Re: understanding postgres issues/bottlenecks - Mailing list pgsql-performance

On Sat, 10 Jan 2009, Luke Lonergan wrote:

> The new MLC based SSDs have better wear leveling tech and don't suffer
> the pauses.  Intel X25-M 80 and 160 GB SSDs are both pause-free.  See
> Anandtech's test results for details.

they don't suffer the pauses, but they still don't have fantasic write
speeds.

David Lang

> Intel's SLC SSDs should also be good enough but they're smaller.
>
> - Luke
>
> ----- Original Message -----
> From: pgsql-performance-owner@postgresql.org <pgsql-performance-owner@postgresql.org>
> To: Gregory Stark <stark@enterprisedb.com>
> Cc: Markus Wanner <markus@bluegap.ch>; Scott Marlowe <scott.marlowe@gmail.com>; Ron <rjpeace@earthlink.net>;
pgsql-performance@postgresql.org<pgsql-performance@postgresql.org> 
> Sent: Sat Jan 10 14:40:51 2009
> Subject: Re: [PERFORM] understanding postgres issues/bottlenecks
>
> On Sat, 10 Jan 2009, Gregory Stark wrote:
>
>> david@lang.hm writes:
>>
>>> On Sat, 10 Jan 2009, Markus Wanner wrote:
>>>
>>>> My understanding of SSDs so far is, that they are not that bad at
>>>> writing *on average*, but to perform wear-leveling, they sometimes have
>>>> to shuffle around multiple blocks at once. So there are pretty awful
>>>> spikes for writing latency (IIRC more than 100ms has been measured on
>>>> cheaper disks).
>>
>> That would be fascinating. And frightening. A lot of people have been
>> recommending these for WAL disks and this would be make them actually *worse*
>> than regular drives.
>>
>>> well, I have one of those cheap disks.
>>>
>>> brand new out of the box, format the 32G drive, then copy large files to it
>>> (~1G per file). this should do almost no wear-leveling, but it's write
>>> performance is still poor and it has occasional 1 second pauses.
>>
>> This isn't similar to the way WAL behaves though. What you're testing is the
>> behaviour when the bandwidth to the SSD is saturated. At that point some point
>> in the stack, whether in the SSD, the USB hardware or driver, or OS buffer
>> cache can start to queue up writes. The stalls you see could be the behaviour
>> when that queue fills up and it needs to push back to higher layers.
>>
>> To simulate WAL you want to transfer smaller volumes of data, well below the
>> bandwidth limit of the drive, fsync the data, then pause a bit repeat. Time
>> each fsync and see whether the time they take is proportional to the amount of
>> data written in the meantime or whether they randomly spike upwards.
>
> if you have a specific benchmark for me to test I would be happy to do
> this.
>
> the test that I did is basicly the best-case for the SSD (more-or-less
> sequential writes where the vendors claim that the drives match or
> slightly outperform the traditional disks). for random writes the vendors
> put SSDs at fewer IOPS than 5400 rpm drives, let along 15K rpm drives.
>
> take a look at this paper
> http://www.imation.com/PageFiles/83/Imation-SSD-Performance-White-Paper.pdf
>
> this is not one of the low-performance drives, they include a sandisk
> drive in the paper that shows significantly less performance (but the same
> basic pattern) than the imation drives.
>
> David Lang
>
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