> The same-ide-channel problem is that you don't get to do that... it's
> one at a time, so you'll get something more like an average (harmonic
> mean I'd guess... don't make me think :) of the two rates, as over time
> you'll be getting data at slow rate, fast rate, slow rate, fast rate, etc
> (and the fast rate parts go quickly, unfortunately, so you spend most
> *time* at the slow rate :)
>
> [algebra lesson]
My experience with identical drive pairs on Promise U/33 is a bit more
straight forward. Here's what you can actually expect in the real EIDE
world:
o 2 drives, 2 controllers, ~2x seq read performance of the slowest
drive.
o 2 drives, 1 controller, ~0.5 seq read performance of the slowest
drive.
For a single dual channel EIDE controller, interleave your drives in
pairs (eg- hde+hdg, hdf+hdh). For two or more dual channel controllers,
use interleaved pairs on sequential controllers. Spend the extra money
on another machine or a masarati.
single dual
- md0 - -------- md0 ----------
hde hdg hde hdg hdi hdk
---|---| ---|---| ---|---|
hdf hdg hdf hdg hdj hdl
- md1 - -------- md1 ----------
[tim@asus tim]# cat /proc/mdstat
Personalities : [2 raid0] [3 raid1]
read_ahead 8 sectors
md0 : active raid0 hde3 hdg3 5221120 blocks 16k chunks
md1 : inactive
md2 : inactive
md3 : inactive
[tim@asus tim]# hdparm -tT /dev/{md0,md0,hde3,hdg3}
/dev/md0:
Timing buffer-cache reads: 64 MB in 0.54 seconds =118.52 MB/sec
Timing buffered disk reads: 32 MB in 1.69 seconds =18.93 MB/sec
/dev/md0:
Timing buffer-cache reads: 64 MB in 0.63 seconds =101.59 MB/sec
Timing buffered disk reads: 32 MB in 1.69 seconds =18.93 MB/sec
/dev/hde3:
Timing buffer-cache reads: 64 MB in 0.52 seconds =123.08 MB/sec
Timing buffered disk reads: 32 MB in 3.36 seconds = 9.52 MB/sec
/dev/hdg3:
Timing buffer-cache reads: 64 MB in 0.51 seconds =125.49 MB/sec
Timing buffered disk reads: 32 MB in 3.35 seconds = 9.55 MB/sec
18.93/((9.55+9.52)/2)
1.9853
kernel 2.0.38 + ide patch, ASUS P3B-F, C300A 4.5x103 MHz, 256 MB
...
ide: Intel 82371 PIIX4 (dual FIFO) DMA Bus Mastering IDE
Controller on PCI bus 0 function 33
ide0: BM-DMA at 0xd800-0xd807
ide1: BM-DMA at 0xd808-0xd80f
ide: PDC20246 UDMA Bus Mastering
Controller on PCI bus 0 function 80
(U)DMA Burst Bit ENABLED Primary PCI Mode Secondary PCI Mode.
ide2: BM-DMA at 0xa400-0xa407
ide3: BM-DMA at 0xa408-0xa40f
hda: Maxtor 88400D8, 8011MB w/256kB Cache, CHS=1021/255/63, UDMA(33)
hdb: ASUS CD-S340, ATAPI CDROM drive
hdc: YAMAHA CRW4416E, ATAPI CDROM drive
hde: IBM-DHEA-36481, 6197MB w/472kB Cache, CHS=12592/16/63, UDMA(33)
hdf: IBM-DTTA-371440, 13783MB w/462kB Cache, CHS=28005/16/63, UDMA(33)
hdg: IBM-DHEA-36481, 6197MB w/472kB Cache, CHS=12592/16/63, UDMA(33)
hdh: IBM-DTTA-371440, 13783MB w/462kB Cache, CHS=28005/16/63, UDMA(33)
ide0 at 0x1f0-0x1f7,0x3f6 on irq 14
ide1 at 0x170-0x177,0x376 on irq 15
ide2 at 0xb800-0xb807,0xb406 on irq 3
ide3 at 0xb000-0xb007,0xa806 on irq 3 (shared with ide2)
...
--
