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Re: Re: Faster CREATE DATABASE by delaying fsync

From: Mark Mielke <mark(at)mark(dot)mielke(dot)cc>
To: Florian Weimer <fw(at)deneb(dot)enyo(dot)de>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andres Freund <andres(at)anarazel(dot)de>, Greg Stark <gsstark(at)mit(dot)edu>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Re: Faster CREATE DATABASE by delaying fsync
Date: 2010-02-14 20:41:02
Message-ID: 4B785FDE.8040308@mark.mielke.cc (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On 02/14/2010 03:24 PM, Florian Weimer wrote:
> * Tom Lane:
>    
>>> Which options would that be? I am not aware that there any for any of the
>>> recent linux filesystems.
>>>        
>> Shouldn't journaling of metadata be sufficient?
>>      
> You also need to enforce ordering between the directory update and the
> file update.  The file metadata is flushed with fsync(), but the
> directory isn't.  On some systems, all directory operations are
> synchronous, but not on Linux.
>    

        dirsync
               All directory updates within the filesystem should be 
done  syn-
               chronously.   This  affects  the  following system calls: 
creat,
               link, unlink, symlink, mkdir, rmdir, mknod and rename.

The widely reported problems, though, did not tend to be a problem with 
directory changes written too late - but directory changes being written 
too early. That is, the directory change is written to disk, but the 
file content is not. This is likely because of the "ordered journal" 
mode widely used in ext3/ext4 where metadata changes are journalled, but 
file pages are not journalled. Therefore, it is important for some 
operations, that the file pages are pushed to disk using fsync(file), 
before the metadata changes are journalled.

In theory there is some open hole where directory updates need to be 
synchronized with file updates, as POSIX doesn't enforce this ordering, 
and we can't trust that all file systems implicitly order things 
correctly, but in practice, I don't see this sort of problem happening.

If you are concerned, enable dirsync.

Cheers,
mark

-- 
Mark Mielke<mark(at)mielke(dot)cc>


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