On Fri, Jan 15, 2010 at 1:59 PM, Jeffry Molanus
<jeffry.mola...@proact.nl> wrote:
>
>> Sometimes people get confused about the ZIL and separate logs. For
>> sizing purposes,
>> the ZIL is a write-only workload. Data which is written to the ZIL is
>> later asynchronously
>> written to the pool when the txg is committed.
>
> Right; the tgx needs time to transfer the ZIL.
>
>
>> The ZFS write performance for this configuration should consistently
>> be greater than 80 IOPS. We've seen measurements in the 600 write
>> IOPS range. Why? Because ZFS writes tend to be contiguous. Also,
>> with the SATA disk write cache enabled, bursts of writes are handled
>> quite nicely.
>> -- richard
>
> Is there a method to determine this value before pool configuration ? Some
> sort of rule of thumb? It would be sad when you configure the pool and have
> to reconfigure later one because you discover the pool can't handle the tgx
> commits from SSD to disk fast enough. In other words; with Y as expected load
> you would require a minimal of X mirror devs or X raid-z vdevs in order to
> have a pool with enough bandwith/IO to flush the ZIL without stalling the
> system.
>
>
All I can tell you (echoed elsewhere in this thread) that a beautiful
ZIL will have two main characteristics: 1) IOPS - it must be an IOPS
"monster" and 2) low latency. On my workloads, adding a ZIL based on
a nice fast 15k RPM SAS disk to a pool of nice 7k2 SATA drives did'nt
provide the kick-in-the-ascii improvement I was looking for. In fact,
the improvement was almost impossible for a typical user to be aware
of. Why? a) not enough IOPS and b) high latency.
Your starting point, at a bare *minimum*, should be an x25-m SSD drive
[1] and only going *up* from this base point. YMMV of course - this
is based on my personal experience on a relatively small ZFS system.
[1] According to Intel, the x25m should last 5 years if you write 20Gb
to it every day. Of course they don't provide a 5 year warranty -
only a 3 year one. Draw your own conclusions.
--
Al Hopper Logical Approach Inc,Plano,TX a...@logical-approach.com
Voice: 972.379.2133 Timezone: US CDT
OpenSolaris Governing Board (OGB) Member - Apr 2005 to Mar 2007
http://www.opensolaris.org/os/community/ogb/ogb_2005-2007/
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