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Cyker
Veteran
Joined: 15 Jun 2006
Posts: 1746
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 Posted: Tue May 15, 2007 6:22 pm Post subject: |
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Slight correction if I may 
RAID1 can offer good read-performance increases, but no write-performance.
On IDE, I tend to find both degrade rather badly on some chipsets but not on others for some reason, even if both the hard disks are on different channels. (Might be something to do with that DMA thing I read in another thread; Never knew that before!)
Agreed on the 80-core cable, although don't use one of those stupid 'rounded cables' - In a proper 80-core cable, 40 of the pins are the regular data wires, but the other 40 are actually Ground wires, and are put in between the data lines as a crude shield to eliminate crosstalk and other EMI nastiness.
In cheap rounded cables, they shred up all the wires so they're moving about freely - This is bad because the crude grounding/shielding is no longer in place, and the high-frequency signals are free to cause cross-talk.
When the BIOS detects that it's an 80-core cable and switches it to UDMA100+, you start getting significant data-corruption which at best slows everything down, and at worse causes real data corruption. |
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thanny
n00b
Joined: 15 Aug 2005
Posts: 55
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| Posted: Wed May 16, 2007 7:37 am Post subject: |
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| Cyker wrote: | Slight correction if I may
RAID1 can offer good read-performance increases, but no write-performance.
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In theory. There's no reason I can think of why it couldn't be treated as a two-drive RAID0 stripe, for reading purposes.
However, in practice, that doesn't happen. Not with every hardware RAID I've tried, and certainly not with the software RAID in Linux.
All reads come from one drive.
The same is true with RAID10 - only one drive of each mirror is read, so the only speed boost you get is from the actual striping. That's true with both software RAID0 on top of software RAID1, and the software RAID10 driver (which is actually slower than layering the other two individually), not to mention the hardware RAID10 in the Intel PCIe SATA/SAS RAID controllers (which actually use MegaRAID from LSI).
But as I said, I agree with you in principle. If I didn't have direct contradictory experience, I'd have to guess that it works exactly as you say. It just wouldn't make sense to not use both drives when reading, right? Seems those actually implementing RAID1 don't agree, but I'll be damned if I know why. |
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bingobob
Apprentice
Joined: 05 Apr 2004
Posts: 296
Location: Hertfordshire, UK
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| Posted: Sat Jun 09, 2007 7:00 am Post subject: |
