6 GB/s SAS RAID 10 (6 Drive) vs. 3 GB/s SAS RAID 10 (14 Drive)

Question

I am running MS SQL 2014 Standard and have a Dell MD 3000i iSCSI SAN. Rough specs:

Dell R610 With PERC H700 controlling 6 x 2.5" 15K SAS drives (6 GB/s) Dell MD3000i with 14 x 3.5" 15K SAS drives (drives are 6 GB/s but MD3000i only supports up to 3 GB/s)

I have been using the SAN with no issues but I have 250 GB unused on a separate partition on the server so I figured why not take advantage of the faster drives for the databases that require better performance, i.e. app sync, fast lookups, etc. I did a backup/restore to move the databases keeping the log files on the SAN and the MDF's on the server.

I'm not so sure this was a good move and I'm surprised to see what may be that the SAN is outperforming the server drives. Maybe due to more spindles or 3.5 vs. 2.5" drives?? I'm going to give it a week (maybe) but I may end up moving the databases back. Using SQL Monitor (Red Gate) here are some stats over the past 24 hour period:

Server:

Disk avg. read time for sql1 > D: Min 0 ms Max 37.2 ms Mean 2.7 ms

Disk avg. write time for sql1 > D: Min 0 ms Max 3.5 ms Mean 0.2 ms

Disk transfers/sec for sql1 > D: Min 0 Max 283.4 Mean 4.0

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SAN

Disk avg. read time for sql1 > S: Min 0 ms Max 117.6 ms Mean 2.1 ms

Disk avg. write time for sql1 > S: Min 1.1 ms Max 118.8 ms Mean 4.5 ms

Disk transfers/sec for sql1 > S: Min 8.6 Max 852.0 Mean 24.9

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In one small test my iOS app that was syncing in 45ms is now syncing in 62ms, I expected that to be faster.

What is the better storage? Server or SAN?

Answer 1

It's a difficult question. I had a similar choice to do five month ago. We had some R610 connected to an iscsi network (one PS6500 and two PS6000) and we had choose to shut down the san (and iscsi at the same time) using severals R720xd. Why :

• iscsi is....
• we had better read/write latency using servers with sas dd attached.
• not necessarely more i/o but we don't need it at this time.

I think that san is better if you need a lot a space and i/o. Moreover, it's easier to backup your data with a san (snapshot, replica). But that sounds logical for me that a server with sas dd (not concidering sas level) has lower latency than a separate storage (direct attached or iscsi).