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Re: WAL "low watermark" during base backup

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: WAL "low watermark" during base backup
Date: 2011-09-09 10:28:18
Message-ID: CABUevEyR3x5Wo-7sAuX_6KEpeUH5KGn6Q2H5SPrhTtibnx5Y+w@mail.gmail.com (view raw or flat)
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Lists: pgsql-hackers
On Tue, Sep 6, 2011 at 22:35, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Mon, Sep 5, 2011 at 11:38 AM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>> On Sun, Sep 4, 2011 at 19:02, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>> On Fri, Sep 2, 2011 at 6:52 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>>>
>>>> Attached patch implements a "low watermark wal location" in the
>>>> walsender shmem array. Setting this value in a walsender prevents
>>>> transaction log removal prior to this point - similar to how
>>>> wal_keep_segments work, except with an absolute number rather than
>>>> relative. For now, this is set when running a base backup with WAL
>>>> included - to prevent the required WAL to be recycled away while the
>>>> backup is running, without having to guestimate the value for
>>>> wal_keep_segments. (There could be other ways added to set it in the
>>>> future, but that's the only one I've done for now)
>>>>
>>>> It obviously needs some documentation updates as well, but I wanted to
>>>> get some comments on the way it's done before I work on those.
>>>
>>> I'm not yet fully available for a discussion on this, but not sure I like this.
>>>
>>> You don't have to guess the setting of wal_keep_segments, you
>>> calculate it exactly from the size of your WAL disk. No other
>>> calculation is easy or accurate.
>>
>> Uh, no. What about the (very large number of) cases where pg is just
>> sitting on one partition, possibly shared with a whole lot of other
>> services? You'd need to set it to all-of-your-disk, which is something
>> that will change over time.
>>
>> Maybe I wasn't entirely clear in the submission, but if it wasn't
>> obvious: the use-case for this is the small and simple installations
>> that need a simple way of doing a reliable online backup. This is the
>> "pg_basebackup -x" usecase altogether - for example, anybody "bigger"
>> likely has archiv elogging setup already, in which case this
>> functionality is not interesting at all.
>
> I understand the need for a reliable backup, problem is they won't get
> one like this.
>
> If your disk fills, the backup cannot end correctly, so you must
> somehow avoid the disk filling while the backup is taken.

The same thing will happen if your archive_command stops working - the
disk fills up. There are plenty of scenarios whereby the disk can fill
up.

There are a lot of cases where this really isn't a risk, and I believe
this would be a helpful feature in many of those *simple* cases.


> Removing the safety that prevents the disk from filling doesn't
> actually prevent it filling.
>
> If you must have this then make pg_basebackup copy xlog files
> regularly during the backup. That way your backup can take forever and
> your primary disk won't fill up. In many cases it actually will take
> forever, but at least we don't take down the primary.

There is a patch to do something like that as well sitting on the CF
page. I don't believe one necessarily excludes the other.

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/

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