Re: [PATCH] 2.4.x write barriers (updated for ext3)

[Jeremy Higdon]

On Mar 4,  8:57am, James Bottomley wrote:
> 
> > 2a) Are the filesystems asking for something impossible?  Can drives
> > really write block N and N+1, making sure to commit N to media before
> > N+1 (including an abort on N+1 if N fails), but still keeping up a
> > nice seek free stream of writes? 
> 
> These are the "big" issues.  There's not much point doing all the work to 
> implement ordered tags, if the end result is going to be no gain in 
> performance.

If a drive does reduced latency writes, then blocks can be written out
of order.  Also, for a trivial case:  with hardware RAIDs, when the
data for a single command is split across multiple drives, you can get
data blocks written out of order, no matter what you do.

I don't think a filesystem can make any assumptions about blocks within
a single command, though with ordered tags (assuming driver and device
support) and no write caching, it can make assumptions between commands.

jeremy