Re: [WIP] Double-write with Fast Checksums

From: Dan Scales <scales(at)vmware(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: PG Hackers <pgsql-hackers(at)postgresql(dot)org>, jkshah(at)gmail(dot)com, David Fetter <david(at)fetter(dot)org>
Subject: Re: [WIP] Double-write with Fast Checksums
Date: 2012-01-17 20:25:41
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We have some numbers for 9.2 runs with and without double writes now. We
are still using the double-write patch that assumes checksums on data
pages, so checksums must be turned on for double writes.

The first set of runs are 50-warehouse 2-processor DBT2 60-minute run,
with checkpoints every 5 minutes. Machine memory is 8G, cache size is
5G. Database size is about 9G. The disks are enterprise Fibre Channel
disks, so there is good disk write-caching at the array. All runs are
for virtual machines. (We expect that the virtual machine numbers would
be representative of performance for non-virtual machines, but we know
that we need to get non-virtual numbers as well.)

orig 9.2| 9.2 + DW patch
FPW off FPW off FPW off FPW on DW on/FPW off
CK off CK on CK on CK on
one disk: 15574 15308 15135 13337 13052 [5G shared_buffer, 8G RAM]
sep log disk: 18739 18134 18063 15823 16033

(First row is everything on one disk, second row is where the WAL log is
on a separate disk.)

So, in this case where cache is large and disks probably have
write-caching, we get about same performance with full_page_write on and
double-writes on. We need to run these numbers more to get a good
average -- in some runs last night, double writes did better, closer to
what we were seeing with 9.0 (score of 17721 instead of 16033).

Note that, for one disk, there is no significant different between the
original 9.2 code and the patched code with checksums (and double-writes)
turned off. For two disks, there is a bigger difference (3.3%), but I'm
not sure that is really significant.

The second set of numbers is for a hard disk with write cache turned off,
closer to internal hard disks of servers (people were quite interested in
that result). These runs are for 50-warehouse 8-processor DBT2 60-minute
run, with checkpoints every 5 minutes. The RAM size is 8G, and the cache
size is 6G.

9.2 + DW patch
FPW off FPW on DW on/FPW off
CK on CK on CK on
one disk: 12084 7849 9766 [6G shared_buffers, 8G RAM]

So, here we see a performance advantage for double writes where the cache
is large and the disks do not have write-caching. Presumably, the cost
of fsyncing the big writes (with full pages) to the WAL log on a slow
disk are traded against the fsyncs of the double writes.

Third set of numbers is back to the first hardware setup, but with much
smaller shared_buffers. Again, the runs are 50-warehouse 2-processor DBT2
60-minute run, with checkpoints every 5 minutes. But shared_buffers is
set to 1G, so there will be a great many more dirty evictions by the

9.2 + DW patch
FPW off FPW on DW on/FPW off
CK on CK on CK on
one disk: 11078 10394 3296 [1G shared_buffers, 8G RAM]
sep log disk: 13605 12015 3412

one disk: 7731 6613 2670 [1G shared_buffers, 2G RAM]
sep log disk: 6752 6129 2722

Here we see that double writes does very badly, because of all the double
writes being done for individual blocks by the backends. With the small
shared cache, the backends are now writing 3 times as many blocks as the

Clearly, the double write option would have to be completely optional,
available for use for database configurations which have a well-sized

It would still be preferable that performance didn't have such a cliff
when dirty evictions become high, so, with that in mind, I am doing some
prototyping of the double-write buffer idea that folks have proposed on
this thread.

Happy to hear all comments/suggestions. Thanks,


----- Original Message -----
From: "Dan Scales" <scales(at)vmware(dot)com>
To: "Heikki Linnakangas" <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: "PG Hackers" <pgsql-hackers(at)postgresql(dot)org>, jkshah(at)gmail(dot)com, "David Fetter" <david(at)fetter(dot)org>
Sent: Wednesday, January 11, 2012 1:25:21 PM
Subject: Re: [HACKERS] [WIP] Double-write with Fast Checksums

Thanks for all the comments and suggestions on the double-write patch. We are working on generating performance results for the 9.2 patch, but there is enough difference between 9.0 and 9.2 that it will take some time.

One thing in 9.2 that may be causing problems with the current patch is the fact that the checkpointer and bgwriter are separated and can run at the same time (I think), and therefore will contend on the double-write file. Is there any thought that the bgwriter might be paused while the checkpointer is doing a checkpoint, since the checkpointer is doing some of the cleaning that the bgwriter wants to do anyways?

The current patch (as mentioned) also may not do well if there are a lot of dirty-page evictions by backends, because of the extra fsyncing just to write individual buffers. I think Heikki's (and Simon's) idea of a growing shared double-write buffer (only doing double-writes when it gets to a certain size) instead is a great idea that could deal with the dirty-page eviction issue with less performance hit. It could also deal with the checkpointer/bgwriter contention, if we can't avoid that. I will think about that approach and any issues that might arise. But for now, we will work on getting performance numbers for the current patch.

With respect to all the extra fsyncs, I agree they are expensive if done on individual buffers by backends. For the checkpointer, there will be extra fsyncs, but the batching helps greatly, and the fsyncs per batch are traded off against the often large & unpredictable fsyncs at the end of checkpoints. In our performance runs on 9.0, the configuration was such that there were not a lot of dirty evictions, and the checkpointer/bgwriter was able to finish the checkpoint on time, even with the double writes.

And just wanted to reiterate one other benefit of double writes -- it greatly reduces the size of the WAL logs.



----- Original Message -----
From: "Heikki Linnakangas" <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: "David Fetter" <david(at)fetter(dot)org>
Cc: "PG Hackers" <pgsql-hackers(at)postgresql(dot)org>, jkshah(at)gmail(dot)com
Sent: Wednesday, January 11, 2012 4:13:01 AM
Subject: Re: [HACKERS] [WIP] Double-write with Fast Checksums

On 10.01.2012 23:43, David Fetter wrote:
> Please find attached a new revision of the double-write patch. While
> this one still uses the checksums from VMware, it's been
> forward-ported to 9.2.
> I'd like to hold off on merging Simon's checksum patch into this one
> for now because there may be some independent issues.

Could you write this patch so that it doesn't depend on any of the
checksum patches, please? That would make the patch smaller and easier
to review, and it would allow benchmarking the performance impact of
double-writes vs full page writes independent of checksums.

At the moment, double-writes are done in one batch, fsyncing the
double-write area first and the data files immediately after that.
That's probably beneficial if you have a BBU, and/or a fairly large
shared_buffers setting, so that pages don't get swapped between OS and
PostgreSQL cache too much. But when those assumptions don't hold, it
would be interesting to treat the double-write buffers more like a 2nd
WAL for full-page images. Whenever a dirty page is evicted from
shared_buffers, write it to the double-write area, but don't fsync it or
write it back to the data file yet. Instead, let it sit in the
double-write area, and grow the double-write file(s) as necessary, until
the next checkpoint comes along.

In general, I must say that I'm pretty horrified by all these extra
fsync's this introduces. You really need a BBU to absorb them, and even
then, you're fsyncing data files to disk much more frequently than you
otherwise would.

Jignesh mentioned having run some performance tests with this. I would
like to see those results, and some analysis and benchmarks of how
settings like shared_buffers and the presence of BBU affect this,
compared to full_page_writes=on and off.

Heikki Linnakangas

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