[CentOS] 3Ware 9550SX and latency/system responsiveness

[Simon Banton]

At 11:16 -0400 14/9/07, Ross S. W. Walker wrote:
>Yes, a write-back cache with a BBU will definitely help, also your config,

The write-cache is enabled, but what I've not known up to now is that 
the absence of a BBU will impact IO performance in this way - which 
seems to be what you and Feizhou are saying. Is there any way to tell 
the card to forget about not having a BBU and behave as if it did?

The main problem here is the latency when under IO load not the 
throughput (or lack of). I don't care if it can't achieve 300MB/s 
sustained write speeds, only that it shouldn't bring the machine to 
its knees in the process of getting 35MB/s.

>  > 4x Seagate ST3250820SV 250GB in a RAID 1 plus 2 hot spare config
>
>is kinda wasteful, why not create a 4 disk RAID10 and get a 5th drive for
>a hot-spare.

Logistics meant that it was more important to be able to cope with a 
disk failure without needing to visit the hosting centre immediately 
afterwards (which we'd have to do if there was only one hot spare).

>Also think about getting 2 internal SATA drives for the OS and keep the
>RAID10 as purely for data, that should make things humm nicely and to be
>able to upgrade your data storage without messing with your OS/application
>installation. It wouldn't cost a lot either, 2 SATA drives + 1 SAS drive.

The server box is a Supermicro AS2020A - there is no onboard SATA nor 
any space for internal disks - there are 6 bays on a hot swap 
backplane and they're all cabled to the 3ware controller.

I've unpacked and fired up one of the other identical machines and 
moved the drives from the original to this one and booted straight 
off them.

The only difference between hardware is that the firmware on the 
3Ware card in this one has not been updated (it's 3.04.00.005 from 
codeset 9.3.0 as opposed to 3.08.02.005 from 9.4.1.2).

# /opt/iozone/bin/iozone -s 20480m -r 64 -i 0 -i 1 -t 1

Original box:
   Initial write		34208.20703
         Rewrite		38133.20313
            Read		79596.36719
         Re-read		79669.22656

Newly unpacked box:
   Initial write		50230.10547
         Rewrite		46108.17969
            Read		78739.14844
         Re-read		79325.11719

... but the new one still shows the same IO blocking/responsiveness issue.

S.