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



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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