Re: [Xen-users] Large server, Xen limitations

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

mark garey wrote:

> On Jan 2, 2009, at 4:07 PM, Robert Dunkley wrote:
> <snip>
> > We actually mix and match as needed, generally I found SAS is worth it
> > for database and web servers but a waste on web servers or backup NAS.
> > SATA is also available in much larger sizes.

> i really appreciate your comments and observations regarding SAS vs
> SATA and the importance of the controller, but can you please clarify this
> last bit about which resources SAS disks are wasted on?

SAS is "SCSI over a serial interface", and does all the stuff SCSI 
has been chosen for over the years - including features like command 
queuing/re-ordering and detach/re-attach. So the controller can throw 
a load of read and write commands at the drive, and the drive can 
re-order them for efficiency and report success/failure for each 
request as it's actually completed. Not applicable to SAS, but 
parallel SCSI allows the controller to (for example) ask a drive to 
fetch some blocks of data, the drive then 'detaches' from the bus, 
and the controller can do other commands with other drives on the bus 
while the first drive is seeking to fetch the data requested.
SATA is "ATA over a serial interface", and while ATA has been getting 
more intelligent over the iterations, it doesn't have some of the 
performance functionality of SCSI. Also, ATA/SATA drives have 
traditionally been 'lower spec' drives than SCSI/SAS drives.

So if you need performance, then SAS is the drive of choice. If 
performance isn't as important, but you want capacity, then choose 
SATA.

A busy database engine puts heavy demands on the storage system with 
a lot of random reads and writes. For these applications, SAS drives 
are preferred as they normally have higher performance.
On the other hand, web servers tend to be read only on the file 
system, and also tend to read files in one go (and in clusters of 
files). For these, the benefits of the high random I/O performance 
from SAS drives are not required and cheaper SATA drives may well 
suffice.
Similarly, random I/O performance of a NAS backup server isn't an 
issue - but space is. So SATA would be the logical choice.

Most good SAS controllers support SATA drives, this is part of the 
spec. So a box with a good SAS controller can have SATA hard drives 
installed (or SATA CD/DVD if required, which aren't available as 
SAS). The reverse is NOT true, SAS drives are NOT supported on SATA 
controllers. The SAS and SATA connectors are similar, but keyed so 
that the above valid combinations are possible.

Just another of those details to consider when speccing up a server !

--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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