Re: Really bad diskio

On Fri, 2005-07-15 at 15:29 -0600, Ron Wills wrote:
> Here's a bit of a dump of the system that should be useful.
>
> Processors x2:
>
> vendor_id : AuthenticAMD
> cpu family : 6
> model : 8
> model name : AMD Athlon(tm) MP 2400+
> stepping : 1
> cpu MHz : 2000.474
> cache size : 256 KB
>
> MemTotal: 903804 kB
>
> Mandrake 10.0 Linux kernel 2.6.3-19mdk
>
> The raid controller, which is using the hardware raid configuration:
>
> 3ware 9000 Storage Controller device driver for Linux v2.26.02.001.
> scsi0 : 3ware 9000 Storage Controller
> 3w-9xxx: scsi0: Found a 3ware 9000 Storage Controller at 0xe8020000, IRQ: 17.
> 3w-9xxx: scsi0: Firmware FE9X 2.02.00.011, BIOS BE9X 2.02.01.037, Ports: 4.
> Vendor: 3ware Model: Logical Disk 00 Rev: 1.00
> Type: Direct-Access ANSI SCSI revision: 00
> SCSI device sda: 624955392 512-byte hdwr sectors (319977 MB)
> SCSI device sda: drive cache: write back, no read (daft)
>
> This is also on a 3.6 reiser filesystem.
>
> Here's the iostat for 10mins every 10secs. I've removed the stats from
> the idle drives to reduce the size of this email.
>
> Linux 2.6.3-19mdksmp (photo_server) 07/15/2005
>
> avg-cpu: %user %nice %sys %iowait %idle
> 2.85 1.53 2.15 39.52 53.95
>
> Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
> sda 82.49 4501.73 188.38 1818836580 76110154
>
> avg-cpu: %user %nice %sys %iowait %idle
> 0.30 0.00 1.00 96.30 2.40
>
> Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
> sda 87.80 6159.20 340.00 61592 3400

These I/O numbers are not so horrible, really. 100% iowait is not
necessarily a symptom of misconfiguration. It just means you are disk
limited. With a database 20 times larger than main memory, this is no
surprise.

If I had to speculate about the best way to improve your performance, I
would say:

1a) Get a better RAID controller. The 3ware hardware RAID5 is very bad.
1b) Get more disks.
2) Get a (much) newer kernel.
3) Try XFS or JFS. Reiser3 has never looked good in my pgbench runs

By the way, are you experiencing bad application performance, or are you
just unhappy with the iostat figures?

Regards,
jwb