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OK, well, I'm going to revisit this - this weekend. Thanks for all the advice but I have to admit to being a bit blinded by science here. I seem to have distilled two main point.
1) I MUST check my cables, I'm pretty sure they're the wrong type (infact they're the rounded variety I think).
2) Seems like I've gone for RAID1 and I should have gone for RAID0 - doh, basic error. As you say, there is a backup in place.
Here is some performance data before I make any changes....
| Code: |
franz ~ # fdisk -l
Disk /dev/hda: 20.0 GB, 20020396032 bytes
255 heads, 63 sectors/track, 2434 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/hda1 * 1 5 40131 83 Linux
/dev/hda2 6 68 506047+ 82 Linux swap / Solaris
/dev/hda3 69 2434 19004895 83 Linux
Disk /dev/hdb: 122.9 GB, 122942324736 bytes
16 heads, 63 sectors/track, 238216 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 1 238216 120060832+ fd Linux raid autodetect
Disk /dev/hdd: 120.0 GB, 120034123776 bytes
255 heads, 63 sectors/track, 14593 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/hdd1 1 14593 117218241 fd Linux raid autodetect
Disk /dev/md0: 120.0 GB, 120031412224 bytes
2 heads, 4 sectors/track, 29304544 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Disk /dev/md0 doesn't contain a valid partition table
Disk /dev/sda: 400.0 GB, 400088457216 bytes
255 heads, 63 sectors/track, 48641 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 48641 390708801 83 Linux
franz ~ # hdparm -tT /dev/hdd1
/dev/hdd1:
Timing cached reads: 894 MB in 2.00 seconds = 446.45 MB/sec
Timing buffered disk reads: 76 MB in 3.02 seconds = 25.13 MB/sec
franz ~ # hdparm -tT /dev/md0
/dev/md0:
Timing cached reads: 892 MB in 2.00 seconds = 446.08 MB/sec
Timing buffered disk reads: 164 MB in 3.01 seconds = 54.50 MB/sec
franz ~ # hdparm -tT /dev/sda1
/dev/sda1:
Timing cached reads: 892 MB in 2.00 seconds = 445.18 MB/sec
Timing buffered disk reads: 80 MB in 3.02 seconds = 26.46 MB/sec
franz ~ # hdparm -tT /dev/hda
/dev/hda:
Timing cached reads: 892 MB in 2.00 seconds = 445.66 MB/sec
Timing buffered disk reads: 70 MB in 3.08 seconds = 22.76 MB/sec
franz ~ #
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and
| Code: |
ranz ~ # dmesg
with idebus=xx
SIS5513: IDE controller at PCI slot 0000:00:02.5
ACPI: PCI Interrupt Link [LNKA] enabled at IRQ 11
PCI: setting IRQ 11 as level-triggered
ACPI: PCI Interrupt 0000:00:02.5[A] -> Link [LNKA] -> GSI 11 (level, low) -> IRQ 11
SIS5513: chipset revision 0
SIS5513: not 100% native mode: will probe irqs later
SIS5513: SiS 962/963 MuTIOL IDE UDMA133 controller
ide0: BM-DMA at 0xa400-0xa407, BIOS settings: hda:DMA, hdb:DMA
ide1: BM-DMA at 0xa408-0xa40f, BIOS settings: hdc:DMA, hdd:DMA
Probing IDE interface ide0...
hda: WDC WD200BB-60BNA1, ATA DISK drive
hdb: Maxtor 6Y120L0, ATA DISK drive
ide0 at 0x1f0-0x1f7,0x3f6 on irq 14
Probing IDE interface ide1...
hdc: Compaq CRD-8484B, ATAPI CD/DVD-ROM drive
hdd: WDC WD1200BB-00DWA0, ATA DISK drive
ide1 at 0x170-0x177,0x376 on irq 15
hda: max request size: 128KiB
