ufs2 / softupdates / ZFS / disk write cache

[Nathanael Hoyle]

Andrew Snow wrote:
> Dan Naumov wrote:
>>>> Or:
>>>> B) use SCSI instead of ATA disks
>>>> C) use UFS+gjournal instead of UFS+SU
>>>> D) use ZFS instead of UFS+SU
>>> All of these solutions still involve disabling of write cache, with 
>>> a performance hit of varying degrees. (I have tried all of those 
>>> except gjournal!)
>
> B) SCSI drives come with write caching disabled by default.  But here, 
> the performance loss is partially made up by Tagged Command Queueing 
> and  faster spindle speeds
>
> C) gjournal needs to flush the disk cache regularly to maintain 
> consistence. It doesn't need to do it as often but on a write-heavy 
> system it isn't ideal for performance because it flushes everything in 
> the cache and not just the journal.
>
> D) ZFS - same as (C)
>

As a minor nitpick to point D, IIRC it is possible to explicitly place 
the ZIL on a different device than the pool it is for.  In this case, if 
the ZIL is on a dedicated device, then it is possible to flush only the 
ZIL, rather than all data pending in cache for the zpool.  I realize 
it's a minor distinction / special case, but the option is worth mentioning.

-Nathanael