zfs discuss - Nov 2008 - ZIL performance on "traditional" HDDs

Hi,

I was wondering is there is a performance gain for an OLTP-like workload 
in putting the ZFS Intent Log (ZIL) on "traditional" HDDs.

Moreover, I''d like to have a "rough" idea of relative
performances for
an OLTP workload with the same storage (e.g. an external storage with 12 
x 300GB 15Krpm HDDs) with this three configurations:

1) UFS on SVM or VxVM (HW RAID)
2) ZFS with a 12 HDDs RAIDZ1(10 + 1 + 1 spare?)
3) ZFS with 2 HDDs for ZIL and 10 HDDs RAIDZ1 (8 + 1 + 1 spare?)

Thanks in advance for any advice...

Rgrds,
Danilo.

On 11/20/08 12:52, Danilo Poccia wrote:
> Hi,
> 
> I was wondering is there is a performance gain for an OLTP-like workload 
> in putting the ZFS Intent Log (ZIL) on "traditional" HDDs.
> 
It''s probably always best to benchmark it yourself, but my
experience has shown that it''s better to only have a separate log
when the log devices are faster. Without a separate log (slog) the log
is allocated dynamically from the pool and at a location where
current allocations are happening for transaction groups.
So there is little head movement needed to write the log.
There may be a problem when the pool is very full and fragmented
as log block allocation will be all over the place and seek latency will
be high. However, this is a problem for the whole pool.
Also, the log is spread across the pool devices so the more devices
in the pool the faster the intent log can be written when the load is heavy.

Hope that helps: Neil.