hda: 39102336 sectors (20020 MB) w/2048KiB Cache, CHS=38792/16/63, UDMA(100)
hda: cache flushes not supported
hda: hda1 hda2 hda3
hdb: max request size: 128KiB
hdb: 240121728 sectors (122942 MB) w/2048KiB Cache, CHS=65535/16/63, UDMA(133)
hdb: cache flushes supported
hdb: hdb1
hdd: max request size: 512KiB
hdd: 234441648 sectors (120034 MB) w/2048KiB Cache, CHS=16383/255/63, UDMA(33)
hdd: cache flushes supported
hdd: hdd1
hdc: ATAPI 48X CD-ROM drive, 128kB Cache, DMA
Uniform CD-ROM driver Revision: 3.20
ehci_hcd: block sizes: qh 128 qtd 96 itd 192 sitd 96
ACPI: PCI Interrupt Link [LNKH] enabled at IRQ 10
PCI: setting IRQ 10 as level-triggered
ACPI: PCI Interrupt 0000:00:03.3[D] -> Link [LNKH] -> GSI 10 (level, low) -> IRQ 10
ehci_hcd 0000:00:03.3: EHCI Host Controller
drivers/usb/core/inode.c: creating file 'devices'
drivers/usb/core/inode.c: creating file '001'
ehci_hcd 0000:00:03.3: new USB bus registered, assigned bus number 1
ehci_hcd 0000:00:03.3: reset hcs_params 0x103206 dbg=1 cc=3 pcc=2 ordered !ppc ports=6
ehci_hcd 0000:00:03.3: reset hcc_params 7070 thresh 7 uframes 1024
PCI: cache line size of 128 is not supported by device 0000:00:03.3
ehci_hcd 0000:00:03.3: supports USB remote wakeup
ehci_hcd 0000:00:03.3: irq 10, io mem 0xe5800000
ehci_hcd 0000:00:03.3: reset command 080002 (park)=0 ithresh=8 period=1024 Reset HALT
ehci_hcd 0000:00:03.3: init command 010001 (park)=0 ithresh=1 period=1024 RUN
ehci_hcd 0000:00:03.3: USB 2.0 started, EHCI 1.00, driver 10 Dec 2004
usb usb1: default language 0x0409
usb usb1: new device strings: Mfr=3, Product=2, SerialNumber=1
usb usb1: Product: EHCI Host Controller
usb usb1: Manufacturer: Linux 2.6.17-gentoo-r4 ehci_hcd
usb usb1: SerialNumber: 0000:00:03.3
usb usb1: uevent
usb usb1: configuration #1 chosen from 1 choice
usb usb1: adding 1-0:1.0 (config #1, interface 0)
usb 1-0:1.0: uevent
hub 1-0:1.0: usb_probe_interface
hub 1-0:1.0: usb_probe_interface - got id
hub 1-0:1.0: USB hub found
hub 1-0:1.0: 6 ports detected
hub 1-0:1.0: standalone hub
hub 1-0:1.0: no power switching (usb 1.0)
hub 1-0:1.0: individual port over-current protection
hub 1-0:1.0: Single TT
hub 1-0:1.0: TT requires at most 8 FS bit times (666 ns)
hub 1-0:1.0: power on to power good time: 20ms
hub 1-0:1.0: local power source is good
hub 1-0:1.0: state 7 ports 6 chg 0000 evt 0000
drivers/usb/core/inode.c: creating file '001'
ohci_hcd: 2005 April 22 USB 1.1 'Open' Host Controller (OHCI) Driver (PCI)
ohci_hcd: block sizes: ed 64 td 64
ehci_hcd 0000:00:03.3: GetStatus port 1 status 001803 POWER sig=j CSC CONNECT
hub 1-0:1.0: port 1, status 0501, change 0001, 480 Mb/s
ACPI: PCI Interrupt Link [LNKE] enabled at IRQ 3
PCI: setting IRQ 3 as level-triggered
ACPI: PCI Interrupt 0000:00:03.0[A] -> Link [LNKE] -> GSI 3 (level, low) -> IRQ 3
ohci_hcd 0000:00:03.0: OHCI Host Controller
drivers/usb/core/inode.c: creating file '002'
ohci_hcd 0000:00:03.0: new USB bus registered, assigned bus number 2
ohci_hcd 0000:00:03.0: created debug files
ohci_hcd 0000:00:03.0: irq 3, io mem 0xe7000000
