building a RAID system

Donald Becker becker at
Wed Oct 8 22:58:17 EDT 2003

On Wed, 8 Oct 2003, Mark Hahn wrote:

> > 	- get those drives w/ 8MB buffer disk cache
> what reason do you have to regard 8M as other than a useless
> marketing feature?  I mean, the kenel has a cache that's 100x
> bigger, and a lot faster.

The larger cache does provide some benefit.  Disks now read and cache up
to a whole track/cylinder at once, starting from when the head settles
from a seek up to when the desired sector is read.  You can't do that
type of caching in the kernel.

As disks become more dense, more memory is needed to save a cylinder's
worth of data, so we should expect the cache size to increase.

But you point is likely "disk cache is mostly legacy superstition".
MS-Windows 98 and earlier had such horrible caching behavior that a few MB
of on-disk cache could triple the performance.  This was also why
MS-Windows would run much faster under Linux+VMWare than on the raw

> > 	- it supposedly can sustain 133MB/sec transfers

Normal disks top out at 70MB/sec read, 50MB/sec write on the outer
tracks.  These numbers drop significantly on the inner tracks.
You might get 10MB/sec better with 10K or 15K RPM SCSI drives, but it's
certainly not linear with the speed.

BTW, 2.5" laptop drives are _far_ worse.
Typical for a modern fast drive is 20MB/sec read and 10MB/sec write.
Older drivers were worse.

> > 	- some say scsi disks are faster ... 
> usually lower-latency, often not higher bandwidth.  interestingly,
> ide disks usually fall off to about half peak bandwidth on inner 
> tracks.  scsi disks fall off too, but usually less so - they 
> don't push capacity quite as hard.

Look at the shape of the transfer performance curve -- the shape is
sometimes the same as the similar IDE drive, but sometimes has a much
different curve.  Wider tracks mean faster seek settling but lower

Donald Becker				becker at
Scyld Computing Corporation
914 Bay Ridge Road, Suite 220		Scyld Beowulf cluster system
Annapolis MD 21403			410-990-9993

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