Re: hardare config question - Mailing list pgsql-performance

UPS does not protect against the tech behind the rack unplugging the
power cable, or an accidental power cycle from exercising the wrong
switch. :)  Both are probably more common causes of failure than a total
power outage.

Erik Myllymaki wrote:
> I have been in discussion with 3ware support and after adjusting some
> settings, the 3ware card in RAID 1 gets better performance than the
> single drive. I guess this had everything to do with the write (and
> maybe read?) cache.
>
> Of course now i am in a dangerous situation - using volatile write
> cache without a BBU.
>
> If I were to use a UPS to ensure a soft shutdown in the event of power
> loss, am I somewhat as safe as if I were to purchase a BBU for this
> RAID card?
>
>
>
> Thanks.
>
> Mark Lewis wrote:
>> It's also possible that the single SATA drive you were testing (or the
>> controller it was attached to) is lying about fsync and performing write
>> caching behind your back, whereas your new controller and drives are
>> not.
>>
>> You'll find a lot more info on the archives of this list about it, but
>> basically if your application is committing a whole lot of small
>> transactions, then it will run fast (but not safely) on a drive which
>> lies about fsync, but slower on a better disk subsystem which doesn't
>> lie about fsync.
>>
>> Try running a test with fsync=off with your new equipment and if it
>> suddenly starts running faster, then you know that's the problem.
>> You'll either have a choice of losing all of your data the next time the
>> system shuts down uncleanly but being fast, or of running slow, or of
>> fixing the applications to use chunkier transactions.
>>
>> -- Mark
>>
>> On Fri, 2006-04-28 at 13:36 -0400, Vivek Khera wrote:
>>> On Apr 28, 2006, at 11:37 AM, Erik Myllymaki wrote:
>>>
>>>> When I had this installed on a single SATA drive running from the
>>>> PE1800's on-board SATA interface, this operation took anywhere
>>>> from  65-80 seconds.
>>>>
>>>> With my new RAID card and drives, this operation took 272 seconds!?
>>> switch it to RAID10 and re-try your experiment.  if that is fast,
>>> then you know your raid controller does bad RAID5.
>>>
>>> anyhow, I have in one server (our office mail server and part-time
>>> development testing box) an adaptec SATA RAID from dell.  it is
>>> configured for RAID5 and does well for normal office stuff, but
>>> when  we do postgres tests on it, it just is plain old awful.
>>>
>>> but I have some LSI based cards on which RAID5 is plenty fast and
>>> suitable for the DB, but those are SCSI.
>>>
>>> For what it is worth, the Dell PE1850 internal PERC4/Si card is
>>> wicked fast when hooked up with a pair of U320 SCSI drives.
>>>
>>>
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