Date: Sun, 26 Aug 2007 21:09:57 -0400
From: Jean-David Beyer <jeandavid8@verizon.net>
Subject: Re: Block size with pg_dump?
In-reply-to: <200708270011.l7R0Bw114136@momjian.us>
To: Bruce Momjian <bruce@momjian.us>
Cc: pgsql-sql@postgresql.org
Message-id: <46D22465.2060306@verizon.net>
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Bruce Momjian wrote:
> Jean-David Beyer wrote:
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>> When I make a backup of a database, I put the output file directly on
>> magnetic tape; i.e., my command looks like this:
>>
>> pg_dump --file=/dev/st0 ....
>>
>> This way I do not have to worry if the total backup exceeds the size of a
>> file system, and it saves me the trouble of copying it to the tape as a
>> separate step. My current tapes will hold 20 GBytes raw or 40GBytes if I
>> enable hardware compression (assuming 2:1 compression happens). Now it says
>> in the documentation that if I use format c it will compress the data in
>> software, so I doubt the hardware compression will do much.
>>
>> I do not know what blocksize pg_dump uses, or if it insists on a particular
>> blocksize on input.
>>
>> Now my tape drive will work with any blocksize, but prefers 65536-byte
>> blocks. I do not see any options for this in pg_dump, but I could pipe the
>> output of pg_dump through dd I suppose to make any blocksize I want.
>>
>> On the way back, likewise I could pipe the tape through dd before giving it
>> to pg_restore.
>>
>> Does pg_dump care what blocksize it gets? If so, what is it?
> 
> I assume you could pipe pg_dump into dd and specify the block size in
> dd.
> 
Of course on the way out I can do that.

The main question is, If I present pg_restore with a 65536-byte blocksize
and it is expecting, e.g., 1024-bytes, will the rest of each block get
skipped? I.e., do I have to use dd on the way back too? And if so, what
should the blocksize be?

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