disk devices speed is ugly

Gary Palmer gpalmer at freebsd.org
Mon Feb 13 15:37:05 UTC 2012

On Mon, Feb 13, 2012 at 03:50:36PM +0100, Alex Samorukov wrote:
> On 02/13/2012 02:28 PM, Gary Palmer wrote:
> >>>
> >>>>Yes. But it will nit fix non-cached access to the disk (raw) devices. 
> >>>>And
> >>>>this is the main reason why ntfs-3g and exfat are much slower then 
> >>>>working
> >>>>on Linux.
> >>>But _that_ can be fixed with the appropriate application of a sensible
> >>>caching layer.
> >>With every application?  :) Are you know anyone who wants to do this? At
> >>least for 3 fuse filesystems.
> >The filesystem is the *BEST* place to do caching.  It knows what metadata
> >is most effective to cache and what other data (e.g. file contents) doesn't
> >need to be cached.  Any attempt to do this in layers between the FS and
> >the disk won't achieve the same gains as a properly written filesystem.
> >e.g. in a UFS implementation the disk layer may see a lot of I/Os for
> >blocks, not necessarily sequential, as a program lists a directory and 
> >stats
> >all the files which pulls in the inode tables.  The filesystem knows that 
> >it
> >needs the inode tables and is likely to need not only the current inode 
> >table
> >disk block but subsequent ones also, and instead of requesting the disk 
> >sector
> >that it needs to service the immediate stat(2) request but maybe the next 
> >few
> >also.  Without that insight into whats going on it is difficult to see how 
> >a
> >highly effective cache could be done at the geom layer.
> I think we are playing in a "captain obvious".
> I have nothing against statement that FS is a "best place for caching". 
> Also - i am absolutely sure that its better to have kernel space fs 
> driver then FUSE one.
> But unfortunately there is no kernel space driver for the exfat, kernel 
> driver for ntfs is ugly and buggy (and r/o) and i don`t think that 
> anyone is going to change this.
> And i really don`t understand why are you trying to tell that it cannot 
> be effective when its so easy to proof that it can. Just try this with 
> fuse based filesystems in Linux, and you will get speed compared to 
> underlying device (especially on relatively slow USB devices). Then try 
> the same code on FreeBSD to see how ugly things are.
> And yes, in ideal world ever fs needs to have good written cache 
> implementation and kernel should not care about caching raw devices at 
> all. But as i mentioned before - there is no kernel-space drivers with a 
> good cache implementation for this 2 widely used systems (and probably 
> not only). Linux is a good example that device-level caching works, and 
> works fine.

Please re-read my message.  At no time did I say that caching below the
FS could not provide speed improvements.  I said it could not be as
effective as a properly implemented filesystem.  I'm sure if you throw
memory at it, a geom layer cache can provide substantial speed ups.  However
I strongly suspect that a proper FS cache would provide a better memory/hit
ratio than a geom layer cache.


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