Mailinglist Archive: opensuse (4498 mails)

["Greg Freemyer" <greg.freemyer@xxxxxxxxx>]

On 11/9/06, Carlos E. R. <robin.listas@xxxxxxxxxxxxxx> wrote:
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> The Thursday 2006-11-09 at 15:54 -0500, Greg Freemyer wrote:
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> ...
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> > I don't know the details of how linux implements it but conceptually
> > for the worse case with raid 5 (ie. scattered writes) to update a
> > single block you have to:
> >
> > Read original block (even if your updating it 100%)
> > Read Parity Block
> > Back-out the original block from the parity calculation  (Simple XOR logic)
> > Put the new blocks data into the parity calculation (Simple XOR logic)
> > Write the new data block
> > Write the new parity block
> >
> > So it takes 4 i/o's to do a single logical write assuming scattered disk
> > writes.
>
> Question.
>
> The data block, is it written to one disk, or is it split over two disks?
Again I don't know how Linux does it, but I just did some detail
reconstruction work on a 3-disk hardware raid5 using a dell perc
controller.

IIRC, the blocks were laid out like:

1 2 p
3 p 4
p 5 6
7 8 p

Each block was 64K (ie. 128 sectors), and full stripe was 2x64K or 128K.

So block 1 is on the first disk in the array.  block 2 is on the
second disk in the array, and the first parity block is on the third
disk in the array.  Block 3 is back on the first disk, and so on and
so on.

Now if only a 4K page is being updated, I don't know if just a single
4K page is read from the data block and parity blocks and then updates
done.  Or if the logic only works with full 64K blocks at a time.

FYI, I was working at this low level because the hardware had failed
but I was able to extract fairly good dd images of the drives and I
used a low-cost windows program to rebuild the discrete dd images into
a single logical dd image.  The tool was "Raid Reconstructor" and I
think is was less than $100.

Greg
--
Greg Freemyer
The Norcross Group
Forensics for the 21st Century