Re: Raid: software or hardware
- From: phil-news-nospam@xxxxxxxx
- Date: 14 Jun 2008 20:19:57 GMT
On Wed, 11 Jun 2008 11:33:03 +0200 Aragorn <aragorn@xxxxxxxxxxxxxxxxxxx> wrote:
| Hardware RAID is always preferable over software RAID *if* it is available.
| However, I sincerely doubt that the onboard SATA RAID of your motherboard
| will be a hardware RAID.
I have a Tyan S2927A2NRF with 6 SATA ports. I had 4 of them connected for
a while to 4 WD 750GB SATA drives. I achieved 96MB/s transfer rate when
accessing 1 drive at a time (linear reading/writing so the drive would be
doing more data transfer than seeking for this test). When I ran all 4
drives at the same time (4 separate processes doing reading), the speed
of each dropped to about 92MB/s, so I was getting an aggregate speed of
368MB/s. I don't know what caused the slowdown (controller, bus, driver,
scheduler) but the machine has 2x dual-core 2.8GHz Opeterons, so there is
at least enough CPU power there to work 4 processes easily. Nevertheless
it's still a decent throughput.
My question is, does your preference for hardware RAID still hold here?
Presumably I could get a 368MB/s peak with software RAID minus whatever
overhead that RAID would incur. Hardware RAID in the controller (it has
it but I've not determined if Linux can configure and work with it, and
it might be the "WinRAID" you mention) might do just as well. External
hardware RAID might be a problem as the aggregate transfer rate (2.944
Gbps) is pushing the limit of a SATA-II connection (hypothetically 3.0
Gbps).
| Most likely, it'll be a hardware-assisted software RAID, or if you will, a
| "WinRAID", just like a Winmodem isn't a real modem.
[*] El-cheapo devices that steal resources from the main CPU(s) so they can
market lower pricing.
|> 1. Using hardware, jmicron (creating raid from bios).
|> Will gentoo see it ? Is it stable ?
|
| I do not know the JMicron, but true hardware SATA RAID from a chip on the
| motherboard is extremely rare and would also make the board more expensive
| than its peers.
Seems to be common on server grade motherboards in the many-hundred-dollar
ranges.
| Even if you enable the RAID functionality of an SATA onboard
| hardware-assisted software RAID, this will only be seen as a RAID by
| Windows, and then will require you to load the appropriate driver for it,
| if Windows doesn't have a driver for it by itself.
|
| In GNU/Linux, such a device is seen as a simple SATA controller, regardless
| of whether the RAID functionality is enabled or not, because basically
| that's what it is.
I'm guessing if it is a "WinRAID" style, it has special queueing commands to
at least improve up having N separate SATA ports doing as software RAID.
|> If one disk fails how will i know ?
|
| Both true hardware RAID and software RAID would report this via /syslogd,/
| or possibly via an additional daemon like Adaptec's Storage Manager for
| Adaptec RAID adapters.
|
|> Will there be no problem with replacing bad disk with new one ?
|
| The replacement of a failed drive in a RAID array is always subject to
| certain delays and restrictions. Not all RAID adapters (or hard disks!)
| support hotplugging.
It appears the ones in the Tyan S2927A2NRF board do not. I connected a
couple ports to an eSATA adapter and tried plugging in an external SATA
drive (A WD 1TB drive with 3 interfaces). It would not recognize the
hotplugging automatically. If I had the drive plugged in while booting
up, it would find the drive OK.
| When a drive has been replaced, most hardware RAID adapters will
| automatically rebuild the array - which takes some time and will slow down
| the machine's performance somewhat - but in software RAID, I believe you
| have to rebuild the array manually using /mdadm./
Presumably some kind of RAID manager daemon could run in the background to
emulate the hardware RAID by doing this chore when it sees the drive being
added.
For a project I am studying which will need a lot of reliable file space,
I'm looking at using RAID only to expand the storage per server, and doing
redundancy between servers, with load balancing between servers as well.
Programs would run on the servers to compare each server tree to the others
and replicate anything found missing on one or more servers.
--
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| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |
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