ohci_hcd 0000:00:03.0: resetting from state 'reset', control = 0x0
ohci_hcd 0000:00:03.0: enabling initreset quirk
ohci_hcd 0000:00:03.0: OHCI controller state
ohci_hcd 0000:00:03.0: OHCI 1.0, NO legacy support registers
ohci_hcd 0000:00:03.0: control 0x083 HCFS=operational CBSR=3
ohci_hcd 0000:00:03.0: cmdstatus 0x00000 SOC=0
ohci_hcd 0000:00:03.0: intrstatus 0x00000004 SF
ohci_hcd 0000:00:03.0: intrenable 0x8000000a MIE RD WDH
ohci_hcd 0000:00:03.0: hcca frame #0003
ohci_hcd 0000:00:03.0: roothub.a 01000202 POTPGT=1 NPS NDP=2(2)
ohci_hcd 0000:00:03.0: roothub.b 00000000 PPCM=0000 DR=0000
ohci_hcd 0000:00:03.0: roothub.status 00008000 DRWE
ohci_hcd 0000:00:03.0: roothub.portstatus [0] 0x00000100 PPS
ohci_hcd 0000:00:03.0: roothub.portstatus [1] 0x00000100 PPS
usb usb2: default language 0x0409
usb usb2: new device strings: Mfr=3, Product=2, SerialNumber=1
usb usb2: Product: OHCI Host Controller
usb usb2: Manufacturer: Linux 2.6.17-gentoo-r4 ohci_hcd
usb usb2: SerialNumber: 0000:00:03.0
usb usb2: uevent
usb usb2: configuration #1 chosen from 1 choice
usb usb2: adding 2-0:1.0 (config #1, interface 0)
usb 2-0:1.0: uevent
hub 2-0:1.0: usb_probe_interface
hub 2-0:1.0: usb_probe_interface - got id
hub 2-0:1.0: USB hub found
hub 2-0:1.0: 2 ports detected
hub 2-0:1.0: standalone hub
hub 2-0:1.0: no power switching (usb 1.0)
hub 2-0:1.0: global over-current protection
hub 2-0:1.0: power on to power good time: 2ms
hub 2-0:1.0: local power source is good
hub 2-0:1.0: no over-current condition exists
hub 1-0:1.0: debounce: port 1: total 100ms stable 100ms status 0x501
drivers/usb/core/inode.c: creating file '001'
ACPI: PCI Interrupt Link [LNKF] enabled at IRQ 5
PCI: setting IRQ 5 as level-triggered
ACPI: PCI Interrupt 0000:00:03.1[B] -> Link [LNKF] -> GSI 5 (level, low) -> IRQ 5
ohci_hcd 0000:00:03.1: OHCI Host Controller
drivers/usb/core/inode.c: creating file '003'
ohci_hcd 0000:00:03.1: new USB bus registered, assigned bus number 3
ohci_hcd 0000:00:03.1: created debug files
ohci_hcd 0000:00:03.1: irq 5, io mem 0xe6800000
ohci_hcd 0000:00:03.1: resetting from state 'reset', control = 0x0
ehci_hcd 0000:00:03.3: port 1 high speed
ehci_hcd 0000:00:03.3: GetStatus port 1 status 001005 POWER sig=se0 PE CONNECT
ohci_hcd 0000:00:03.1: enabling initreset quirk
ohci_hcd 0000:00:03.1: OHCI controller state
ohci_hcd 0000:00:03.1: OHCI 1.0, NO legacy support registers
ohci_hcd 0000:00:03.1: control 0x083 HCFS=operational CBSR=3
ohci_hcd 0000:00:03.1: cmdstatus 0x00000 SOC=0
ohci_hcd 0000:00:03.1: intrstatus 0x00000004 SF
ohci_hcd 0000:00:03.1: intrenable 0x8000000a MIE RD WDH
ohci_hcd 0000:00:03.1: hcca frame #0003
ohci_hcd 0000:00:03.1: roothub.a 01000202 POTPGT=1 NPS NDP=2(2)
ohci_hcd 0000:00:03.1: roothub.b 00000000 PPCM=0000 DR=0000
ohci_hcd 0000:00:03.1: roothub.status 00008000 DRWE
ohci_hcd 0000:00:03.1: roothub.portstatus [0] 0x00000100 PPS
ohci_hcd 0000:00:03.1: roothub.portstatus [1] 0x00000100 PPS
usb usb3: default language 0x0409
usb usb3: new device strings: Mfr=3, Product=2, SerialNumber=1
usb usb3: Product: OHCI Host Controller
