Re: Two sanity checks

"Keith Boulton" <kboulton_at_ntlworld.com> wrote in message news:<4SI88.6986$as2.1086165_at_news6-win.server.ntlworld.com>...

<sorry, had to reply via google, my ISP has eaten your reply...>

>
> This is approximately the same as the maximum (theoretical) throughput as
> 3x160MB/s SCSI controllers.
>
> Why then should a SAN outperform (as has been said to me) a local storage
> approach?

It does. As you said, the above is the "maximum (theoretical) throughput"
of 3 SCSI controllers. You need a LOT more than 3 to approach these numbers with "max (practical) throughput". And a SAN *starts* at a single fibre connection: they can have a lot more too.

>
> When I asked, I was told that the SAN had an enormous cache and it was this
> that was responsible for the performance.
>

yes, that affects overall I/O time. But it has nothing to do with transfer
speed.

> It it me, but having the data cache at the wrong side of a 500MB/s link
> compared with local memory at GB's/s seems mad.

not really. the cache in the SAN is optimized to handle impedance mismatches between different drives in a RAID logical volume and such. It can also be partitioned per fraction of logical volume in some models.
And it does a few things more which are hard to handle by a local cache
(as in local system cache).

> The response that followed to this statement was: well I would still use a
> SAN because of ease of administration.
>
> But surely, that's a simple trade off, how much time do I save at c£10,000
> per person-month compared with the additional cost of the SAN?

quite a lot. to give you an idea, administration of disks/controllers/partitions consumed the largest section of time of a sysadmin and a DBA at a site I was in about 5 years ago. That was UNIX,
with a LVM. Nowadays, I work with a EMC in a HP site and we have a part-time sysadmin that can handle the whole lot. All I have to do is send
an e-mail to the fella and ask for such and such a block of disk with/out
x-amount of cache and Bang: next day it is there ready for me, with *predictable* performance!
That is worth a lot of my time and money!

>
> In addition, the SAN bought was some HP thing of 750GB and you have to pay
> extra licence fees if you go above 1TB (as they want to now), and buy
> additional cache. It sounds like a licence to print money for the vendor. No
> wonder they're dead keen on the things.

Yup. As I said, they're expensive. It's all a matter of juggling ALL the numbers involved and see if it's worth it. Sometimes it isn't.

Cheers
Nuno Souto Received on Sat Feb 09 2002 - 05:24:04 CST