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Re: ReiserFS vs XFS or JFS?

 Interesting.  I hadn't thought of it that way. 

I understand the concept of journaling a filesystem or database, but I 
haven't looked at the source code to any of the implementations.  This 
conversation has got me wondering - where is the journaled data 
typically stored?  It's clear that the more separate you can keep the 
journal from the thing being journaled, the safer you are in the case of 
catastrophic failure.  Do any of these journaled filesystems or 
journaled databases allow you to write the journal to a separate disk 
from the one that contains the filesystem or database, on the theory 
that both disks aren't likely to die simultaneously? 


[hidden email] wrote: 
>> If the raw file containing your database is represented inside the 
>> structures used by a filesystem and something in the filesystem gets 
>> trashed, having a journaled database isn't going to help at all because 
>> the journaling information will be inaccessible, since it's also stored 
>> in the trashed filesystem. 
> Depending on the database that may be true, but a block level journaling 
> database like Oracle or PostgreSQL that is likely not true. I know 
> PostgreSQL better, so I explain using it as an example. 
> Assuming an active database, one is which there may  be re-usable space in 
> existing blocks and PostgreSQL WAL files are fairly constant in size. 
> Assume that the file system will not be obliterated on a power failure. 
> That it is robust enough (or simple enough) to merely have lost chains. I 
> would use something like DOS FAT as a model. Simple to the point of being 
> stupid. 
> When a database table grows, it grows in fixed sized blocks (like FAT). 
> After each database write, fsync is called. Far more often than not, only 
> the data within a file changes while the disk allocation and file system 
> information remain constant. 
> The meta information and file system never needs to be journaled because 
> the file's "file system" characteristics change relatively infrequently 
> and when they do, fsync will be called immediately. There will never be 
> any real benefit from the file system journal and you'll end up doing the 
> same work twice. 
>> So, if you store your database inside a filesystem, double journaling is 
>> unavoidable if you want your data safe.  The preferable alternative is 
>> to avoid the structures involved in a filesystem, and allocate an entire 
>> partition to your database.  Then all you have to worry about is if the 
>> partition table were to get trashed, so make a backup of block 0 of the 
>> disk. 
>>     Mark R. 
>> [hidden email] wrote: 
>>> IMHO: 
>>> EXT2 is great for a database journal in that you won't be double 
>>> journalling. (I often speculate that a very minimal UNIX file system 
>>> designed for purely for speed and regularly sized blocks, something like 
>>> a 
>>> streamlined FAT system, would be awesome for databases.) 

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