usb usb3: Manufacturer: Linux 2.6.17-gentoo-r4 ohci_hcd
usb usb3: SerialNumber: 0000:00:03.1
usb usb3: uevent
usb usb3: configuration #1 chosen from 1 choice
usb usb3: adding 3-0:1.0 (config #1, interface 0)
usb 3-0:1.0: uevent
hub 3-0:1.0: usb_probe_interface
hub 3-0:1.0: usb_probe_interface - got id
hub 3-0:1.0: USB hub found
hub 3-0:1.0: 2 ports detected
hub 3-0:1.0: standalone hub
hub 3-0:1.0: no power switching (usb 1.0)
hub 3-0:1.0: global over-current protection
hub 3-0:1.0: power on to power good time: 2ms
hub 3-0:1.0: local power source is good
hub 3-0:1.0: no over-current condition exists
usb 1-1: new high speed USB device using ehci_hcd and address 2
ehci_hcd 0000:00:03.3: port 1 high speed
ehci_hcd 0000:00:03.3: GetStatus port 1 status 001005 POWER sig=se0 PE CONNECT
drivers/usb/core/inode.c: creating file '001'
ACPI: PCI Interrupt Link [LNKG] enabled at IRQ 6
PCI: setting IRQ 6 as level-triggered
ACPI: PCI Interrupt 0000:00:03.2[C] -> Link [LNKG] -> GSI 6 (level, low) -> IRQ 6
ohci_hcd 0000:00:03.2: OHCI Host Controller
drivers/usb/core/inode.c: creating file '004'
ohci_hcd 0000:00:03.2: new USB bus registered, assigned bus number 4
ohci_hcd 0000:00:03.2: created debug files
ohci_hcd 0000:00:03.2: irq 6, io mem 0xe6000000
ohci_hcd 0000:00:03.2: resetting from state 'reset', control = 0x0
ohci_hcd 0000:00:03.2: enabling initreset quirk
ohci_hcd 0000:00:03.2: OHCI controller state
ohci_hcd 0000:00:03.2: OHCI 1.0, NO legacy support registers
ohci_hcd 0000:00:03.2: control 0x083 HCFS=operational CBSR=3
ohci_hcd 0000:00:03.2: cmdstatus 0x00000 SOC=0
ohci_hcd 0000:00:03.2: intrstatus 0x00000004 SF
ohci_hcd 0000:00:03.2: intrenable 0x8000000a MIE RD WDH
ohci_hcd 0000:00:03.2: hcca frame #0003
ohci_hcd 0000:00:03.2: roothub.a 01000202 POTPGT=1 NPS NDP=2(2)
ohci_hcd 0000:00:03.2: roothub.b 00000000 PPCM=0000 DR=0000
ohci_hcd 0000:00:03.2: roothub.status 00008000 DRWE
ohci_hcd 0000:00:03.2: roothub.portstatus [0] 0x00000100 PPS
ohci_hcd 0000:00:03.2: roothub.portstatus [1] 0x00000100 PPS
usb usb4: default language 0x0409
usb usb4: new device strings: Mfr=3, Product=2, SerialNumber=1
usb usb4: Product: OHCI Host Controller
usb usb4: Manufacturer: Linux 2.6.17-gentoo-r4 ohci_hcd
usb usb4: SerialNumber: 0000:00:03.2
usb usb4: uevent
usb usb4: configuration #1 chosen from 1 choice
usb usb4: adding 4-0:1.0 (config #1, interface 0)
usb 4-0:1.0: uevent
hub 4-0:1.0: usb_probe_interface
hub 4-0:1.0: usb_probe_interface - got id
hub 4-0:1.0: USB hub found
hub 4-0:1.0: 2 ports detected
hub 4-0:1.0: standalone hub
hub 4-0:1.0: no power switching (usb 1.0)
hub 4-0:1.0: global over-current protection
hub 4-0:1.0: power on to power good time: 2ms
hub 4-0:1.0: local power source is good
hub 4-0:1.0: no over-current condition exists
ohci_hcd 0000:00:03.0: suspend root hub
usb 1-1: default language 0x0409
usb 1-1: new device strings: Mfr=1, Product=2, SerialNumber=3
usb 1-1: Product: Samsung ML-2550 Series
usb 1-1: Manufacturer: SAMSUNG ELECTRONICS CO., LTD
usb 1-1: SerialNumber: 2M37B1EA100142H.
