From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, apoc9009(at)yahoo(dot)de, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: [FEATURE REQUEST] Streaming Onlinebackup (Maybe OFFTOPIC) |
Date: | 2007-09-07 00:03:42 |
Message-ID: | 1189123422.9243.29.camel@dogma.ljc.laika.com |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
On Thu, 2007-09-06 at 19:23 -0400, Tom Lane wrote:
> Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> > On Thu, 2007-09-06 at 12:08 +0100, Heikki Linnakangas wrote:
> >> With file-based log shipping, you can get down to 1 second, by using the
> >> archive_timeout setting. It will produce a lot of log files with very
> >> little content in them, but they will compress well.
>
> > I tried doing a couple pg_switch_xlog() in quick succession, and the WAL
> > files that represent less than a second of actual data don't seem much
> > more compressible than the rest.
>
> That's because we do not try to zero out a WAL file when recycling it,
> so the part after what's been used is not any more compressible than the
> valid part.
>
That was my point.
> pg_switch_xlog is not, and was never intended to be, a solution for a
> low-latency log-shipping scheme. The right solution for that is to make
> a smarter log-shipping daemon that transmits only part of a WAL segment
> at a time. (As Hannu is getting tired of repeating, you can find a
> working example in SkyTools.)
I think that in addition to WalMgr (which is the tool I assume you're
referring to), there's still room for a simple tool that can zero out
the meaningless data in a partially-used WAL segment before compression.
It seems reasonable to me, so long as you keep archive_timeout at
something reasonably high.
If nothing else, people that already have a collection of archived WAL
segments would then be able to compact them.
I agree that something like WalMgr is better for low-latency, however.
Regards,
Jeff Davis
From | Date | Subject | |
---|---|---|---|
Next Message | Kevin Grittner | 2007-09-07 00:20:13 | Re: Just-in-time Background Writer Patch+Test Results |
Previous Message | Charlie Savage | 2007-09-07 00:01:24 | Re: Trouble with the PL/pgSQL debugger and VC++ |