cache" submenu of the "format -e" command. If you disable
the write cache where it could safely be enabled you will only reduce the
performance of the system. If you enable the write cache where it should not be
enabled, you run the risk of data loss and/or corruption in the event of a power
loss.

4) I wouldn''t assume any particular setting for FRU parts, although I
believe that Sun parts generally ship with the write caches disabled. Better to
explicitly check using "format -e".
-- 
This message posted from opensolaris.org

Something caused my original message to get cut off. Here is the full post:

1) Turning on write caching is potentially dangerous because the disk will
indicate that data has been written (to cache) before it has actually been
written to non-volatile storage (disk). Since the factory has no way of knowing
how you''ll use your T5140, I''m guessing that they set the disk
write caches off by default.

2) Since ZFS "knows" about disk caches and ensures that it issues
synchronous writes where required, it is safe to turn on write caching when the
*ENTIRE* disk is used for ZFS. Accordingly, ZFS will attempt to turn on a
disk''s write cache whenever you add the
*ENTIRE* disk to a zpool. If you add only a disk slice to a zpool, ZFS will not
try to turn on write caching since it doesn''t know whether other
portions of the disk will be used for applications which are not write-cache
safe.

zpool create pool01 c0t0d0        ZFS will try to turn on disk write cache since
using entire disk

zpool create pool02 c0t0d0s1      ZFS will not try to turn on disk write cache
(only using 1 slice)

To avoid future disk replacement problems (e.g. if the replacement disk is
slightly smaller), we generally create a single disk slice that takes up almost
the entire disk and then build our pools on these slices. ZFS doesn''t
turn on the write cache in this case, but since we know that the disk is only
being used for ZFS we can (and do!) safety turn on the write cache manually.

3) You can change the write (and read) cache settings using the
"cache" submenu of the "format -e" command. If you disable
the write cache where it could safely be enabled you will only reduce the
performance of the system. If you enable the write cache where
it should not be enabled, you run the risk of data loss and/or corruption in the
event of a power loss.

4) I wouldn''t assume any particular setting for FRU parts, although I
believe that Sun parts generally ship with the write caches
disabled. Better to explicitly check using "format -e".
-- 
This message posted from opensolaris.org