usb 1-1: uevent
usb 1-1: configuration #1 chosen from 1 choice
usb 1-1: adding 1-1:1.0 (config #1, interface 0)
usb 1-1:1.0: uevent
drivers/usb/core/inode.c: creating file '002'
ehci_hcd 0000:00:03.3: GetStatus port 2 status 001803 POWER sig=j CSC CONNECT
hub 1-0:1.0: port 2, status 0501, change 0001, 480 Mb/s
drivers/usb/core/inode.c: creating file '001'
USB Universal Host Controller Interface driver v3.0
ohci_hcd 0000:00:03.1: suspend root hub
hub 1-0:1.0: debounce: port 2: total 100ms stable 100ms status 0x501
ehci_hcd 0000:00:03.3: port 2 high speed
ehci_hcd 0000:00:03.3: GetStatus port 2 status 001005 POWER sig=se0 PE CONNECT
usb 1-2: new high speed USB device using ehci_hcd and address 3
ohci_hcd 0000:00:03.2: suspend root hub
ehci_hcd 0000:00:03.3: port 2 high speed
ehci_hcd 0000:00:03.3: GetStatus port 2 status 001005 POWER sig=se0 PE CONNECT
usb 1-2: default language 0x0409
usb 1-2: new device strings: Mfr=1, Product=2, SerialNumber=3
usb 1-2: Product: Freecom Hard Drive
usb 1-2: Manufacturer: Freecom
usb 1-2: SerialNumber: 01BC0E4B0053
usb 1-2: uevent
usb 1-2: configuration #1 chosen from 1 choice
usb 1-2: adding 1-2:1.0 (config #1, interface 0)
usb 1-2:1.0: uevent
drivers/usb/core/inode.c: creating file '003'
hub 2-0:1.0: state 7 ports 2 chg 0000 evt 0000
hub 1-0:1.0: state 7 ports 6 chg 0000 evt 0000
hub 3-0:1.0: state 7 ports 2 chg 0000 evt 0000
hub 4-0:1.0: state 7 ports 2 chg 0000 evt 0000
usblp 1-1:1.0: usb_probe_interface
usblp 1-1:1.0: usb_probe_interface - got id
drivers/usb/core/file.c: looking for a minor, starting at 0
drivers/usb/class/usblp.c: usblp0: USB Bidirectional printer dev 2 if 0 alt 0 proto 2 vid 0x04E8 pid 0x324A
usbcore: registered new driver usblp
drivers/usb/class/usblp.c: v0.13: USB Printer Device Class driver
Initializing USB Mass Storage driver...
usb-storage 1-2:1.0: usb_probe_interface
usb-storage 1-2:1.0: usb_probe_interface - got id
scsi0 : SCSI emulation for USB Mass Storage devices
usbcore: registered new driver usb-storage
USB Mass Storage support registered.
usb-storage: device found at 3
usb-storage: waiting for device to settle before scanning
md: linear personality registered for level -1
md: raid1 personality registered for level 1
md: multipath personality registered for level -4
md: faulty personality registered for level -5
md: md driver 0.90.3 MAX_MD_DEVS=256, MD_SB_DISKS=27
md: bitmap version 4.39
Advanced Linux Sound Architecture Driver Version 1.0.11rc4 (Wed Mar 22 10:27:24 2006 UTC).
ACPI: PCI Interrupt Link [LNKC] enabled at IRQ 10
ACPI: PCI Interrupt 0000:00:02.7[C] -> Link [LNKC] -> GSI 10 (level, low) -> IRQ 10
intel8x0_measure_ac97_clock: measured 51728 usecs
intel8x0: clocking to 48000
ALSA device list:
#0: SiS SI7012 with CS4299 at 0x9400, irq 10
ip_conntrack version 2.4 (7168 buckets, 57344 max) - 224 bytes per conntrack
ip_conntrack_pptp version 3.1 loaded
ip_nat_pptp version 3.0 loaded
ip_tables: (C) 2000-2006 Netfilter Core Team
ipt_recent v0.3.1: Stephen Frost <sfrost@snowman.net>. http://snowman.net/projects/ipt_recent/
ClusterIP Version 0.8 loaded successfully
arp_tables: (C) 2002 David S. Miller
TCP bic registered
NET: Registered protocol family 1
NET: Registered protocol family 17
Using IPI Shortcut mode
md: Autodetecting RAID arrays.
md: autorun ...
md: considering hdd1 ...
md: adding hdd1 ...
md: adding hdb1 ...
md: created md0
md: bind<hdb1>
md: bind<hdd1>
md: running: <hdd1><hdb1>
raid1: raid set md0 active with 2 out of 2 mirrors
md: ... autorun DONE.
kjournald starting. Commit interval 5 seconds
EXT3-fs: mounted filesystem with ordered data mode.
