Edward Ned Harvey wrote:
>> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
>> boun...@opensolaris.org] On Behalf Of Brandon High
>>
>> Write caching will be disabled on devices that use slices. It can be
>> turned back on by using format -e
>
> My experience has been, despite what the BPG (or whatever) says, this is not
> true.
> When I create pools using slices or not using slices, it doesn't seem to
> make any difference to the cache status of the drives. Also, when I go into
> format -e, I attempt to toggle the cache status, and toggling also fails.
>
> Which brings me back to my former argument: Who cares about the drive cache
> anyway. The system ram makes it irrelevant.
ZFS only uses system RAM for read caching, as all writes must be written to
some form of stable storage before acknowledged. If a vdev represents a whole
disk, ZFS will attempt to enable write caching. If a device does not support
write caching, the attempt to set wce fails silently.
As you made reference to above, one would need to use 'format -e' to manually
inquire about this capability on a per disk type basis. If a disk does support
write caching, and ZFS enables it, there should be some measurable write I/O
performance, although how much is unclear.
For those interested, one can trace back the ZFS code starting here:
http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/fs/zfs/vdev_disk.c#276
Jim
>
>
>>> 3. Assuming I want to do such an allocation, is this done with quota &
>> reservation? Or is it snapshots as you suggest?
>>
>> I think Edward misspoke when he said to use snapshots, and probably
>> meant reservation.
>
> I meant if you are able to reduce or eliminate your reservation, that should
> free up enough space to enable you to destroy a snapshot, and re-enable your
> reservation.
>
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