Re: ext3 file system

[Hans Reiser]

jshankar wrote:

> Hello,
>
> Please provide some more insight.
>
> Suppose a filesystem issues a write command to the disk with around 10 4K 
> Blocks  to be written. SCSI device point of view i don't get what is the 
> parallel I/O.
> It has only 1 write command. If some other sends a write request it needs to 
> be queued. But the next question arises how the write data would be handled. 
> Does it mean the SCSI does not give a response for the block of data written. 
> In otherwords does it mean that the response would be given after all the 
> block of data is written for a single write request.
>
> Thanks
> Jay
>
>
>
>
>  
>
> > ===== Original Message From Mike Fedyk <mfedyk@xxxxxxxxxxxxx> =====
> > On Wed, Dec 17, 2003 at 05:25:49PM -0500, Richard B. Johnson wrote:
> >    
> >
> > > to the physical media. There are special file-systems (journaling)
> > > that guarantee that something, enough to recover the data, is
> > > written at periodic intervals.
> > >      
> > Most journaling filesystems make guarantees on the filesystem meta-data, but
> > not on the data.  Some like ext3, and reiserfs (with suse's journaling
> > patch) can journal the data, or order things so that the data is written
> > before any pointers (ie meta-data) make it to the disk so it will be harder
> > to loose data.
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>  
Filesystems don't usually wait on the IO to complete before submitting 
more IO in response to the next write() syscall.  They can do this by 
batching a whole bunch of operations into one committed transaction.

In reiser4 we do this more carefully than other filesystems such as 
reiserfs v3, and as a result every fs operation is fully atomic.

--
Hans


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