VFS: Mounted root (ext3 filesystem) readonly.
Freeing unused kernel memory: 164k freed
usb usb2: uevent
usb 2-0:1.0: uevent
usb usb3: uevent
usb 3-0:1.0: uevent
usb usb4: uevent
usb 4-0:1.0: uevent
usb usb1: uevent
usb 1-0:1.0: uevent
usb 1-1: uevent
usb 1-1:1.0: uevent
usb 1-2: uevent
usb 1-2:1.0: uevent
Vendor: SAMSUNG Model: HD400LD Rev: WQ10
Type: Direct-Access ANSI SCSI revision: 00
SCSI device sda: 781422768 512-byte hdwr sectors (400088 MB)
sda: Write Protect is off
sda: Mode Sense: 03 00 00 00
sda: assuming drive cache: write through
SCSI device sda: 781422768 512-byte hdwr sectors (400088 MB)
sda: Write Protect is off
sda: Mode Sense: 03 00 00 00
sda: assuming drive cache: write through
sda: sda1
sd 0:0:0:0: Attached scsi disk sda
usb-storage: device scan complete
EXT3 FS on hda3, internal journal
kjournald starting. Commit interval 5 seconds
EXT3 FS on md0, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
kjournald starting. Commit interval 5 seconds
EXT3 FS on sda1, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
Adding 506036k swap on /dev/hda2. Priority:-1 extents:1 across:506036k
eth0: Media Link On 100mbps full-duplex
eth0: Media Link Off
eth0: Media Link On 100mbps full-duplex
eth0: Media Link Off
eth0: Media Link On 100mbps full-duplex
eth0: Media Link Off
eth0: Media Link On 100mbps full-duplex
eth0: Media Link Off
eth0: Media Link On 100mbps full-duplex
eth0: Media Link Off
eth0: Media Link On 100mbps full-duplex
eth0: Media Link Off
eth0: Media Link On 100mbps full-duplex
eth0: Media Link Off
eth0: Media Link On 100mbps full-duplex
spurious 8259A interrupt: IRQ7.
eth0: Media Link Off
eth0: Media Link On 100mbps full-duplex
franz ~ #
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Cyker
Veteran
Joined: 15 Jun 2006
Posts: 1746
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| Posted: Sat Jun 09, 2007 2:34 pm Post subject: |
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| thanny wrote: | | Cyker wrote: | Slight correction if I may
RAID1 can offer good read-performance increases, but no write-performance.
|
In theory. There's no reason I can think of why it couldn't be treated as a two-drive RAID0 stripe, for reading purposes.
However, in practice, that doesn't happen. Not with every hardware RAID I've tried, and certainly not with the software RAID in Linux.
All reads come from one drive.
The same is true with RAID10 - only one drive of each mirror is read, so the only speed boost you get is from the actual striping. That's true with both software RAID0 on top of software RAID1, and the software RAID10 driver (which is actually slower than layering the other two individually), not to mention the hardware RAID10 in the Intel PCIe SATA/SAS RAID controllers (which actually use MegaRAID from LSI).
But as I said, I agree with you in principle. If I didn't have direct contradictory experience, I'd have to guess that it works exactly as you say. It just wouldn't make sense to not use both drives when reading, right? Seems those actually implementing RAID1 don't agree, but I'll be damned if I know why. |
Oooh, then I stand corrected! 
I didn't realise this was the case; Must confess I've yet to use RAID1 in software Linux, but the old PCI Adaptec RAID card we had in an old server seemed to perform noticable better for reads than a single drive so I just assumed that was why...
If the kernel devs haven't done it that way in the kernel, the only reason I can think of would be for complexity reasons (Scheduling and splitting reads between the two+ disks could potentially get quite complicated (Block level? File level? Pseudo-Stripe? Sector?? Nearest?? Ping-pong??), whereas with striped RAID0/4/5/6 etc. the split-reads are a side-effect of the striping so no extra read-scheduling and routing code is needed) |
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