From: | david(at)lang(dot)hm |
---|---|
To: | Decibel! <decibel(at)decibel(dot)org> |
Cc: | Henrik <henke(at)mac(dot)se>, Jeff <threshar(at)threshar(dot)is-a-geek(dot)com>, Luke Lonergan <LLonergan(at)greenplum(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Filesystem benchmarking for pg 8.3.3 server |
Date: | 2008-08-17 00:15:41 |
Message-ID: | alpine.DEB.1.10.0808161714280.12859@asgard.lang.hm |
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Thread: | |
Lists: | pgsql-performance |
On Sat, 16 Aug 2008, Decibel! wrote:
> On Aug 13, 2008, at 2:54 PM, Henrik wrote:
>>> Additionally, you need to be careful of what size writes you're using. If
>>> you're doing random writes that perfectly align with the raid stripe size,
>>> you'll see virtually no RAID5 overhead, and you'll get the performance of
>>> N-1 drives, as opposed to RAID10 giving you N/2.
>> But it still needs to do 2 reads and 2 writes for every write, correct?
>
>
> If you are completely over-writing an entire stripe, there's no reason to
> read the existing data; you would just calculate the parity information from
> the new data. Any good controller should take that approach.
in theory yes, in practice the OS writes usually aren't that large and
aligned, and as a result most raid controllers (and software) don't have
the special-case code to deal with it.
there's discussion of these issues, but not much more then that.
David Lang
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