Thread

1. Re: understanding postgres issues/bottlenecks

   Luke Lonergan <llonergan@greenplum.com> — 2009-01-10T22:54:35Z

   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.
   
   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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2. Re: understanding postgres issues/bottlenecks

   Mark Kirkwood <markir@paradise.net.nz> — 2009-01-10T23:44:16Z

   david@lang.hm wrote:
   > 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.
   >>
   
    From what I can see, SLC SSDs are still quite superior for reliability 
   and (write) performance. However they are too small and too expensive 
   right now. Hopefully the various manufacturers are working on improving 
   the size/price issue for SLC, as well as improving the 
   performance/reliability area for the MLC products.
   
   regards
   
   Mark
   
   

3. Re: understanding postgres issues/bottlenecks

   david@lang.hm — 2009-01-11T00:03:32Z

   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
   >
   > --
   > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
   > To make changes to your subscription:
   > http://www.postgresql.org/mailpref/pgsql-performance
   >
   
   

4. Re: understanding postgres issues/bottlenecks

   david@lang.hm — 2009-01-11T01:54:11Z

   On Sun, 11 Jan 2009, Mark Kirkwood wrote:
   
   > david@lang.hm wrote:
   >> 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.
   >>> 
   >
   > From what I can see, SLC SSDs are still quite superior for reliability and 
   > (write) performance. However they are too small and too expensive right now. 
   > Hopefully the various manufacturers are working on improving the size/price 
   > issue for SLC, as well as improving the performance/reliability area for the 
   > MLC products.
   
   the very nature of the technology means that SLC will never be as cheap as 
   MLC and MLC will never be as reliable as SLC
   
   take a look at 
   http://www.imation.com/PageFiles/83/SSD-Reliability-Lifetime-White-Paper.pdf 
   for a good writeup of the technology.
   
   for both technologies, the price will continue to drop, and the 
   reliability and performance will continue to climb, but I don't see 
   anything that would improve one without the other (well, I could see MLC 
   gaining a 50% capacity boost if they can get to 3 bits per cell vs the 
   current 2, but that would come at the cost of reliability again)
   
   for write performance I don't think there is as much of a difference 
   between the two technologies. today there is a huge difference in most of 
   the shipping products, but Intel has now demonstrated that it's mostly due 
   to the controller chip, so I expect much of that difference to vanish in 
   the next year or so (as new generations of controller chips ship)
   
   David Lang