Mailinglist Archive: opensuse (3156 mails)

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Re: [opensuse] Re: [OT] vmware and fake scsi devs
  • From: Russell Jones <russell.jones@xxxxxxxxxxxx>
  • Date: Mon, 10 Dec 2007 15:13:29 +0000
  • Message-id: <475D5799.1030607@xxxxxxxxxxxx>
Aaron Kulkis wrote:

You must be assuming that striping across a RAID is somehow a serial operation.
That doesn't matter. Your assumption that the data blocks
from several disks can be XOR'ed together, and written
to one of those disks, and the parity partition on yet
another disk is FASTER than not doing so is just patently
ridiculous.
So you think CPUs operates more slowly than disks, Aaron? Or do you think you need to read data from a disk every time you write it?

That doesn't even count the matter of increasing the
bandwidth usage by a factor of N for N disks in the
RAID 5 configuration.
Aaron said they are "NOT in parallel". It seems reasonable to infer that he thinks they are in serial.

No, I'm asuming that you're using RAID 5, which
is what you said.
So what did you mean by this:
No, they are NOT in parallel. The are issued sequentially.
They may overlap (only if you have SCSI, or if all of the
disks are on different controllers), but they are not in
parallel.
If they occur at the same time, they are happening in parallel. Not synchronised, true, but simultaneous, i.e. parallel, even if the data is sent to the disks serially. All that's required is for the bus speed to be faster than the write speed and for the drives to have a write cache.
You're obviously a CS major, not an engineer.
And you're a psychology major? ;-)
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